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KS4 Physics
Energy Transfers
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Contents
Energy Transfers
Useful energy
Conservation of energy
Wasted energy
Efficiency
Summary activities
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Useful energy transfer
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?
electrical
energy
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kinetic
energy
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What is the useful energy transfer?
What energy transfer is a speaker designed to carry out?
electrical
energy
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sound
energy
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What is the useful energy transfer?
What energy transfer is a wind farm designed to carry out?
kinetic
energy
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electrical
energy
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What is the useful energy transfer?
What energy transfer is a hydroelectric power station
designed to carry out?
gravitational
energy
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electrical
energy
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Useful energy transfers
Complete the input and useful output energies for the
devices in this table.
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Device
Input
energy
Output
energy
kettle
electrical
heat
solar cell
light
electrical
catapult
elastic
kinetic
coal fire
chemical
heat
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Contents
Energy Transfers
Useful energy
Conservation of energy
Wasted energy
Efficiency
Summary activities
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Law of conservation of energy
All energy transfers follow the law of conservation of energy:
Energy cannot be created or destroyed,
just changed in form.
 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 that is transferred to unwanted forms of energy
is wasted.
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Energy transfer in a television
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:
light
electrical
sound
heat
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Energy transfer in a radio
What are the main energy transfers in a radio?
(Don’t forget the wasted energy.)
electrical
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sound
heat
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Energy transfer in a car engine
What are the main energy transfers in a car engine?
(Don’t forget the wasted energy.)
kinetic
chemical
sound
heat
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Identifying energy transfers
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Falling objects
When an object falls in a gravitational field:
Gravitational potential energy (GPE) at the top
equals the kinetic energy (KE) of the object
just before it hits the ground.
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?
4000J
_________
Assuming no energy is lost by air resistance!!!
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Gerald the Human Cannonball
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Contents
Energy Transfers
Useful energy
Conservation of energy
Wasted energy
Efficiency
Summary activities
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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:
 heat energy;
 sound energy.
 All electrical devices produce a small amount of heat
energy, which is normally wasted.
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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 that waste energy as sound include:
car engine
washing
machine
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 that waste energy as heat include:
car engine
television
mobile phone
charger
radio
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Which type of energy is wasted?
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What happens to wasted energy?
What happens to the wasted energy produced
whenever energy is changed from one form to another?
(Remember that energy cannot be created or destroyed.)
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.
This is why it is important to reduce the amount of energy
that is wasted.
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Contents
Energy Transfers
Useful energy
Conservation of energy
Wasted energy
Efficiency
Summary activities
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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%.
The more efficient a device
is the less energy is wasted.
Energy efficient light bulbs are more
efficient than normal light bulbs
aste less energy.
because they w____
More of the e_______
lectrical energy that
they are supplied with is converted
into l____
ight energy and not h___.
eat
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Calculating efficiency
The efficiency of a device can be calculated using this
formula:
useful energy out
efficiency =
total energy in
Useful energy is measured in joules (J).
Total energy is measured in joules (J).
This means that efficiency does not have any units.
It is a number between 0 and 1 or a percentage.
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Efficiency of a bulb
This filament bulb is
supplied with 100 J of
electrical energy which
it converts to 45 J of
light energy.
a) How much energy
is wasted? 55 J
Wasted energy = Total – Useful
= 100 J – 45 J
= 55 J
b) In what form is the
energy wasted? heat
c) What is the efficiency
of the bulb? 0.45 or 45%
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Efficiency = Useful
Total
= 45 J
100 J
= 0.45 or 45%
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Efficiency of a radio
This radio is supplied
with 300J of electrical
energy which it converts
to 96J of sound energy.
a) How much energy
is wasted? 204 J
Wasted energy = Total – Useful
= 300 J – 96 J
= 204 J
b) In what form is the
energy wasted? heat
c) What is the efficiency
of the radio? 0.32 or 32%
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Efficiency = Useful
Total
= 96 J
300J
= 0.32 or 32%
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Efficiency of a television
This television converts
2 000J of electrical
energy into useful energy
at an efficiency of 65%.
a) What useful energy does
a television produce?
light and sound
b) How much useful
energy is produced?
1 300 J
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Efficiency = Useful
Total
Useful = Efficiency x Total
= 0.65 x 2 000 J
= 1 300 J
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Efficiency questions
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Contents
Energy Transfers
Useful energy
Conservation of energy
Wasted energy
Efficiency
Summary activities
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Glossary
 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 that is wanted.
 wasted energy – Energy in a form that is not wanted or
is not used.
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Anagrams
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Energy transfers – summary
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Multiple-choice quiz
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