BRITISH INTERNATIONAL SCHOOL, AJMAN
Worksheet: Energy efficiency
Name:
Sub: SCIENCE
Gr: 7
Date:
Learning objective: Recapitulate the concepts for energy flow diagram and understand the differences between
energy efficient and inefficient systems.
In every system, there is an input energy and output energy. For example, in a hairdryer, the input energy is
electrical energy and the output energy is the heat energy and sound energy.
Some output energy is useful to us, such as the heat energy from a hairdryer. We use it because the heat
generated will cause the water in our hair to evaporate, and dry our hair. Other output energy is not useful to us;
the sound generated from the hairdryer when it is switched on is not useful to us, and so, it is wasted. We can
draw a diagram like the one below, to represent the energy flow in a system. We will use the hairdryer as our
example:
The useful output energy is always less than the input energy, because some of the output energy that is
produced is useless to us and is thus wasted.
Energy Efficient Systems
Have you heard of “energy saving light bulbs”? You may have heard that they are more energy efficient than
normal incandescent light bulbs. This means that more of the input energy is transferred into useful output
energy, and less output energy is wasted.
Think about this:
In an incandescent light bulb, 100J of electrical energy is needed every second to make it work. Of this, 5J of
light energy is produced per second. 95J of heat energy is also produced per second. We use a light bulb for
LIGHT; so, only 5J of the 100J of input energy is useful output energy (light) and 95J is wasted (heat). In energy
saving light bulbs (or compact fluorescent light bulbs), the amount of useful output energy (light) is higher and
the wasted output energy (heat) is lower. This makes them more energy efficient than an incandescent light bulb.
Energy Inefficient Systems
Unfortunately, many of the systems we use are not very energy efficient. A car only uses about 60% of the
chemical energy put into it in the form of petrol. The other 40% of the energy is wasted in the form of heat and
sound.
A coal power station burns coal to produce electrical energy. In this process, up to Coal power station.
60% of the output energy is wasted as it is ‘lost’ to its surroundings as heat energy. That means only 40% is
useful output energy (electrical energy). This 40% of useful output energy is transferred through the grid system
to our homes. At each point along the grid, at each place where a change or conversion happens, energy is ‘lost’
to the surroundings as heat and sound energy. By the time the energy is fed through your meter- box into your
home, it is much less than 40%.
We need to find alternative ways to generate and conserve energy so that the amount of wasted output energy is
less. We need to find more energy efficient alternatives for producing and using energy, particularly electrical
energy.
Answer these questions:
Q1. What are the two types of output energy that any system will produce?
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Q2. What is the difference between an energy ‘efficient’ appliance and an energy ‘inefficient’ appliance?
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Q3. Which is more energy efficient: an incandescent light bulb or a compact fluorescent light bulb?
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Q4. Write any 3 different ways by which we can conserve electricity at home?
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Q4. Write the energy input and output for the following examples and draw an energy transfer diagram (With
simple arrows): (you have to decide which of the output energies are useful and which are wasted).
a)
Television
Input energyOutput energy-
b)
Electric fan heater
Input energyOutput energy-
c)
Washing machine
Input energyOutput energy-