9Id
Wasted energy
How is energy wasted in energy changes?
Energy cannot be made or destroyed. It can
only be turned from one form into another.
This is known as the law of conservation
of energy. However, when this happens not
all of the energy is turned into a form that
we want.
When we run, some of our chemical energy is
turned into kinetic energy. But we also get
hot. Not all of the chemical energy from our
food has been turned into kinetic energy.
Some of it has been turned into heat energy.
The heat energy is wasted energy.
?
1 Draw an energy flow diagram showing the
energy changes when someone runs.
Our muscles are not very efficient. For every 100 J of
chemical energy a muscle receives, only 25 J are used for
moving. 75 J of useless heat energy is also made. This means
that only 25% of the chemical energy has been used to do
something useful. Our muscles are only 25% efficient.
The unit for measuring
energy is named after
James Joule (1818–1889)
who was from Salford,
England. Although he
was a brewer he did a lot
of work on how energy
was changed from one
form to another and how
it was never destroyed.
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?
2 If a man eats 12 000 kJ in
one day and all of this is
used up by his muscles,
how much heat energy
does he produce?
Useless heat energy is produced by
machinery too. The moving parts of a car
engine heat up, due to the force of friction.
In a car only about 25% of the chemical
energy stored in the fuel is turned into useful
kinetic energy. Most of the rest is wasted as
heat energy. The car is 25% efficient.
?
3 a) For each 100 J of chemical energy in the
fuel of the car, how much becomes useful
kinetic energy?
b) What happens to the rest of the energy?
25%
chemical energy
in the fuel
kinetic energy
75%
wasted energy
A fossil fuel power station has an efficiency of 30%. The rest
is wasted as heat energy. For example, the generator
produces useless heat energy as it turns.
We can make our energy
resources last longer if we
cut down on the amount of
energy we waste. For
instance, an ordinary light
bulb uses 60 J of energy
every second, but only about
10 J of this is converted to
light energy. New energysaving bulbs produce the
same amount of light energy,
but only use 10 J of energy
each second.
?
4 a) What is the efficiency of a normal light bulb?
b) What form of energy is the wasted energy produced by a
light bulb?
5 a) The bulb in a torch needs electrical energy. What produces
this?
b) What type of energy is the electrical energy transferred from?
c) What types of energy does the bulb produce?
d) Which of these types of energy is the useful energy?
e) Draw an energy flow diagram to show the energy changes
in a torch.
f) 70% of the energy made by the torch is wasted. How efficient
is the torch?
!
Generators in power stations
are filled with hydrogen. This
gives less resistance (or drag)
than air, and so the generator
produces less heat than if it
were turning in air.
You should know…
● When energy is changed
from one form to another,
useless heat energy is
often produced.
● We can make energy
resources last longer by
using more efficient
machines.
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