Physical Science Lesson 2

Page 1

Energy is a measure of capacity to do work. Energy is a state,

Work is done when a change in energy occurs and work is a process occurring in time. In physical science, work is defined as the product of a force and the distance through which the force acts on a body. As an equation, it’s expressed as follows:

W = Fs

In this formula, W stands for work in joules, F stands for force in newtons, and s stands for distance in meters. the force is assumed to be in the same direction as the displacement of the body, or distance if work is done. If no motion results from the application of a force, no work is done.

The metric unit of work and energy is the joule is derived as a product of newtons and meters and is sometimes indicated as a newton meter (Nm).

The centripetal force is perpendicular to the direction of movement and doesn’t cause any change in the body’s energy; it only causes an accelerated movement of the body or a change in the direction of the velocity of the body.

Kinetic energy is the energy of motion.

The three basic types of kinetic energy are:

Vibrational energy, kinetic energy due to vibration

Rotational energy, kinetic energy due to rotating motion

Translational energy, kinetic energy due to motion from one location to another.

The amount of kinetic energy that an object has depends on its mass and its speed and can be calculated using this formula:

Ek=mv^2/2

In the formula, Ek stands for kinetic energy in joules, m stands for the mass of the object in kilograms, and v stands for the object’s velocity in meters per second. Note that kinetic energy values are always positive.

Potential energy is stored energy that has the potential of doing work. think of a bow and arrow. A bow lying on a table has no potential energy. If you pick up the bow and stretch back its string, it now has the potential ability to do work (fire an arrow). After releasing the arrow, the string has kinetic energy because it's in motion.

Assume that the force applied is equal to the force of gravity acting on the box but in the opposite direction. There will be no acceleration, and the work done results only in a change of position. The resulting potential energy acquired by the lifted box is gravitational potential energy. If the box is allowed to fall from the high position to the ground, it will develop kinetic energy. The ground is usually the reference level for the height of the body.

Gravitational potential energy is expressed by the following formula:

Ep = mgh

In this formula, Ep stands for potential energy in joules, m stands for mass in kilograms, g stands for the acceleration of gravity (9.8m/s^2), and h stands for height in meters.

Work doesn’t have to be performed for the body to have potential energy. If the box is held at a certain height without any change in position or velocity, it will have a potential energy depending on its mass, gravity acceleration, and height.

steam in an enclosure under high pressure; it contains stored thermal energy (or heat energy) that can be transformed into kinetic energy, such as the motion of a turbine.

Gravitational potential energy and kinetic energy are both forms of mechanical energy. A body can have both kinds of mechanical energy, and the two kinds can change.

The bodies considered up to this point have been ideal bodies because any loss from friction isn't considered.

Note that the examples in this lesson disregard frictional effects even though they’re present in all practical applications.

The law of energy conservation states that energy can’t be created or destroyed—it can only be changed from one form to another. The law of energy conservation is a universal law of physics; it applies to all forms of energy. When applied to mechanical energy, it means that the sum of potential and kinetic energy before transformation equals the sum of potential and kinetic energy after transformation.

.The law of energy conservation can be expressed mathematically as

Original energy = Final energy

or with the following formula:

Epo + Eko = Epf + Ekf

In this formula, Epo stands for original potential energy, Eko stands for original kinetic energy, Epf stands for final potential energy, and Ekf stands for final kinetic energy.

Each form of energy—heat, sound, light, electrical, or nuclear—can be transformed into any other form of energy, the total energy is theoretically conserved. Small losses of energy that occur in practical applications aren’t considered here. In practical applications, there’s always some loss of energy, mostly due to friction. The energy isn’t actually lost; it’s just changed to a form that’s not useful in a particular process.

Power is the time rate at which work is performed. The worker who completes the job more quickly uses more power.

Power can be calculated with the following formula:

P=W/t

In this formula, P stands for power in watts, W stands for work in joules, and t stands for time in seconds.

The metric unit of power is the watt. One watt of power is exerted when the work of one joule is performed during one second. in the metric system, it’s used for all types of power, including mechanical power. In the English system, mechanical energy is measured in foot-pounds. One foot-pound is the energy exerted when one pound of force moves a body one foot on a smooth surface.

Horsepower, a widely used English unit of power, was originally defined as the time rate of work that an average horse is capable of doing. Scientifically, one horsepower is the work of 550 foot-pounds per second.

1 horsepower (hp) = 746 watts (W)

1 watt (W) = 0.00134 horsepower (hp)

Calculations are easier using metric units because the same unit is used for all kinds of power.

The ratio of useful output power to total input power in a machine, expressed in

percent, is called the efficiency of a machine. Efficiency can be calculated by using the following formula:

Efficiency=Output power/Input power×100

The efficiency of some machines may be very high, but it can never be 100 percent. A machine with perpetual action (that is, nonstop operation) isn’t possible in real practice; any machine will require a continued input of power to keep it running.

you might have to find mass first

kinetic energy = mass x velocity^2 / 2

(velocity squared mass is not)

to find horsepower divide watts by 746

the input may be described as "supplied" or "provided" power or energy. Output described as "useful work" or "useful power"

What amount of work is done when a force of 20 N is used to drag the block in the figure above a distance of 10 meters across a floor?

Solution: Use the work formula to calculate the answer.

W = Fs

W = 20 N × 10 m

W = 200 J

Find the kinetic energy of an object that has a mass of 4 kg and moves with a velocity of 5 m/s.

Solution: Use the kinetic energy formula. Substitute the known values into the formula and solve.

Ek=mv^2/2

Ek=4kg×(5m/s)^2/2

Ek=4×25/2

Ek=100/2

Ek = 50 J

the force F = mg = 50 N, and the potential energy is calculated as follows:

Ep = mgh

Ep = 50 N × 5 m

Ep = 250 J

A mass of 5 kilograms is lifted slowly to a height of 10 meters. How much work is performed during the lifting?

Solution: Use the work formula to calculate the answer. Substitute the known values into the formula and solve.

W = mgs

W = 5 kg × 9.8 m/s^2 × 10 m

W = 49 × 10

W = 490 J

Suppose that 1,000 bricks, each with a mass of 3 kilograms, must be carried up to a third floor, 10 meters above the ground. The work necessary to accomplish the task can be calculated as follows:

W = Fs

W = mgs

W = (1,000 × 3 kg) × 9.8 m/s^2 × 10 m

W = 3,000 kg × 9.8 m/s^2 × 10 m

W = 29,400 × 10

W = 294,000 J

How much horsepower is required to lift a mass of 50 kilograms through a height of 5 meters in 2 seconds?

Solution: Use the power formula to calculate the answer. Since W = mgs, you can use substitution in the power formula.

P=W/t

P=mgs/t

P=50kg×9.8m/s2×5/2s

P=490×5/2

P=2,450/2

P = 1,225 W

Now convert watts to horsepower. One watt equals 0.00134 horsepower, so multiply 1,225 W by 0.00134 to convert the value to horsepower.

1,225 W × 0.00134 = 1.64 hp

assume ideal conditions (conditions with 100 percent efficiency).

Example: If a machine is supplied energy at a rate of 2,000 W and it does useful work at a rate of 1,900 W, what is the efficiency of the machine?

Solution: Use the formula to calculate the efficiency of the machine.

Efficiency=Output power/Input power×100

Efficiency=1,900/2,000 ×100

Efficiency=0.95×100

Efficiency=95

Energy may be defined as a measure of capacity to do work; work is done when a change in energy occurs.

Work equals force times distance (W = Fs)

The metric unit of work and energy is the joule, sometimes expressed as the newton meter (Nm).

Potential energy is stored energy.

Three types of kinetic energy are vibrational, rotational, and translational.

Gravitational potential energy and kinetic energy are both forms of mechanical energy.

The law of energy conservation states that energy can’t be created or destroyed; it can be changed from one form to another.

Power is the time rate at which work is performed. The metric unit of power is the watt.

Efficiency can be calculated by dividing output power by input power and multiplying this by 100.

Page 2

General notes:

Heat is a form of energy. Because the molecules of a body (object) are continually in motion, everything possesses a certain amount of heat.

In the solid state, molecules of the body move slowly because they’re packed close together and attract each other with a relatively great force.

When the amount of heat energy in a solid body increases enough, the matter usually changes to a liquid state, where the molecules have greater freedom of motion.

If the amount of heat energy in a liquid rises sufficiently, the matter changes from liquid to the gaseous state.

Terms such as “hot,” “warm,” “cool,” “cold,” and “icy” are used to describe the relative amounts of heat energy possessed by an object.

Heat always transfers from warmer to cooler. This transfer of heat is usually fairly rapid

Heat energy can be obtained from physical, chemical, electrical, mechanical, and nuclear sources.

The term temperature indicates the hotness or coldness of a body. The temperature of a body depends on the velocity (speed) of the body’s molecules only. The amount of heat energy in a body depends on both the temperature of the body and its mass.

A thermometer measures temperature using the principle that most substances expand when heated and shrink when cooled.

When the English system is used, the temperature is measured in degrees Fahrenheit (°F). When using the metric system, the temperature is measured in either degrees Celsius (°C) or kelvins.

Mathematically, one Celsius degree is equal to 9⁄5 Fahrenheit degrees (180 ÷ 100 = 9⁄5). One Fahrenheit degree is equal to 5⁄9 Celsius degree (100 ÷ 180 = 5⁄9).

°F = (9⁄5 × °C) + 32Formula 1 celsius to Fahrenheit. °C = 5⁄9 × (°F – 32)Formula 2 Fahrenheit to celsius.

The point at which all molecular motion ceases in a body—at which the body has absolutely no heat left in it—is defined as absolute zero, the lowest temperature possible. It has been determined that absolute zero equals approximately –273°C or –460°F.

The Kelvin scale (also called Celsius Absolute) is used when working with absolute temperatures. they're used only for scientific, engineering, and manufacturing applications.

Temperatures in the Kelvin scale are represented with the letter K, but the degree symbol isn’t used with Kelvin temperatures.

The Rankine scale, also called Fahrenheit Absolute, was used in the past but has been replaced with the Kelvin scale, even in countries that normally use Fahrenheit to express temperature. The absolute temperature in degrees Rankine is obtained by adding 460 degrees to the Fahrenheit temperature.

Although the Kelvin scale has the same degree divisions as the Celsius scale, temperatures expressed in the Kelvin scale are 273° higher than temperatures in the Celsius scale. To convert a Celsius temperature to Kelvin, add 273 to the number of degrees Celsius. Zero degrees in the Kelvin scale (0 K) is –273°C. To express a temperature given in kelvins as degrees Celsius, subtract 273 from the number of kelvins.

Standardizing temperature points are the boiling temperature and the freezing temperature of water.

Standardizing Points of Temperature and Absolute Zero

Celsius, Kelvin, Fahrenheit

The boiling point of water 100°C 373K 212°F

The freezing point of water 0°C 273K 32°F

Absolute zero–273°C 0K –460°F

there's no direct way to convert between two scales; two steps are needed. to change kelvins to degrees Fahrenheit, you must first convert the measure in kelvins to degrees on the Celsius scale. Then you can convert from Celsius to Fahrenheit.

The amount of heat energy, or thermal energy, contained in a body depends not only on the temperature but also on the body’s mass and the material of which it’s made. Water is the standard material to which all other materials are compared, and it’s used as a standard for the definition of a constant called specific heat. In countries where the English system of units is used, the basic unit of heat energy is the British thermal unit (Btu). One Btu is the amount of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit at one atmosphere of pressure. One atmosphere of pressure is a standard used for scientific calculations that are based on the atmospheric pressure at sea level. Atmospheric pressure decreases as altitude increases.

Where the metric system is used, the basic unit of heat energy is either the gram calorie or the kilogram calorie (kilocalorie). One gram calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius at one atmosphere of pressure. One kilocalorie is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius at one atmosphere.

1,000 gram calories = 1 kilogram calorie

1 gram calorie = 0.001 kilogram calorie

The kilocalorie is used in reference to the “calorie content” of foods. when a package label indicates that one cup of ice cream contains 200 calories, it actually means 200 kilocalories. So, if you eat the cup of ice cream, your body will have to produce 200 kilocalories of heat energy to burn up the ice cream.

The science of heat was developed separately from the science of mechanics and other branches of physics. some of the measures are expressed in different units. heat is measured in Btus and calories, mechanical energy is measured in foot-pounds, and electrical energy is measured in watt-hours.

Although the calorie was defined using metric units (kilograms and degrees Celsius), it should not be used with the Systeme International. In the SI system, energy is measured in joules, regardless of the form the energy takes.

when a known amount of work is done on a system where all the work done shows up in an increase in the system’s water temperature, the amount of work, in joules, is always in the same relationship to the amount of heat energy, in kilogram calories. This relationship can be expressed as follows:

1 kilogram calorie = 4,187 joules

To convert to joules if you know the number of kilogram calories, use the following formula:

joules = kilogram calories × 4187

To convert to kilogram calories if you know the number of joules, use the following formula:

kilogram calories = joules ÷ 4187

It’s also helpful to know the following relationship:

1 Btu = 1,055 joules

To convert to joules if you know the number of Btus, use the following formula:

joules = Btu × 1055

To convert to Btus if you know the number of joules, use the following formula:

Btu = joules ÷ 1055

When greater quantities of energy need to be expressed, it’s more practical to use larger units that are multiples of the joule. These include the kilojoule (kJ), the megajoule (MJ), and the gigajoule (GJ).

1 kilojoule = 1,000 joules

1 megajoule = 1,000,000 joules

1 gigajoule = 1,000,000,000 joules

The amount of heat energy required to raise the temperature of a unit mass of a material one degree is called the heat capacity of the material. The heat capacity of water, for example, is 4,187 joules per kilogram degrees Celsius (J/kg°C).

The ratio of the heat capacity of any material to the heat capacity of water is called the specific heat (symbol c) of the material. the specific heat of a material is a ratio, or a pure number, not expressed in any measuring units. Its numerical value is the same no matter what units are used to express the heat capacities of the material and water. The specific heat of water is 1.

Specific Heats of Solids and Liquids page 36-37 in my digital notebook.

the specific heat of a gas depends on the conditions in which the heating process occurs. If the gas is confined while it’s being heated so that its volume can’t change as the temperature changes, the specific heat is called the specific heat at constant volume. If the gas is allowed to expand while being heated so that its pressure remains constant, the specific heat is called the specific heat at constant pressure. The specific heat values at constant pressure for several gases are given in the table that follows. For most common gases, it may be assumed that the ratio of the specific heat at constant pressure to the specific heat at constant volume is about 1.4. When the kind of specific heat (that is, constant volume or constant pressure) isn’t stated, it’s generally understood that specific heat at constant pressure is meant.

Specific Heats of Gases at Constant Pressure page 38 in my digital notebook.

If a tank with vertical sides and an open-top is filled with a liquid, the bottom of the tank must support the weight of the column of liquid over any area.

the figure is subjected to a force equal to the weight of the liquid in the column above it, or

F = mg

In this formula, m stands for mass in kilograms, and g stands for the acceleration of gravity in meters per second squared (m/s2). Acceleration of gravity is a constant value of 9.8 m/s2.

The volume of a column of liquid is obtained by finding the product of the base area and the height of the column, or

V = Ah

In this formula, V stands for volume in cubic meters, A stands for the area of the base surface in square meters, and h stands for the height of the column in meters.

The mass of the liquid column is determined by finding the product of the volume and the density, or

m = Vd

In this formula, m stands for mass in kilograms, V stands for volume in cubic meters, and d stands for density in kilograms per cubic meter.

The density of a body is defined as the mass of a unit of volume, here measured in kilograms per cubic meter (kg/m3). If the liquid is water, its density is 1,000 kg/m^3. The known densities of matter in solid, liquid, or gaseous states can be found in reference books; to solve a problem, you simply look up the density for that liquid.

The force of gravity on the column of liquid, or the weight of that column, is found by the following equation:

F = Ahdg

Pressure is the force exerted on a unit of area and can be calculated with the following formula:

P=F/A

If the force is expressed in newtons and the area is expressed in square meters, the pressure is measured in newtons per square meter, or pascals (abbreviated Pa). One pascal is the pressure of one newton on one square meter (m2). One kilopascal (kPa) is equal to 1,000 pascals.

To determine the pressure at a point in a liquid, the value of the force exerted on the liquid column can be substituted in the general formula for pressure as follows:

P=F/A=Ahdg/A

or when simplified,

P = hdg

In this formula, P stands for the pressure in kilopascals, h stands for the height of the liquid column in meters, d stands for the density of the liquid in kg/m^3, and g stands for the gravity acceleration.

For measuring pressure in a liquid, the preferred unit is the kilopascal. (1 kilopascal is equal to 1,000 pascals.)

In the English system, pressure may be expressed in pounds per square foot or pounds per square inch.

Atmospheric pressure also acts downward on the surface of the water. The pressure due to the air above the liquid (the atmosphere) should be added to the liquid pressure to get precise results. Atmospheric pressure will be discussed later, but in the following problems, the atmospheric pressure isn’t considered.

When a liquid in a container is at rest, the direction of the pressure at any point on the surface of the container is at right angles, or normal, to the surface. when an object is immersed in a liquid, the liquid exerts pressure on any surface of the object that’s in contact with the liquid.

In the same liquid, the pressures are equal at all points that are the same distance below the liquid surface. That’s true because h and d are the same for all such points. The pressure at a particular point on a vertical or an inclined surface is the same as the pressure at any point on a horizontal surface that’s at the same level below the liquid surface as the point under consideration.

In addition to gravity, a confined liquid may be subjected to additional pressure by the application of an external force. The feature of a confined liquid that allows the liquid to transmit undiminished pressure is known as Pascal’s principle. It can be stated as follows: Whenever the pressure in a confined liquid is increased or decreased at any point, the change in pressure is transmitted equally throughout the entire liquid. In this statement, it’s important to realize that pressure rather than force is transmitted.

To illustrate the application of Pascal’s principle, consider the two cylinders shown in the figure here. One is a cylinder of a solid material, concrete. Assume that it has a cross-sectional area of 0.05 m2. Further, assume that the concrete cylinder is compressed between the surfaces AB and CD by a force of 10,000 N, applied as shown. If that force is uniformly distributed over the top of the cylinder, the pressure on the top of the cylinder can be calculated as follows:

P = F/A

P = 10,000N/0.5m^2

P = 200,000 Pa, or 200 kPa

Pressure is transmitted from the top of the cylinder undiminished to the bottom of the cylinder. Unless a force large enough to cause the concrete to crumble is applied, the cylinder will retain its shape and there will be no need to apply vertical or sideways forces to help it do so.

One type of device in which Pascal’s law is usefully applied is the hydraulic press.

A surface liquid that’s subjected to atmospheric pressure is called a free surface. When a liquid is at rest, its free surface must be horizontal. If a liquid has a free surface, the pressure at a point in the liquid can be calculated by using the following formula:

P = hdg

In freshwater, which has a density of 1,000 kg/m3, the pressure at a point 3 meters below the surface is:

P = hdg

P = 3 m × 1,000 kg/m3 × 9.8 m/s2

P = 3,000 × 9.8

P = 29,400 Pa, or 29.4 kPa

A pressure gauge is any device used to measure the pressure at a point in a liquid. One such type of gage, the manometer. The manometer is a U-shaped glass tube partially filled with a suitable liquid that’s heavy and permits the measurement of a fairly high pressure with a relatively short tube.

A widely used type of pressure gage called the Bourdon gage. The Bourdon gage is widely used because it’s easily handled and read and is adaptable to a wide range of pressures. For most purposes, it gives a satisfactory degree of accuracy.

When an object is immersed or floating in a liquid, it weighs less than it does in air by an amount equal to the weight of the liquid displaced by it.

Keep in mind that “weight,” as used here, means the force of gravity acting upon the object and the displaced liquid. The upward force, called the buoyant force, is equal to the weight of the cork, causing it to float. when ice equal in volume to the cork is placed in water, the ice floats (as shown in the figure), but it sinks deeper into the water than the cork does because it’s heavier than the cork. When concrete equal in volume to the cork is placed in the water, it sinks to the bottom because it can’t displace enough water to weigh as much as it does. the water does exert a buoyant force on it equal to the weight of the water displaced by it. Because of the buoyant force, the concrete appears to weigh less in water than it does in air. If measurements are made, it’s found that the difference in the weight of the concrete block in air and in water is exactly equal to the weight of the water displaced by the block.

An important difference between a liquid and a gas is that the volume of a liquid is only slightly affected by a large increase or decrease in pressure, whereas the volume of a gas changes considerably under the same circumstances. The property of a gas that permits a decrease in volume without any change in mass is called compressibility. All gases have high compressibility; that is, they can be compressed easily. The property of a gas that permits an increase in volume without any change in mass is called expansibility. Gases have high expansibility; that is, they expand very easily.

Assume that a cylinder-shaped gas vessel contains a piston. The piston fits the cylinder tightly enough to prevent gas from leaking around it but loosely enough to move up and down easily when a force is applied.

Since air has a mass and a gravity force is exerted on that mass, the layer of air produces pressure. A simple way to measure that pressure is to use a mercury barometer. In a mercury barometer, a glass tube of mercury is placed in an open container of mercury. The air pressure on the open surface of mercury is sufficient to support the column of mercury in the tube. The weight of a column of air that’s about 1,000 kilometers high is the same as the weight of a column of mercury approximately 760 millimeters high with the same cross-sectional area.

At any one location, atmospheric pressure varies slightly from day to day. the standard atmospheric pressure is defined as a pressure equivalent to that produced by a column of mercury that’s 760 millimeters high at 0 degrees C (Celsius) usually expressed as 760 millimeters Hg because Hg is the chemical symbol for mercury. The density of mercury at 0 degrees is 13,600 kg/m3.

Two general types of barometers are used today: the liquid type just described and the aneroid type. The liquid barometer is the standard type used in the laboratory and isn’t usually portable. An aneroid barometer consists of a partially evacuated corrugated cylinder. As the atmospheric pressure changes, the top of the cylinder moves up and down. The motion is magnified by the linkage system and moves a pointer over a scale. The aneroid barometer is portable and usually calibrated to agree with a standard mercury barometer.

Atmospheric pressure decreases with increasing altitude.

When the atmospheric pressure on a liquid isn't taken into account, the liquid pressure is called gauge pressure. With a gas, however, atmospheric pressure must usually be taken into account. A pressure obtained by adding atmospheric pressure (100 kPa) to the gauge pressure is called absolute pressure. An absolute pressure is usually greater than atmospheric pressure, but it may be equal to or less than atmospheric pressure.

When the absolute pressure of a gas in a container is less than atmospheric pressure, a partial vacuum exists in the container. A partial vacuum can be produced by first enclosing a certain mass of air in a container and then removing some of the air.

Conduction is the transfer of heat through a substance from molecule to molecule as they collide. Conductivity, then, is a measure of how easily heat passes through a material. Solids are better conductors of heat than liquids because the molecules in solids are closer together. Silver is one metal that conducts heat very well. An insulator is a solid that’s a poor conductor of heat. Insulation is the ability of a material to prevent the flow of heat. Solids that contain lots of tiny air pockets, such as wood and cork, are good insulators because stagnant air is a good insulator. Good insulation materials can increase the efficiency of a home’s heating and cooling systems.

Any solid object exerts pressure on the surface it rests on. The amount of pressure relates to both its mass and the area of its bottom surface.

Video notes:

Temperature is based on the average speed of the molecules f a body. Total heat energy is affected by the velocity of the molecules and by the object's mass.

nuclear, chemical, electrical, and mechanical energy can all be turned into thermal energy. the most important source of thermal energy is the sun, which is fueled by nuclear energy.

to convert BTUs to Joules the equation is: Joules=number of BTUs x 1055 to convert Joules to BTUs the equation is: BTUs=Joules / 1055

heat capacity: the amount of energy it takes to heat up the material by one degree. Specific heat compares the heat capacity to the heat capacity of water. Specific heats less than 1 take less energy to heat up than water. For gasses, the gas must be kept at either a constant volume or constant pressure to measure the specific heat.

if the mass of the water is known use force equals mass times gravity. if the mass of the water is unknown use the formula force equals area x height x density x gravity.

the density of water is always 1000 kg/m^3

pressure is equal to force divided by area. pascal is equal to 1-newton spread over 1 meter squared.

pressure equals height x density x gravity.

the area doesn't affect the pressure only height.

to find kiloPascals divide pascals by 1000.

when finding pressure if force is not given to find the force take the mass and multiply it by 9.8 then use the result for force.

apparent weight is equal to the weight of the object minus the buoyant force.

Pressure high in the atmosphere is less than pressure lower in the atmosphere.

Examples:

A Celsius thermometer indicates a temperature of 40°C. What’s the current temperature in degrees Fahrenheit?

Solution: Use Formula 1 to convert degrees Celsius to degrees Fahrenheit. Substitute the known value into the formula and solve.

°F = (9⁄5 × °C) + 32

°F = (9⁄5 × 40) + 32

°F = (360⁄5) + 32

°F = 72 + 32

°F = 104

What’s the equivalent temperature, in degrees Celsius, to a temperature of 100°F?

Solution: To convert Fahrenheit to Celsius, use Formula 2. Substitute the known values into the formula and solve.

°C = 5⁄9 × (°F – 32)

°C = 5⁄9 (100 – 32)

°C = 5⁄9 × 68

°C = 340⁄9

°C = 37.78°C

What’s the value of 10°C in kelvins?

Solution: Add 273 to the Celsius temperature to obtain the value in kelvins.

K = °C + 273

K = 10°C + 273

K = 283

What’s the value of 300 K in degrees Celsius?

Solution: Subtract 273 from the Kelvin temperature to obtain the value in Celsius degrees.

°C = K – 273

°C = 300 K – 273

°C = 27

What’s the value of 2,843 Btus in joules?

Solution: One Btu is equal to 1,055 joules. Multiply the number of Btus by 1,055 to convert the value to joules.

1 Btu = 1,055 joules

2,843 Btus × 1,055 = 2,999,365 joules

What’s the value of 30,478 joules in Btus?

Solution: 1 Btu is equal to 1,055 joules. Divide the number of joules by 1055 to get Btus.

30,478 Joules ÷ 1055 = 28.889 Btus

What’s the value of 4,592,000 joules when converted to megajoules?

Solution: One megajoule is equal to 1,000,000 joules. Divide the number of joules by 1,000,000 to convert the value to megajoules.

1 megajoule = 1,000,000 joules

4,592,000 J ÷ 1,000,000 = 4.592 MJ

If the liquid in the tank in the figure above is water, the height of the liquid column is 5 meters, and the area is 1 square meter (m2), calculate the force of gravity on the column of water.

Solution: Use the formula F = Ahdg to calculate the force of gravity. Substitute the known values into the formula and solve.

F = Ahdg

F = 1 m2 × 5 m × 1,000 kg/m3 × 9.8 m/s2

F = 5 × 1,000 × 9.8

F = 5,000 × 9.8

F = 49,000 N

For the tank shown earlier, calculate the pressure on area A at the bottom of the tank.

Solution: Use the pressure formula. Substitute the known values into the formula and solve.

P = hdg

P = 5 m × 1,000 kg/m3 × 9.8 m/s2

P = 5,000 × 9.8

P = 49,000 Pa

What's the water pressure at the bottom of a cup in which the water stands 0.03 meters high?

Solution: Use the pressure formula. Substitute the known values into the formula and solve.

P = hdg

P = 0.03 m × 1,000 kg/m3 × 9.8 m/s2

P = 30 × 9.8

P = 294 Pa, or 0.294 kPa

If a total force exerted by the water in a container with a bottom area of 0.5 square meters is 600 newtons, what's the water pressure at the bottom of the container?

Solution: Because the total force is known, the general pressure formula can be applied here. Substitute the known values into the formula and solve.

P=F/A

P=600N/0.5m2

P = 1,200 Pa, or 1.2 kPa

If the tank shown in the figure below has a flat bottom and is filled with water to a height of 3 meters, calculate the pressure at any point on the bottom of the tank.

Solution: Use the pressure formula. Substitute the known values into the formula and solve.

P = hdg

P = 3 m × 1,000 kg/m3 × 9.8 m/s2

P = 3,000 × 9.8

P = 29,400 Pa, or 29.4 kPa

If the liquid in the figure is water, and if the height (h) of the water above point A is 1 meter, calculate the pressure at point A.

Solution: Use the pressure formula. Substitute the known values into the formula and solve.

P = hdg

P = 1 m × 1,000 kg/m3 × 9.8 m/s2

P = 1,000 × 9.8

P = 9,800 Pa, or 9.8 kPa

Assume that the surface of the piston shown in the figure above has an area of 0.01 m2 and that the piston exerts a force of 2,000 newtons on the liquid. What's the external pressure on the upper surface of the liquid?

Solution: Use the general pressure formula. Substitute the known values into the

formula and solve.

P = F/A

P = 2,000N/0.01m^2

P = 200,000 Pa, or 200 kPa

Suppose that the surface of the piston shown in the image above has an area of 0.05 m2 and that the piston carries a load of 500 kg. What will be the external pressure on the upper surface of the liquid?

Solution: Because F = mg, you can use the following variation of the pressure formula to solve this problem.

P = mg/A

P = 5kx9.8m/s^2/0.05m2

P = 4,900/0.05

P = 98,000 Pa, or 98 kPa

The cylinder in the figure above is filled with water 0.4 meter high. What's the pressure at the bottom of the container if the internal pressure is considered?

Solution: Use the formula to calculate the internal pressure of the confined liquid at the bottom of the container. Substitute the known values into the formula and solve.

P = hdg

P = 0.4m x 1,000/m3 x 9.8m/s2

P = 400 x 9.8

P = 3,920 Pa, or 3.92 kPa

The total pressure at the bottom is the sum of the external pressure and the internal pressure of the confined liquid. This can be calculated as follows:

200 kPa + 3.92 kPa = 203.92 kPa

The pressure exerted by a column of mercury 0.760 meters high can be calculated using the same pressure formula as for a liquid.

P = hdg

P = 0.760 m × 13,600 kg/m3 × 9.8 m/s2

P = 10,336 × 9.8

P = 101,292.8 Pa, or 101.3 kPa

In practical applications, it may be assumed that the atmospheric pressure is approximately 100 kPa, or 760 mm Hg. This is the value of normal atmospheric pressure. The pressure is equivalent to 14.7 psi (pounds per square inch) if expressed in English units.

Object 1 in the figure has a bottom surface area of 0.2 m2, and Object 2 has a bottom surface area of 0.8 m2. What's the pressure exerted by each object on the tabletop?

Solution: First, use the formula F = mg to calculate the force for each object.

F = mg

F = 20 kg × 9.8 m/s2

F = 196 N

Next, use the formula to calculate the pressure for Object 1.

P = F/A

P = 196N/0.2m^2

P = 980 Pa

Use the formula again to calculate the pressure for Object 2.

P F/A

P = 196N/0.8m^2

P = 2 Pa

Answer: Object 1 exerts a pressure of 980 Pa, and Object 2 exerts a pressure of 245 Pa. Thus, you can see that when both objects have the same mass, the object with the smaller bottom surface area will exert more pressure on the tabletop.

Key points:

Heat is a form of energy related to the motion of the molecules in a body. Whenever a change of state occurs in a substance, heat energy is either gained or released.

Molecules in the solid state move slowly because they’re packed close together; molecules in a liquid have greater freedom of motion; the forces of attraction between the molecules of a gas are relatively small, so a gas will tend to expand its volume and occupy any space to which it has access.

Heat energy can be obtained from physical, chemical, electrical, mechanical, and nuclear sources.

Thermometers measure temperature. Their operation is based on the principle that most substances expand when heated and shrink when cooled.

In the English system, temperature is measured in degrees Fahrenheit (°F). In the metric system, temperature is measured in either degrees Celsius (°C) or kelvins (K).

On the Celsius scale, the freezing point of water is designated as 0°C, and its boiling point is 100°C.

On the Fahrenheit scale, the freezing point of water is designated as 32°F, and its boiling point is 212°F.

Absolute zero is the lowest temperature possible, when all molecular motion ceases in a body. Absolute zero equals 0K, –273°C, or –460°F.

A Btu is the amount of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit at one atmosphere of pressure.

In the SI system, energy is measured in joules. 1 joule = 0.0002388 kilogram calorie = 0.000948 Btu

Specific heat is the ratio of the heat capacity of a material to the heat capacity of water.

If the temperature of a solid is increased, the solid will expand; if the temperature is reduced, the solid will contract.

Boyle’s Law states that when the pressure of a gas increases, its volume decreases; when the volume of the gas increases, its pressure decreases (PoVo = PfVf).

Charles’ Law states that when a gas is heated while keeping its volume constant, both its temperature and pressure will rise; when the same gas is cooled, both its pressure and temperature will drop.

If constant pressure is maintained while the temperature of a gas is raised, the volume increases; if the pressure is maintained while the temperature is decreased, the volume decreases.

In conduction, heat is transferred from one molecule to another molecule by direct contact.

In convection, heat is transferred by the movement of molecules in a liquid or gas.

Convection can’t take place in a solid.

In radiation, a hot body sends out rays that pass through a medium without raising the temperature of that medium appreciably. Neither contact nor a conducting medium is required.

When waves of radiant energy strike the surface of a body, part of the heat energy is absorbed by the body, and the remainder is reflected by the surface. Absorption occurs when energy is taken into matter from outside. Reflection occurs when energy is bounced off a surface.

Page 3

In a solid, the bonds are strong and hold the molecules so they can’t move. In liquids, the molecules have more energy and move more freely. In a gas, the energy of the molecules almost completely overcomes the intermolecular bonds, and they’re able to move freely. In a solid, the bonds are strong and hold the molecules so they can’t move. In liquids, the molecules have more energy and move more freely. In a gas, the energy of the molecules almost completely overcomes the intermolecular bonds, and they’re able to move freely. A few substances exist in only one state at a reasonable temperature. Most substances exist as a solid or a liquid, or as a liquid or a gas, at reasonable temperatures.

The critical temperatures of a substance are the points at which the substance changes its state. The same substance is a solid below its freezing point, a liquid between its freezing and boiling points, and a gas above its boiling point. The critical temperature at which a material changes from solid to liquid is called the melting point, or the fusion point. The critical temperature at which evaporation occurs is called the boiling point or the evaporation point. Molecules of a gas become restricted in their movement to such a degree that they change to the liquid state at a critical temperature called the condensation point.

Molecules of a liquid become more restricted in their movement so that they change to the solid-state at a critical temperature called the freezing point. If a substance forms crystals when it changes from a liquid to a solid, the freezing point and the melting point of the substance are the same. A substance that doesn’t form crystals when it solidifies is called an amorphous substance; it doesn’t have a definite freezing point or melting point. For every substance, the boiling point and the condensation point are the same.

The mass of a substance in the solid-state occupies less space than the same mass of that substance in the liquid state. the exception to this rule is water—just before the temperature of the water reaches the freezing point, its volume increases.

The process by which a liquid changes into a gas is called vaporization or evaporation. Under normal conditions of temperature and pressure, the gaseous state of a liquid is called vapor. The heat energy required to vaporize liquid is obtained from the air, and the air becomes cooler. A liquid that evaporates rapidly at a low temperature is called a volatile liquid.

When the temperature of liquid in a closed container is raised, at least a small amount of vapor will form above the liquid. The vapor above the liquid in the container will produce vapor pressure on the upper surface of the liquid. As the temperature is raised, more and more molecules of the liquid will evaporate. However, many of the molecules in the lower portion of the vapor will return to liquid (condense). As more molecules enter the vapor, more molecules also return to the liquid.

If the temperature of a liquid is raised to a certain value and then kept constant, eventually the number of molecules returning to the liquid from the vapor and the number of molecules leaving the liquid will be the same. In other words, the rate of evaporation and the rate of condensation will be the same. The liquid is then said to be at its saturation point. The vapor pressure for a liquid at a particular temperature at the saturation point is called the vapor tension, which has a definite value.

When a substance is heated or cooled and the temperature changes, this is called sensible heat, because a person can sense the difference. When heat is added or removed at a critical temperature, the temperature of the substance does not change. It is instead used to make or break bonds between molecules, changing the state of the substance. The heat absorbed or lost by a substance changing state without changing from the critical temperature is called latent heat. This heat energy can be used in a variety of practical systems, such as refrigeration, home heating, and air conditioning.

The frozen-food compartment of the refrigerator is surrounded by coils of pipes that make up the evaporator. The coils of the evaporator contain a volatile liquid called refrigerant. Since the refrigerant is under low pressure, it expands easily and absorbs heat energy from the air in the freezing compartment. The absorbed heat serves as the vaporization heat for the refrigerant, and the liquid evaporates. As a result of this evaporation, the air in the frozen-food compartment is cooled rapidly. After the refrigerant has evaporated into a gas, it stays at a low pressure. A compressor moves the gas from the evaporator into the condenser while increasing the gas pressure. The condenser is made up of a series of coils. The gas cools as it passes through the coils and condenses back into a liquid, giving up the heat of condensation to the outside air. The liquid, under high pressure, forces its way through the expansion valve back to the evaporator. The pressure of the liquid is decreased, and the process is repeated.

In a hot-air heating system, fuel such as oil, natural gas, or coal is burned in a firebox inside a furnace. An air space surrounds the firebox. The heat from the burning fuel warms the inner walls of the furnace and the air in the outside jacket of the furnace. This heated air passes by convection (either freely or forced by a fan) through pipes that lead to the areas being heated. it is usually the least expensive type of central heating system to install. Its pipes require more space than hot-water or steam systems, but these same pipes can easily be utilized for an air-conditioning system.

In a hot-water heating system, a water jacket surrounds the firebox section of the furnace instead of an airspace. Pipes lead from the water jacket to radiators or baseboard heating units in various parts of the house. As the water is heated, it’s circulated by simple convection or by a water pump. The rooms are heated by hot-water circulation—cooler water flows back to the water jacket, where it’s heated again. A hot-water system maintains a more uniform temperature in the rooms than a hot-air system. However, it usually responds more sluggishly when a sudden change in temperature occurs.

In a steam heating system, the water in the jacket surrounding the furnace’s firebox is heated to the boiling point, and the resulting steam is piped to the rooms. Heat is given off to radiators in two ways. First, heat is released when the steam condenses; second, heat is released when the hot water cools before flowing to the boiler to be changed back to steam.

Another heating system, the heat pump, uses the same principles that are applied in refrigerators. A refrigerant with a low boiling point (such as ammonia) is circulated through the system, which can be used to heat or cool the air in a building.

Some homes use electrical heating systems instead of furnaces that burn fuel. In an electrical heating system, the heating units consist of electrical resistors that are distributed throughout the building and that are connected to electrical circuits. Electrical heating systems are also clean and produce fewer environmental pollutants than other heating systems. The initial cost of installing the system is relatively low because there’s no need for a furnace. However, the electrical energy is more costly than conventional fuels.

One of the most promising energy sources, at least for partial energy needs, is solar energy. The heating of buildings by solar energy has two basic problems: collecting enough of the sun’s energy and storing it for future use.

the compressor maintains the circulation of a volatile fluid through various parts of the system. The fluid enters the compressor as a gas under low pressure and is forced into the condenser as a gas under high pressure. The gas in the condenser is cooled from the outside by a fan. In this cooling process, the gas loses heat to the surrounding air and returns to the liquid state. The fan pushes the warm air to the outside. The liquid under high pressure is forced into the receiver and is then filtered through the drier. the liquid passes through the regulating valve, where it expands in a series of coils called the evaporator. The air that’s to be cooled is forced over the outside of these coils by a fan. As the volatile liquid expands in the evaporator, it absorbs heat energy from this air and changes into a gas. The air becomes cooler as it loses heat in the process. The gas in the evaporator is drawn into the compressor, and the cycle is repeated. In an air-conditioning system, the cooled inside air passes through a humidifier, which controls the moisture content, and through a filter, which removes the impurities. The air then enters a set of ducts that carries it to various rooms. Another set of ducts carries the used air back to the unit for recirculation.

Under normal atmospheric pressure, the boiling temperature of water is 100°C. However, if the water is placed under higher pressure, the boiling point of the water will be higher than 100°C. Therefore, water under pressure can exist as a liquid at a higher temperature. This principle is applied in a common kitchen appliance called a pressure cooker. Food is placed in the pressure cooker with a small amount of water. The lid of the cooker is fastened securely in position so that pressure can build up in the cooker as the water is heated. Under these conditions, the water temperature can be raised to about 121°C before the water boils, and food can be cooked more quickly than if the water boiled at 100°C. The same principle is applied in a car radiator. A car radiator uses a pressure cap to keep the pressure inside higher than the atmospheric pressure. The same principle is applied in a car radiator. A car radiator uses a pressure cap to keep the pressure inside higher than the atmospheric pressure.

Video Notes:

Examples:

Key Points:

https://lessons.pennfoster.edu/savi/transcripts/IN00736_T.pdf

The critical temperatures of a substance are the points at which the substance changes its state.

Change from solid to liquid is called the melting point or the fusing point; change to the gaseous state is the boiling point or evaporation point; gas changes to the liquid state at the condensation point; liquid changes to the solid-state at the freezing point.

In a freezer, coils of pipe contain a volatile liquid called refrigerant. A compressor moves the gas from the evaporator into the condenser while increasing the gas pressure. In a refrigerator, the cooling process takes place mainly by convection.

In a hot-air heating system, fuel is burned in a firebox inside a furnace. Heat from the burning fuel warms the inner walls of the furnace and the air in the outside jacket of the furnace. This heated air passes by convection through pipes that lead to the areas being heated.

In a hot-water heating system, a water jacket surrounds the firebox of the furnace. As the water is heated, it’s circulated by simple convection or by a water pump.

In a steam heating system, the water in the jacket surrounding the furnace’s firebox is heated to the boiling point, and the resulting steam is piped to the rooms to be heated.

A heat pump uses the same principles that are applied in refrigerators.

Page 4

A fuel is a substance containing stored chemical energy that can be released when the temperature of the fuel becomes high enough. The process by which this energy is released is called combustion. The most important chemical elements in fuel are hydrogen, carbon, and sulfur. The combustion process usually involves the chemical combination of one or more of these elements with oxygen. In the combustion process, the fuel is burned, and most of the chemical energy stored in the fuel is converted to light and heat. the temperature of the fuel must be raised to a certain point, called the ignition point. The ignition point for a given fuel depends on the nature of the fuel. A relatively small amount remains in the substances that are formed as a result of chemical reaction. For a fuel to burn completely, a sufficient quantity of oxygen must be supplied. If the oxygen is obtained from the air (which is 23 percent oxygen by mass), the mass of air needed to permit complete combustion of a fuel may be found by dividing the required mass of oxygen by 0.23.

A practical fuel must meet the following requirements: It must contain a large amount of stored energy; it must be composed mainly of elements that combine readily with oxygen; it must either occur in large quantities in nature or be easily produced from natural materials. A substance that’s used as a fuel may be in the solid, liquid, or gaseous state. The most common solid fuels are coal, charcoal, and wood. The most common liquid fuel is produced from petroleum and is known as fuel oil. Some liquid fuels are obtained by subjecting natural petroleum gases to sufficient pressure. The gaseous fuels in common use are natural gas and manufactured gas.

Coal is generally classified as anthracite or bituminous. The difference between the two types of coal is in the carbon content. The carbon content of anthracite coal is between 92.5 and 97 percent, and the carbon content of bituminous coal is between 50 and 75 percent. Coke is formed by heating bituminous coal in a coke oven until all the volatile matter is driven off. The coke that remains consists of carbon and a small amount of ash, sulfur, and phosphorous. charcoal is obtained by heating wood until all of the volatile matter is driven out. The remaining charcoal consists of carbon and ash. It's practically free of sulfur and phosphorus.

Fuel oil is the substance that remains as a liquid when crude petroleum is distilled and the gasoline, kerosene, and naphtha are removed. An important property of fuel oil is its flash point, the lowest temperature at which the oil gives off a vapor that can be readily ignited. For a fuel oil to be safe for use in an ordinary furnace, the flashpoint shouldn’t be lower than 66°C. The liquid fuel that’s obtained by compressing natural petroleum gases has propane and butane as its principal compounds. These substances are in the gaseous state under normal atmospheric pressure, but they become liquid when subjected to moderate pressure in a suitable container.

The most important combustible substances usually present in natural petroleum gas are methane, hydrogen, ethane, and carbon monoxide. Coal gas, which is produced by heating bituminous coal under suitable conditions, consists mainly of methane, ammonia, and tar. The relative density of coal gas with respect to air is about 0.45. Water gas, which is produced by bringing steam in contact with carbon in the form of anthracite or coke, is a mixture of hydrogen and carbon monoxide. The density of the class of water gas that possesses the most heat energy is about 0.625.

The heat energy that can be developed from the complete combustion of a unit mass of a fuel is called the heating value of the fuel and can be expressed in joules per kilogram. The heating values of the combustible elements in fuels are given in the table in my digital notebook on page 103.

The approximate heating value of various solid and liquid fuels are given in the table in my digital notebook on page 103.

For gaseous fuels, the approximate heating values at normal atmospheric pressure and at 15°C are given in the table in my digital notebook on page 104.

In an internal-combustion engine, a mixture of air and fuel is burned inside the engine to produce power. The engines used in automobiles are all gasoline-powered, internal-combustion engines. The engines used in large trucks are usually powered by diesel fuel. These diesel engines are different in construction and operation from gasoline engines. Gasoline is volatile at ordinary temperatures, whereas diesel fuel doesn’t vaporize until the temperature becomes relatively high. Natural gas (methane), propane, and alcohol fuels are also used in some specialized vehicles. For example, propane fuel is often used in forklifts, and alcohol fuel is used in some racing cars. Natural gas and propane fuels are being increasingly used instead of gasoline for road vehicles as well.

The cylinder is a hollow metal tube that’s drilled into the solid metal engine block. The piston is a can-shaped component that can move smoothly up and down inside the cylinder. The piston is the main moving part in an engine. The top of the cylinder is sealed by a metal cover called the cylinder head.

When the piston is positioned at the very top of the cylinder, an open area is still left above the piston called the combustion chamber, a mixture of air and gasoline is burned to produce power. Fuel from the car’s fuel tank is vaporized and mixed with air in the engine. The air-and-fuel mixture is then delivered to the cylinder through the intake valve. After combustion takes place, the burned gases that are the waste products of combustion are vented out of the cylinder through the exhaust valve.

the spark plug is screwed into a hole in the cylinder head. The end of the spark plug protrudes through the cylinder head and into the combustion chamber. The spark plug is used to make sparks that will ignite the air-and-fuel mixture in the cylinder and cause it to burn. When the air-and-fuel mixture is ignited by a spark in the combustion chamber, the mixture burns suddenly with an explosive force. This force pushes down on the piston and gets it moving in the cylinder. The ignition of the air-and-fuel mixture by the spark plug will occur over and over again for as long as the engine is running, keeping the piston moving up and down constantly. The total distance that the piston moves from the top of the cylinder to the bottom is called the stroke. The piston is connected to the crankshaft by the connecting rod. When the piston is forced downward in the cylinder, the piston’s downward motion is transferred through the connecting rod to the crankshaft. The rod and crankshaft convert the up-and-down motion of the piston to rotary motion. The crankshaft is connected to other engine components that use this rotary motion to make the car move forward.

for the purposes of this discussion, we’re looking at a simplified drawing of just one engine cylinder. a real automotive engine will usually contain either six or eight cylinder-and-piston assemblies, all moving up and down at the same time to produce a great deal of turning power in the crankshaft.

To produce power, an engine must continually go through the same four stages of operation:

Intake. A mixture of air and fuel is drawn into the cylinder.

Compression. The piston rises and compresses the fuel mixture in the combustion chamber.

Power. The fuel mixture is ignited by the spark plug, and the resulting explosion forces the piston down in the cylinder.

Exhaust. The exhaust gases left over from the combustion are released from the cylinder.

In a four-stroke engine, the piston requires four movements (two upward and two downward) to complete one full engine cycle. In a four-stroke engine, the four piston strokes complete the four stages of operation as follows:

Piston stroke 1: Intake stage

Piston stroke 2: Compression stage

Piston stroke 3: Power stage

Piston stroke 4: Exhaust stage

During the intake stage, air that has been mixed with fuel is delivered to the cylinder. The intake valve opens, and the piston moves down in the cylinder, creating a vacuum. This vacuum draws the air-and-fuel mixture into the cylinder through the open intake valve. The intake stage continues until the piston reaches its lowest position in the cylinder. the engine’s exhaust valve stays closed at this time.

When the piston reaches its lowest position in the cylinder, the intake stage is completed, and the compression stage of the cycle begins. During this stage, both the intake and the exhaust valves are closed. The air-and-fuel mixture is now trapped inside the sealed combustion chamber. At this point, the piston begins to rise, compressing the air-and-fuel mixture tightly. Compressing the air-and-fuel mixture makes it ignite more easily and burn more efficiently, thus producing more power.

The compression stage continues as the piston rises. The temperature and pressure of the mixture are increased very rapidly and reach their highest values at the end of the compression stroke. As the piston approaches its highest position in the cylinder, the engine’s ignition system causes the spark plug to “fire” when a spark is applied to a compressed mixture of air and fuel, a contained explosion immediately occurs in the cylinder.

When the air-and-fuel mixture is ignited, the temperature and pressure of the mixture increase rapidly, and the burning gases expand with great force inside the cylinder. As a result, the piston moves down hard and fast. Because the piston is connected to the crankshaft through the connecting rod, the piston’s downward motion causes the crankshaft to turn around. (This downward motion is similar to the pressure of a person’s foot pressing down on the pedal of a bicycle, causing the wheels to turn and the bike to go forward.) This stage of burning the mixture in the cylinder and forcing the piston down is called the power stage.

The power stage continues until the piston reaches its lowest position in the cylinder. At this point, the piston begins to move upward in the cylinder again, and the exhaust valve opens. (The intake valve remains closed at this time.) As the piston moves upward in the cylinder, it forces the remaining burned gases in the cylinder out through the open exhaust valve. This is the exhaust stage

The exhaust stage continues until the piston reaches its highest point in the cylinder. Once the exhaust stage is completed, the four stages of engine operation are finished, and the cycle begins all over again. The intake valve opens, the piston moves downward to draw in more fuel, and a new intake stage begins. One engine cycle is a complete run through all four stages of operation—intake, compression, power, and exhaust. All the stages must occur for the engine to function properly. One engine cycle is a complete run through all four stages of operation—intake, compression, power, and exhaust. All the stages must occur for the engine to function properly.

If an automobile engine had only one cylinder, the vehicle wouldn’t move very smoothly because power would be supplied only once out of every four strokes of the piston. a typical automobile engine contains six or eight cylinders in which the corresponding cycles occur at different times. The connecting rods for all the pistons are attached to a single crankshaft, and therefore, the rotation of the crankshaft is practically uniform and very smooth.

Diesel engines, powered by diesel fuel rather than gasoline, are found in some automobiles but more commonly in large commercial trucks, buses, and other heavy-duty vehicles. Diesel engines contain crankshafts, connecting rods, and pistons just like gasoline engines, but a diesel engine has no ignition system. it uses very high compression to create heat in the cylinder, and the heat ignites the air-and-fuel mixture. The compression inside the cylinder is so high, and the air-and-fuel mixture is squeezed so tightly, that the mixture begins to heat up, igniting the fuel in the cylinder without any assistance from a spark plug. Because of its high compression, the four-cycle diesel engine is more powerful and fuel-efficient than the four-cycle gasoline engine. these advantages are offset by the problems of hard starting and sluggish operation when the diesel engine is cold. When the engine cylinder is cold, a lot of heat is lost during compression. This is why diesel engines are usually found on larger vehicles or outdoor power equipment that must run for long periods of time.

A diesel engine’s internal components are very similar in operation and appearance to those of the gasoline engine. Diesel components are usually larger and heavier than gasoline engine components. When compared to a gasoline engine, the diesel engine's combustion chamber is small. The smaller the combustion chamber, the more tightly the fuel mixture will be squeezed, and thus, the higher the compression. Sometimes, a diesel engine cylinder won’t even have a real combustion chamber space. Instead, a small depression at the top of the piston serves as the combustion chamber. Usually, a diesel engine has a small, separate chamber called a precombustion chamber. The precombustion chamber helps to produce a swirling motion in the combustion chamber that mixes the air and fuel more thoroughly for combustion. The fuel begins burning in the precombustion chamber, and the expanding mixture then rushes into the combustion chamber to mix with more air and finish burning.

A diesel engine uses a fuel-injection system to spray fuel directly into the combustion chamber. The fuel injector must spray fuel in a precise pattern that causes the fuel to vaporize thoroughly and burn completely. The engine’s speed and power output are controlled by the amount of fuel that’s injected into the combustion chamber. the fuel injector is usually installed so that it extends into the precombustion chamber. The diesel fuel can’t ignite without sufficient heat, so a completely cold engine can’t start. A glow plug is used to add heat to the combustion chamber. The glow plug is a resistive electrical device that produces heat when an electrical current is passed through it. Glow plugs in modern diesel engines are designed to turn on automatically when the ignition switch is turned on and the engine is cold.

During the intake stroke, the inlet valve is open to permit pure air to enter the cylinder, whereas the exhaust valve and the oil-supply valve are closed. The pressure in the cylinder is nearly the same as that of the atmosphere. During most of the compression stroke, both the intake valve and the exhaust valve are closed, and the air is subjected to a pressure of between 3,100,500 kPa and 3,789,500 kPa. The temperature of the air at this pressure is about 538°C, which is higher than the ignition point of the fuel. Shortly before the piston reaches the top of the cylinder, the oil-supply valve is opened, and oil is pumped into the cylinder under sufficient pressure. Because of the temperature of the compressed air in the cylinder, the fuel oil ignites as soon as it enters the cylinder.

For a short time at the beginning of the power stroke, the oil-supply valve remains open while the rest of the fuel oil is pumped into the cylinder. The expansion of the burning mixture of oil and air forces the piston to move downward. During the first portion of the power stroke, while oil is being admitted to the cylinder and combustion continues, the pressure within the cylinder remains almost unchanged. After the complete combustion of the mixture, the pressure drops rapidly until it’s a little greater than that of the atmosphere.

All of the valves remain closed while most of the expansion of the mixture takes place. However, just before the end of the power stroke, the exhaust valve is opened. During practically the entire exhaust stroke, the intake valve remains closed, and the exhaust valve remains open. Just before the piston’s direction of motion is changed at the end of the exhaust stroke, the intake valve is opened and a new cycle is started.

All of the valves remain closed while most of the expansion of the mixture takes place. However, just before the end of the power stroke, the exhaust valve is opened. During practically the entire exhaust stroke, the intake valve remains closed, and the exhaust valve remains open. Just before the piston’s direction of motion is changed at the end of the exhaust stroke, the intake valve is opened and a new cycle is started.

Another type of internal-combustion engine is called a gas turbine. Air is fed through the inlet pipe into a set of rotating blades in the compressor. The compressed air passes through the pipe. Some goes to the outer combustion chamber, and some goes to its inner oil-burning chamber. Fuel oil from the pump enters the oil-burning chamber, where it’s mixed with some of the compressed air from the pipe. This mixture is then ignited and burned in the chamber. The gaseous products of combustion mix with the compressed air in the outer combustion chamber, where these gases are cooled to a suitable temperature for use in the turbine wheel. The turbine wheel consists of a large number of blades mounted on a shaft. The turbine wheel consists of a large number of blades mounted on a shaft.

Gas turbines are commonly used in aircraft. The most suitable fuels for gas turbines are aviation-grade gasoline and a proper blend of gasoline, kerosene, and fuel oil.

According to Newton’s third law of motion, every action must be accompanied by an equal and opposite reaction. Jet propulsion depends primarily on the application of Newton’s third law. The principal vehicles that are propelled by jet action are aircraft and rockets. In any jet-propelled vehicle, the thrust that propels the vehicle forward is developed by discharging jets of gas at a high velocity backward. In the engines that are used to propel aircraft, oxygen from the air is used to support combustion of the fuel, and the discharge of the hot gases resulting from the combustion process develops the thrust. A rocket engine also develops thrust by discharging hot gases, but the combustion of the fuel doesn’t depend on the oxygen from the air.

A longitudinal section of a turbojet engine When the engine is in motion, air from the atmosphere enters the inlet case at atmospheric pressure and passes through the compressor, which is driven by the turbine. In the combustion chamber, some of the compressed air is used to support combustion of the fuel. The fuel is supplied to this chamber by the fuel injectors. The heat produced by the combustion of the fuel causes all of the gases in the chamber to expand and to move through the chamber with a greater velocity. The force required to obtain this velocity develops a reaction that tends to propel the combustion chamber forward. This reaction helps to propel the entire engine. Before the exhaust gases are discharged from the engine, they must strike the blades of the turbine. Since the turbine is mounted on the same shaft as the compressor, the rotation of the blades drives the compressor. In some types of turbojet engines, the exhaust gases must also pass through an afterburner before they’re discharged through the exhaust nozzle. The afterburner is simply another combustion chamber. As a stream of gases enters the afterburner, it’s injected with fuel. The combustion of this fuel reheats the exhaust gases. These reheated gases pass through the exhaust nozzle with greatly increased velocity. As a result, the forward thrust on the turbojet engine is greater than it would have been without the afterburner.

the combustion of the fuel in a rocket engine doesn’t depend on oxygen from the air. The rocket carries its own supply of oxygen in the form of an oxidizer. An oxidizer is a substance that can support the combustion of a fuel. The fuel used in a rocket engine either may be a liquid or a solid. When a liquid fuel is used, the oxygen required for fuel combustion is generally obtained from an oxidizing liquid carried in a separate tank. The fuel and the oxidizer must be fed gradually into the combustion chamber to control the rate of combustion. When a solid fuel is used, the oxidizer can be diffused through the fuel in a mixture. The entire mixture is then placed in the combustion chamber before the mixture is ignited. The most suitable liquid fuels for rocket engines are ethyl alcohol, gasoline, kerosene, liquid hydrogen, and ammonia. Liquid oxygen is commonly used as the oxidizer for these fuels.

in the gas turbine plant the fuel and oxidizer are stored under atmospheric pressure in separate tanks. These liquids are passed through their respective pumps, through the control valve, and later into the combustion chamber under high pressure. The pumps are mounted on a common shaft that’s driven by a gas turbine. The gases for propelling the rocket are discharged from the exhaust nozzle. The exhaust gases leaving the turbine can also be used to increase the thrust that propels the rocket. In the second power plant, the fuel and the oxidizing liquid are subjected to the required pressure by an inert gas that’s admitted to the fuel tank and the oxidizer tank. This inert gas is stored under very high pressure in a separate tank and passes through a pressure regulator before it’s fed into the fuel and oxidizer tanks. The control valve, the combustion chamber, and the exhaust nozzle are similar to those in the type of plant shown in the first plant. An inert gas is a gas that doesn’t react with other substances or combust.

A rocket engine is used in some cases to assist the main engine of a jet-propelled aircraft when it’s taking off from the ground. For this purpose, a rocket engine that uses solid fuel is usually preferred to one that uses liquid fuel. The mixture of solid fuel and the oxidizer is placed in the combustion chamber. Combustion is started by an ignition device near the exhaust nozzle, and the combustion process continues until the entire charge is consumed. The rocket thrust comes from the pressure created by the expanding hot gases being discharged from the nozzle.

Fossil fuels come from long dead (fossilized) biomatter, such as oil and coal. Fossil fuels rely on reserves (coal in mountains and oil in oil fields) that would take many lifetimes for the earth to replace. Fossil fuels are carbon-rich—they have a lot of carbon atoms and hydrogen atoms. The chemistry of those bonds enables them to provide a lot of energy when they're burned in your car engine or in a power plant. That carbon is released into the air and combines with oxygen to make carbon dioxide (CO2). In our atmosphere, carbon dioxide traps the sun's heat more than the earth would normally, causing the average temperature on the planet to rise over time. This is similar to the glass in a greenhouse. Even in the winter. Scientists have named this effect on the planet the greenhouse effect.

There are many other sources of energy besides fossil fuels. Dams such as the Hoover Dam use running water to turn engines and make electricity. Nuclear power plants use the heat given off by nuclear material to spin turbines and create electricity. Solar power uses the sun's light to make electricity. Wind turbines also generate electricity, but only in windy areas.

Biofuels are made out of organic plant material such as algae or corn. Currently, ethanol made from corn is a popular biofuel present in gasoline in small quantities. Look for E-85 labels the next time you're at a gas station. Ethanol helps make gasoline burn a little more efficiently, but it uses corn.

A practical fuel must store a large amount of oxygen, combine readily with oxygen, and be easy to produce or found in abundance in nature.

For example, one kilogram of hydrogen requires eight kilograms of oxygen to form water; therefore, the amount of air needed for complete combustion can be calculated as follows:

Amount of air needed = mass of oxygen needed ÷ 0.23

Amount of air needed = 8 kg ÷ 0.23

Amount of air needed = 34.8 kg

A fuel contains stored chemical energy that can be released when the temperature becomes high enough. The process by which this energy is released is called combustion. For a fuel to burn completely, a sufficient quantity of oxygen must be supplied.

A practical fuel must contain a large amount of stored energy, be composed mainly of elements that combine readily with oxygen, and either occur in large quantities in nature or be easily produced from natural materials.

The most common solid fuels are coal, charcoal, and wood. The most common liquid fuel, fuel oil, is produced from petroleum. The gaseous fuels in common use are natural gas and manufactured gas.

The heat energy that can be developed from the complete combustion of a unit mass of a fuel is called the heating value of the fuel; it’s expressed in joules per kilogram.

In an internal-combustion engine, a mixture of air and fuel is burned inside the engine to produce power.

The fuels most commonly used in internal-combustion engines are gasoline and diesel fuel, which are normally in a liquid state. Gasoline is very volatile at ordinary temperatures, whereas diesel fuel doesn’t vaporize until the temperature becomes relatively high.

To produce power, an engine must continually go through the same four stages of operation: Intake (air and fuel are drawn into the cylinder); compression (the piston rises and compresses the fuel mixture in the combustion chamber); power (the fuel mixture is ignited and forces the piston down in the cylinder); and exhaust (the gases left over from the combustion are released from the cylinder).

Diesel engines have no ignition system. Instead, very high compression creates heat in the cylinder, igniting the air-and-fuel mixture.

A glow plug is needed to add heat to the combustion chamber of a diesel engine in cold weather.

The gas turbine is an internal-combustion engine in which high-pressure gas strikes the blades of the turbine and causes the turbine and shaft to rotate, which provides the mechanical energy needed to drive a generator.

In a jet-propelled vehicle, the thrust that propels the vehicle forward is developed by discharging jets of gas backward at a high velocity.

In turbojet engines, oxygen from the air is used to support fuel combustion.

Rocket engines carry their own supply of oxygen in the form of an oxidizer.