physics revision 2014

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ENERGY AND ELECTRICITY:

energy can be useful when it is transformed from one form to another, for example electrical energy in a kettle is transformed into heat energy to heat up the water in the kettle. other examples of this are:

a whistle: chemical potential is put in (you blow into it) this turns into kinetic energy and then sound and heat energy.

a yoyo: gravitational potential energy is put in and kinetic and heat energy are brought out (although this isn't an example of a useful transformation as the heat and kinetic energy are both wasted)

a wind up toy:chemical energy is put in which turns into kinetic energy which then turns into elastic energy which finally turns into sound and heat energy which are wasted.

stored energy is called potential energy and can be chemical, gravitational or elastic energy.

electrical energy is transferred around circuits and can be transformed into components. transformed in this case means that it goes into the component and powers the component and allows it to fulfil its function.

a voltmeter is connected across components to measure voltage, voltage is the push of current around the circuit.

a good model of a circuit is: a factory represents the battery of the circuit, shops represent bulbs or buzzers, trucks represent electrons, boxes of crème eggs inside the trucks represent energy, the roads represent wires, a traffics light represents the switch and road works represent resistors, trucks have to slow down at road works and the shops slowing the current flow and have to stop at traffic lights which is why it represents the switch,the factory represents the battery because the truck get all there supplies from there. the rest explains itself.

a cell or battery has stored chemical energy which is transformed by a chemical change to electrical energy in the circuit.

high voltages are very dangerous to people, the higher the voltage is the more risky it is to tamper with it, the higher voltage the brighter or louder the bulb or buzzer will become.

electrical appliances transform electrical energy from the mains into other forms of energy, like the kettle as I mentioned earlier, another example is the toaster which converts electrical energy into kinetic and thermal energy.

some electrical appliances such as heating appliances (e.g. radiators) use more energy in a given time than other appliances.

to work out the power rating of electrical appliances you: do (wattage times hours per day) divided by 1000. then to work out the cost of using the appliance you: (do however many watts you worked out above times however many hours you use it a day times how many days you use it a week divided by 1000.

electrical generators are essentially coils of electric conductors usually copper wire that are wrapped around a metal core and mounted to spin around inside an array of large magnets. anytime an electric conductor moves in the magnetic field the magnetic field will interact with the electrons inside the conductor and induce a flow of electronic current within it.

fossil fuels, nuclear fuels and renewable energy sources can all be used to power generators.

pollution and other environmental effects are caused by electricity generation in different types of power station.

not all energy input is usefully transformed and energy can be wasted as heat and other forms

some appliances are more efficient (waste less energy) than others.

see book/sheet for flow diagrams on appliances.

GRAVITY AND SPACE

mass is how heavy an object always is no matter where it is, the mass of an object always stays the same /the amount of material an object is made up of. mass is measured in kg.

the weight of an object is the force acting on a mass due to gravitational field. weight is measured in newtons (N)

How to calculate weight form mass: every 100g is 1 newton/every 0.1 kg is one newton weight=mass times 10 (on earth)

gravity is a universal force of attraction which acts towards the centre of earth and towards the centre of other planets.

the mass of an object is the same on any planet but its weight is different on different planets, depending on the gravitational field.

the gravitational attraction between two objects depends on their mass and distance between them.

gravitational attraction between two objects decreases as distance between them increases.

the sun exerts a very large gravitational force which keeps all the planets In orbit around it.

how have our ideas about the solar system changed over time:

1.ancient civilizations studied the movement of the stars and sun and were able to predict what there movements would be hundreds of years into the future, people also believed that the earth was in the centre of the universe.

2.Copernicus (1473-1543) believed that the sun was at the centre of the solar system with planets orbiting around it. Galileo, a very important scientist at the time said he agreed with Copernicus.

3.in the 17th century Johanno Kepler developed Galileo's model further and found out that the planets orbits were elliptical (not quite round) in shape.

4.sir Isaac newton created a mathematical model using the movements and masses of the planets in the solar system to create the model we still use today though it has been extended and slightly changed.

the force of gravity keeps planets and satellites in orbit.

to keep something in a circular path such as planets and satellites requires an inward force towards the centre of the circle.

the moon is a natural satellite of the earth, whose orbit is maintained by the earths gravitational force.

how to work out weight on different planets: mass times gravity on the planet will give you the weight of that object on that planet for example: if you put 100g on mercury it will weight 0.37N because the gravity of mercury is 3.7 so 0.1 times 3.7 is 0.37.

uses of artificial satellites are: television, studying the weather, navigation , communications.

why some satellites need to be in geostationary or polar orbits: some satellites need to be in a geostationary orbit because geostationary satellites remain at a fixed point above the earths surface and so are ideal for communications navigation and TV signal. some satellites need to be in polar orbit because they are intended purely for studying the weather or providing navigational signals.

scientists use satellites to gain information about the solar system by pointing them out into space as giant telescopes or x rays etc. they're much steadier and more powerful than satellites on earth (bar a select few) and don't have the problem of trying to see through our atmosphere.

the forces on rockets and satellites vary as they travel away from the earth by:

1.the thrust force is much larger than the weight of the rocket so the rocket accelerates

2.the rocket speeds up to 40,000km per hour to escape most of earths gravity, the thrust force is much larger so the rocket accelerates.

3. the moons gravity pulls the rocket but it is still slowed down slightly by the earth, the rocket accelerates.

4.the earth and the moons gravitational pull are no the same but the rocket accelerates as the moons gravitational pull is slowly getting stronger as the rocket gets closer to it.

5.the rocket slows down slightly as it enters the moons orbit but the moons gravitational pull still pulls them in.

SPEEDING UP

to work out distance you do speed times time

to work out speed you do distance divided by time.

a speed at a point is exactly how fast the thing is going at that moment in time and an average speed is how fast it goes throughout its whole journey which it was not traveling at a constant speed all the way through.

some measurements need to be more precise than others because a measurement such as the average of something does not need to be as precise as the exact time it took something or some one to get to a certain place.

if an object has more mass then it increases the time it would take to change speed, and also increases the force required to make it change speed in the first place.

forces affect speed by slowing down (sometimes) the object such as the force air resistance other forces speed the object up such as the acceleration of a car powered by the engine etc. etc.

if forces are balanced the object remains at a steady speed or stays stationary.

if the forces are unbalanced the object either speeds up or slows down.

water and air resistance oppose motion by slowing down the object as it has to force its way through air and water particles with water particles being tightly packed and harder to get through.

streamlining reduces water and air resistance by decreasing how wide you are and how many particles hit you which allows you to cut through air and water with greater ease.

factors that effect air resistance are: the objects mass, the objects surface area and the objects acceleration.

then energy required to keep an object moving depends on its air resistance e.g. if there is a lot of air resistance on an object it will take a lot more force to get it moving.

PRESSURE AND MOMENTS

If you apply a larger force to a smaller area then the pressure will be greater whereas if you apply a smaller force to a larger area then the pressure exerted will be weaker.

pressure=force divided by area

if you push down a syringe with liquid in which connects to another syringe the liquid will not go anywhere (disappear) it will simply change shape and be forced into the other syringe which will push it up.

gas pressure is caused by particles in the gas moving around and causing the gas particle to bump into a wall and bounce off, this creates forces which creates pressure because force divided by area =pressure.

liquid can be used in the hydraulic system to push up heavier objects using a small force. and gasses are used to pump up tyres and keep them inflated as the gas makes sure the tyres do not get flattened by the weight of the car.

liquids and gases have internal pressure which depends on the weight of the substance above a particular point.

a lever is a simple machine which uses a pivot.

using a long lever to do something makes the job easier if you hold the lever by the very end and push down, this makes the turning effect of the force greater. by applying the force at a distance the turning effect of the force is greater.

a door is an example of a lever in everyday life, a seesaw is also an example as well as tweezers or pliers. and an example of a lever in the human body is the joint between the head and the neck and between the arms.

the force applied to one end of the lever tries to turn the lever, tis is called the turning effect.

an object can be kept in balance by holding it right in the middle of balancing both sides out evenly with objects if you are holding it unevenly. the object may topple around a pivot because it is heavier on one side and will swing around, and if a person is holding that objects he will have to use more force to keep the object balanced.

the turning effect of a force is its movement.

movement=force times distance from pivot.

unit: Nm

Ncm

Nmm

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