ENERGY, WORK AND POWER

ENERGYIf something has energy then it can move or it is capable of making something move. For example an airplane has energy because it is moving but a can of petrol also has energy. Although it may not be moving it is capable of making a car move if you put it into the engine. In other words if a force is moved energy is being used.

ENERGY = FORCE X DISTANCE MOVED (in direction of force)

E = fd

As force is measured in newtons and distance is measured in metres, energy is measured in newton metres. One newton metre is a JOULE (J).

There are different types of energy:

1. KINETIC - This is the energy an object has because it is moving. It is sometimes called MOTION ENERGY. An example of something that has kinetic energy is a moving car.

The formula for kinetic energy is:

Kinetic Energy = mass x velocity 2 K.E. = mv2

2 2

2. INTERNAL - This is sometimes called THERMAL energy or HEAT energy. An example of something that gives out heat energy is a fire.

3. SOUND - An example of something that gives out sound energy is a radio.

4. CHEMICAL - Here chemicals store energy and can then release it. Examples of chemical energy are oil, coal, gas, a battery and food.

5. NUCLEAR - This comes from the nucleus of an atom. Nuclear reactions make the sun give out heat and light and nuclear bombs and power stations use nuclear energy.

6. GRAVITATIONAL POTENTIAL – This is the energy an object has because it is high up. Sometimes it is called energy of POSITION. An example is a parachutist falling through the air.

The formula for gravitational potential energy is:

Potential Energy = mass x gravity x height

P.E. = mgh

7. STRAIN - This is sometimes called ELASTIC energy. It occurs when things are stretched, squashed or bent. Examples of strain energy are in a slingshot or a wound up spring.

8. ELECTRICAL - A computer, television, radio and light bulb all use electrical energy. A battery and generator give out electrical energy.

9. LIGHT - A light bulb, the sun and a fire all give out light energy.

CONSERVATION OF ENERGYThis means that you cannot destroy energy but you can change it from one form to another.

Examples of energy conversion are:

ELECTRIC HEATER

ELECTRICAL HEAT

FLASHLIGHT

CHEMICAL ELECTRICAL LIGHT + HEAT

SLINGSHOT

STRAIN KINETIC

SLED GOING DOWNHILL

GRAVITATIONAL KINETIC

MAN CLIMBING MOUNTAIN

CHEMICAL KINETIC GRAVITATIONAL

In energy conversions, heat is often given out. For example a car is designed to convert the chemical energy in petrol into kinetic energy but the engine also gives out heat. A light bulb is designed to convert electrical energy into light but the bulb also gives out a lot of heat.

The word EFFICIENCY is often used to describe how good a device is at converting one form of energy into another. A car is about 30 % efficient which means that about 30% of the energy in the fuel is used to make the car move, the other 70% is mostly given out as heat. This can be used to make the car warm inside during the winter but in the summer the heat is not needed.

The formula for efficiency is:

EFFICIENCY = ENERGY OUT X 100% ENERGY IN

Suppose 70 J of electrical energy went into a light bulb. This then gave out 64 J of heat and 6J of light. As bulbs are designed to give out light and not heat we calculate efficiency using the numbers:

6 x 100 = 8.6%70

NUCLEAR ENERGYThis energy comes from the nucleus of the atom.

In nuclear FISSION the nucleus of a large atom like uranium is split up into smaller nuclei plus energy.

This occurs in a nuclear power station.

LARGE SMALLER NUCLEUS NUCLEI

In nuclear FUSION two smaller nuclei such as that of hydrogen combine to form a larger nucleus plus energy.

This is how the sun gives out heat and light.

SMALL LARGENUCLEI NUCLEUSTHE POWER STATIONThis converts the chemical energy in coal, oil or gas into electricity. The fuel is burnt to in a BOILER to produce heat. The heat is then used to turn water into steam which is fired against big fans called TURBINES. These spin and then turn a GENERATOR to produce electricity.

The energy changes are: CHEMICAL HEAT KINETIC ELECTRICAL

FOSSIL FUELSMost of the energy we use now comes from coal, oil or gas. These fuels are called FOSSIL FUELS. They were formed millions of years ago from dead animals and plants that were compressed under layers of rock that built up above them. We burn these fuels to release heat and in a power station this heat can be used to produce electricity. However when we burn fossil fuels we also release harmful gases at the same time. The carbon dioxide released builds up in the atmosphere and it prevents the Earth losing heat to space by reflecting the heat back to the ground. As a result the Earth gets warmer which means that the polar ice caps will start to melt and coastal regions could get flooded. This is known as the GREENHOUSE EFFECT. Also impurities such as sulphur in the fossil fuels (especially coal) produce sulphur dioxide gas when they are burnt and this dissolves in the rain to produce ACID RAIN. The other problem with fossil fuels is that eventually they will run out so we must start looking now into other ways of providing us with energy for the future that will not run out. This is known as RENEWABLE ENERGY.

GEOTHERMAL ENERGYThis comes from hot rocks under the ground. These rocks are hot because the core of the Earth is about 4,000oC. This heat is produced from radioactive decay. The hot rocks naturally heat water under the ground which reaches the surface in the form of geysers and hot springs. You can drill two holes under the ground in these areas and pump cold water down one hole which gets heated up and comes up as hot water in the other hole. The hot water can be used to heat houses. If the water comes to the surface as steam then it can be used to turn turbines that drive generators that produce electricity.

Advantages of Geothermal Energy1. The energy source is free.2. There is little pollution

Disadvantages of Geothermal Energy1. It is only available in some parts of the world with hot rocks underground2. Some of these areas are volcanically active e.g. Hawaii, Iceland3. The heat is used up after a few years.

SOLAR ENERGYThis comes from the sun in the form of heat and light.

Solar cells can turn the light from the sun into electricity. These solar cells were first used to provide electricity for

space vehicles but now you commonly see them in solar powered calculators and watches. However large solar cells are expensive.

Solar water heaters absorb the heat energy from the sun that to heat water that passes through pipes painted black running through a box covered in glass. The glass helps trap the heat and the black pipes help the water absorb the heat.

Advantages of Solar Energy1. The energy source is free2. There is no pollution

Disadvantages of Solar Energy1. It doesn’t work at night2. It doesn’t work so well in regions that are not hot 3. It doesn’t work so well in winter

HYDROELECTRIC POWERWater, stored high up in lakes in a mountain or behind a dam, flows rapidly downhill and turns a turbine as it does so which then turns a generator to produce electricity.

Advantages of Hydroelectric Energy Disadvantages of Hydroelectric Power1. The energy source is free 1. You need an area that has a lot of water and

mountains2. There is no pollution 2. It floods the areas behind the dam

3. Silt deposited by the river raises the level of the river bed4. The dam does not allow boats and fish to pass without locks being built

WAVE ENERGYThe waves are caused by the wind blowing over the surface of the sea. You could put floats in the water that move up and down with the waves. When they do this, the floats turn a generator to make electricity. These floats are called ‘Ducks’.

Advantages of Wave Energy1. The energy source is free2. There is no pollution3. The seas are roughest in the winter and the rough seas generate more electricity. Winter is the time you need most electricity.

Disadvantages of Wave Energy1. You do not always have waves that are very large2. It will be difficult to do maintenance on them as they are out to sea3. They may affect shipping4. The high waves in winter could damage the machines5. Countries that do not have a coast cannot use wave energy

TIDE ENERGYThis is different from wave energy. The tides are caused by the gravitational attraction of the moon on the oceans. There are normally two tides a day in coastal areas. If you build a dam (barrage) in the mouth of an estuary you can hold the water behind it as the tide comes in or out and the difference in water levels can be used to turn a turbine which

will turn a generator to generate electricity.

Advantages of Tide Energy1. The energy source is free2. The tide is regular and you can predict when it will occur

Disadvantages of Tide Energy1. You can only generate 8 hours of electricity a day 2. The tides may not occur when you need the most electricity3. You must have a large difference in sea level between low and high tide4. The barrage causes problems for boats to go past it and it will alter the coast which is a habitat for many seashore animals5. Countries which do not have a coast cannot use tide energy

WORKWork is done when a force moves an object. The greater the force and the further the object moves the more work is done.

WORK = FORCE X DISTANCE (in newtons) (in metres)

Work is measured in newton metres (Nm) and one newton metre is known as a JOULE (symbol J)

POWERThis means how much work is done in a second. If a machine does a lot of work in a short time then the machine is said to be powerful.

The equation for power is:

POWER = WORK (in joules)TIME (in seconds)

Power is measured in joules per second (J/s) and one joule per second is known as a WATT (symbol W).

WORKED EXAMPLEA man of mass 80 kg runs up 20 steps in 8 seconds. If each step is 25 cm high, what is his power?

Weight of man = 80 kg x 10 = 800 N

Total height in metres = 20 x 25 = 500 cm = 5.0 metres

Work = force x distance

= 800 N x 5.0 m = 4000 J

Power = work / time= 4000 J / 8= 500 W

CHANGING POTENTIAL ENERGY INTO KINETIC ENERGYIf an object is high up it has gravitational potential energy. When it falls this is converted into kinetic energy. The kinetic energy just before it lands is equal to the gravitational energy before it was dropped.

Example: A ball of mass 2 kg is held 6 m off the ground. 2 kg

a) What is its gravitational energy before it was dropped?

GPE = mgh 6 m= 2 x 10 x 6= 120 J

b) What is its kinetic energy before impact?

Same as GPE = 120 J

c) How fast is the ball moving before it lands?

GPE = KE 120 J = mv 2 120 = 2 x v 2 v = 2 x 120 = 11 m/s 2 2 2