Chapter 7 Energy

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Chapter 7 Energy

7.1 Work

Work: is the

effort exerted on something that will change its energy

2 factors whenever work is done

1.

The application of a force

2.

The movement of something by that force

Work=

force X distance; W= Fd

-if

the distance is twice as much or the weight is twice as much we are doing twice

as much work to move it

-when

lifting a barbell you are doing work on a barbell, when just holding the

barbell still you are not doing work on the barbell

-when

pushing against a wall you are using your own energy to push against the wall,

but no work is done on the wall because it does not move

2

categories of work

1.

Work is done against another force

-

You do work on something when you force it to

move against the influence of an opposing force (often friction)

2.

Work done to change the speed of an object

-

(work against a force or changing speed), work

involves a transfer of energy (to another object)

Unit

measurement for work: combines a force (N) with a unit of distance (m); the

unit of work is the newton-meter (N*m) aka the joule (J)

-

One joule of work is done when a force of 1

newton is exerted over a distance of 1 meter (smaller values)

-

kJ kilojoules or megajoules MJ (larger values)

Power:

-

we do the same amount of work weather we walk or

run up the stairs; the power is different

Power: is the amount of work done per time it takes

to do it

-

Power= work done/ time interval

-

Unit of Power: joule per second J/s; also known

as a watt. 1 watt of power is expended when 1 joule of work is done in 1 second

o

1 horsepower is 746 watts, e.g. 134 horsepower=

100 kW

Mechanical

Energy:

-

E.g. when pulling back a bow the archer gives

the bow the ability to do work on the arrow; when work is done to wind a spring

mechanism, the spring acquires the ability to do work on various gears to run a

clock

-

Something is acquired that enables the object to

do work.

Energy: enable an object to do work

(measured in joules)

Mechanical energy: the energy due to

the position of something and the movement of something

7.2

Potential Energy

Potential energy (PE): energy that is stored and has the potential do

work

- a bow string has the potential to do work. When it is drawn, the energy is stored in the bow.

-chemical energy in fuels is also potential energy. (energy of position at the submicroscopic level)

- this energy is available when the positions of electric charges within and between molecules are altered (when the reaction occurs)

gravitational potential energy: the potential energy due to elevated positions of objects

-whenever work is done energy is exchanged.

gravitation potential energy possessed by an elevated object is = to the work done against gravity in lifting it.

W=Fd; the work done = the force required to move it upward X the vertical distance it moved

Gravitational potential energy =weight X height; PE= mgh

-it does not depend on the path it takes to reach that level but it does depend on the height

only changes in potential energy are meaningful.

7.3 Kinetic Energy:

Kinetic energy: an object that is moving, has energy of motion, and is capable to do work.

-depends on it's mass and it's speed of the object

Kinetic energy = 1/2 mass X Speed (squared)

KE = 1/2 mv(squared)

The kinetic energy of a moving object is = to the work required to bring it to its speed from rest, or work the object can do while being brought to rest:

Net force X distance = kinetic energy or Fd= 1/2mv(squared)

-notice: ***the speed of an object is squared so if the speed of an object is doubled, its kinetic energy is quadrupled, it also takes 4 X the work to double the speed

-whenever work is done, energy changes

7.4 Work-Energy Theorem

Work= (delta) KE

work = the change in kinetic energy

work-energy theorem: the net work that is the work based on the net force. ( is work done by an outside force or work is not a form of energy but a transferring of energy from one place to another)

e.g: when you push on an object and friction also acts on the object --> KE = the work done by the net force - friction

-if the force of friction = and opposite your push ---> the net force on the object is zero and no net work is done

kinetic energy depends on speed squared.

7.5 Conservation of Energy:

Law of conservation of mass: energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes

-it can change form or be transferred from one place to another; total energy stays the same

Recycled Energy:

- reused energy that would otherwise be wasted. -fuel power plants and thermal energy waste

7.6 Machines:

machine: is a device that multiplies force or simply changes the direction of force.

-principle is the conservation of energy

lever: causes the direction of force to change

if the friction forces and the unbalanced weight of the lever are small enough to ignore work input= work output

(force X distance (input)) = (force X distance (output))

fulcrum: the point of support on which a lever rotates

when the fulcrum of a lever is relatively close to the load, a small input force will produce a large output force because the input force is exerted through a large distance and the load has moved through a correspondingly short distance.

lever= force multiplier

any machine that multiplies force does so at the expense of distance.

any machne that multiplies distance, does so at the expense of force

-no machine/device can pt out more energy than what is put in; it can only transfer energy from on form to another

7.7 Efficiency:

an ideal machine would operate at 100% effiency

-in an transformation, some energy is dissipated to molecular kinetic energy- thermal energy- which makes the machine and its surrounding warmer

100J of energy of work on a lever that puts out 98J is 98% effective.

some of the input energy may also be "wasted" by going into the potential energy of the pulley system itself

the lower the efficiency of a machine, the greater the percentage of energy that is degraded to thermalenergy

Efficiency= energy output/total energy input

in any transformation, there is a dilution of available useful energy.

-the amount of usable energy decreases with each transformation until there is nothing left but thermal energy at ordinary timperature

7.8 Sources of Energy

sun is the source of practically all out energy (except energy from nuclear power

all our fuels were created from the sun (photosynthesis

solar energy- photovoltalic cells

sun to heat water to heat turbines

wind is caused by unequeal warming of the Earth's surface- is a form of solar power

wind-used to turn generator turbines

hydrogen is the least polluting of all fuels but causes the production of carbon dioxide