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Physics
Energy as mechanical work

Vehicles must burn fuels to be able to move. Many toys use the energy
stored in a chemical battery. Lifts use electricity to transport people up
and down the different floors of a building, and people eat food to use
and store energy. To make objects move and to perform mechanical
work, energy must be transferred.
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There are other ways of storing and transferring energy, but we will
explore these in more detail in 10.3.

Conserving energy

The most important thing for you to remember is that energy does not disappear . Energy
is not generated from ‘nowhere’ and does not disappear – though it can be stored,
mainly momentarily, in different ways. For example, fossil fuels store chemical energy
until we burn them. This chemical energy is then transferred as light and heat (thermal
energy). We are then able to harness this light and heat for other purposes (i.e. generating
electricity, heating homes, etc.). The energy that is not used for useful purposes will be
transferred to the surroundings (air, objects) and heat them up, increasing the temperature.
But most importantly, all the energy stored in the fossil fuel burnt will be transferred
somewhere or transformed – it will NOT disappear! The total amount of energy put in is the
same amount we get out – all that varies is how it is used and where it goes.

Visualizing energy transfer
Energy transfer can be shown graphically in
light
a Sankey diagram. Sankey diagrams show
electrical energy energy
the amount of energy IN and the amount of
100 J 75 J
energy OUT, illustrating where it flows, and how.
The width of the arrows is proportional to the
amount of energy transferred.
A Sankey
ACT thermal energy diagram for an
energy-saving
24 J lightbulb
1. Explore the Sankey diagram and discuss in
pairs what types of energy transfer you see. Using key words, write these
transformations down in your notebook.

2. Which energy transformations are useful and which are not?

ACT Look around the classroom and list objects that use energy. Analyze where their
energy comes from and how they use it. Draw Sankey diagrams for three different objects.
Estimate the percentage of energy transferred.

OVER TO YOU!

1. A TV has an energy input of about 90 joules every 1 second when it is on. Describe
what you think happens with the energy during this transfer.
2. Scientists recommend that adults have a daily intake of about 85 000 joules of
energy per day. Why do humans need so much energy?

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