E NE RG Y MAT TER S
FACT SHEET 1: WHAT IS ENERGY?
It seems an obvious and simple question,
though the generally accepted answer –
the capacity to do work – asks more
questions than it answers. For example,
what is work? Its generally accepted
definition is as mechanical work:
movement of an object. Mathematically it
is the value of the distance that an object
is moved multiplied by the force acting on
it.
sound etc, but that between stored and
released energy. Stored energy contains
the ability (potential) to do work.
Released energy does the work.
EXERCISE 3
What are some examples of stored
energy?
Energy creates force which does work.
EXERCISE 1
Consider a windmill or wind turbine. How
does the above definition apply to this
specific case?
Stored energy when it is released will
produce movement in some form, whether
it is visible to the observer or not.
EXERCISE 4
When the examples in Exercise 3 release
their stored energy, where is the
movement produced?
EXERCISE 2
When you heat water in a kettle, you are
quite definitely using energy. However,
nothing seems to be moving, just that the
water increases in temperature. Why is
this not violating the definition of energy?
Forms of energy
The most important distinction between
the various forms of energy, is not as you
might expect, the difference between heat,
Energy changes
Energy cannot be created or destroyed, the
First Law of Thermodynamics says, only
converted into different forms. When
energy is expended to do work, the
amount of energy is much greater than
the work done. This is because the
conversion of energy into moving the
object is not very efficient. Some of the
energy is converted into other forms, such
as heat, light and sound.
EXERCISE 5
Classify the following general forms of
energy as stored or released.
Form
Type
Heat boils water
From
To
Electrical
Steam turns turbines
Chemical
From
Nuclear
To
Gravitational
Generation of electricity
Heat
From
Light
To
Sound
Boiling water in the kettle
Kinetic
From
EXERCISE 6
Trace the energy interconversions from
the formation of coal through to heating
water in an electrical kettle.
To
Growth of plants
From
To
Formation of coal
From
To
Burning of coal in power station furnace
From
To
Every time energy is converted from one
form to another, some is lost, which means
a lot of sunlight has gone into making your
cup of coffee!
Power
Another relevant term here is power. It is
the rate at which energy is used or
converted into a new form. Running
consumes more energy in a given time
than does walking, therefore your power is
greater. The heat energy required to boil a
litre of water is a constant. However, the
rate at which it reaches boiling point is
not, and depends on the amount of energy
per second that is being supplied to the
water. Trying to boil the water with a lit
match will take a very long while
compared to the much higher powered
electric kettle.