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KS4 Physics

Energy Transfers

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Energy Transfers

Useful energy

Conservation of energy

Efficiency

Summary activities

Wasted energy

Contents

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Useful energy transfer

electricalenergy

kineticenergy

Many everyday objects are designed to transfer energy from one form into another useful form.

What energy transfer is an electric fan designed to carry out?

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What is the useful energy transfer?

electricalenergy

soundenergy

What energy transfer is a speaker designed to carry out?

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What is the useful energy transfer?

kineticenergy

electricalenergy

What energy transfer is a wind farm designed to carry out?

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What is the useful energy transfer?

gravitationalenergy

electricalenergy

What energy transfer is a hydroelectric power station designed to carry out?

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Useful energy transfers

DeviceInput

energyOutput energy

kettle

solar cell

catapult

coal fire

light electrical

elastic kinetic

chemical heat

electrical heat

Complete the input and useful output energies for the devices in this table.

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Energy Transfers

Useful energy

Conservation of energy

Efficiency

Summary activities

Wasted energy

Contents

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Law of conservation of energy

This means that energy never just ‘disappears’.

The total amount of energy always stays the same,i.e. the total input energy = total output energy.

In most energy transfers, the energy is transferred to several different forms, which may be useful or not useful.

The energy which is transferred to unwanted forms of energy is wasted.

All energy transfers follow the law of conservation of energy:

Energy cannot be created or destroyed, just changed in form.

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Energy transfer in a television

electrical

light

sound

heat

An energy transfer diagram shows the input energy and the output energies for a device. This includes all the useful and wasted forms of energy.

For example, in a television:

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Energy transfer in a radio

electricalsound

heat

What are the main energy transfers in a radio?

(Don’t forget the wasted energy.)

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Energy transfer in a car engine

chemical

kinetic

sound

heat

What are the main energy transfers in a car engine?

(Don’t forget the wasted energy.)

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Identifying energy transfers

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Falling objects

When an object falls in a gravitational field:

If a diver has 4 000 J of GPE at the top of a cliff, how much KE will she have just before she hits the sea?

_________4 000 J

Gravitational potential energy (GPE) at the top equals the kinetic energy (KE) of the object

just before it hits the ground.

Assuming no energy is lost by air resistance!!!

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Gerald the Human Cannonball

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Energy Transfers

Useful energy

Conservation of energy

Efficiency

Summary activities

Wasted energy

Contents

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Wasted energy

Whenever energy is changed from one form to another, some energy is always wasted.

Energy is usually wasted as:

All electrical devices produce a small amount of heat energy which is normally wasted.

heat energy;

sound energy.

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Wasted sound energy

If you can hear a device that is not designed to make noise then energy is wasted as sound.

Examples of devices which waste energy as sound include:

washing machine

car engine

hairdryer

jet engine

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Wasted heat energy

If a device gets warm and it is not designed to get warm then energy is wasted as heat.

Examples of devices which waste energy as heat include:

radio

mobile phone charger

television

car engine

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Which type of energy is wasted?

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What happens to wasted energy?

Wasted energy spreads out into the surroundings – this is called dissipated energy.

This dissipated energy is too spread out to do useful work and so cannot be re-used.

(Remember energy cannot be created or destroyed.)

What happens to the wasted energy produced whenever energy is changed from one form to another?

This is why it is important to reduce the amount of energy that is wasted.

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Energy Transfers

Useful energy

Conservation of energy

Efficiency

Summary activities

Wasted energy

Contents

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Energy efficiency

Efficiency is a measure of how good a device is at changing energy from one form to another.

All devices waste energy, so the efficiency of a device is never 100%.

Energy efficient light bulbs are more efficient than normal light bulbs because they w____ less energy. More of the e_______ energy that they are supplied with is converted into l____ energy and not h___.

astelectrical

ight eat

The more efficient a device is the less energy is wasted.

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Calculating efficiency

The efficiency of a device can be calculated using this formula :

This means that efficiency does not have any units.

It is a number between 0 and 1 or a percentage.

Useful energy is measured in joules (J).

Total energy is measured in Joules (J).

efficiency = useful energy out

total energy in

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Efficiency of a bulb

0.45 or 45%

a) How much energy is wasted?

b) In what form is the energy wasted?

c) What is the efficiency of the bulb?

Efficiency = Useful Total

= 45 J100 J

= 0.45 or 45%

This filament bulb is supplied with 100 J of electrical energy which it converts to 45 J of light energy.

55 J

heat

Wasted energy = Total – Useful = 100 J – 45 J

= 55 J

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Efficiency of a radio

a) How much energy is wasted?

b) In what form is the energy wasted?

c) What is the efficiency of the radio?

This radio is supplied with 300J of electrical energy which it converts to 96J of sound energy.

Efficiency = Useful Total

= 96 J 300J

= 0.32 or 32%

204 J

Wasted energy = Total – Useful = 300 J – 96 J

= 204 J

heat

0.32 or 32%

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Efficiency of a television

a) What useful energy does a television produce?

b) How much useful energy is produced?

This television converts 2 000J of electrical energy into useful energy at an efficiency of 65%.

Efficiency = Useful Total

Useful = Efficiency x Total

= 0.65 x 2 000 J = 1 300 J

light and sound

1 300 J

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Efficiency questions

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Energy Transfers

Useful energy

Conservation of energy

Efficiency

Summary activities

Wasted energy

Contents

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dissipated – Energy that spreads out into its surroundings so that it is no longer useful.

efficiency – A measure of how good a device is at changing energy from one form to another.

energy transfer – The process of changing energy from one form into another.

input energy – The energy going into a device.

output energy – The forms of energy produced by a device.

useful energy– Energy in a form which is wanted.

wasted energy – Energy in a form which is not wanted or is not used.

Glossary

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Anagrams

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Energy transfers – summary

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Multiple-choice quiz