Scientists define energy as the ability to do work. Energy
makes things happen. The energy in sunlight makes plants grow, the energy in
food enables us to move and helps us to keep warm, and the energy in fuel
powers engines. Energy comes in many different forms and can be converted from
one form into another. The main types include POTENTIAL ENERGY
, KINETIC ENERGY, and CHEMICAL ENERGY.
Energy that is stored up ready to be used in the future
is called potential energy, because it has the potential (or ability) to do
something useful later on. An object usually has potential energy because a
force has moved it to a different position or changed it in some other way.
When an object releases its stored potential energy, this energy is converted
into energy of a different form.
ELECTRICAL POTENTIAL ENERGY
When thunderclouds move through the sky, they build up a large
amount of electricity inside themselves. This is known as static electricity,
which is a store of energy. When a cloud builds up more static electricity than
it can store, some of the electricity flows from the cloud to Earth in a bolt
of lightning.
This type of potential energy powers bows and catapults. It takes
effort to stretch a piece of elastic or rubber because the forces between its
molecules try to resist being pulled apart. As the elastic stretches, the
molecules move away from one another and gain potential energy. The energy
stored in stretched elastic can also be used to power such things as toy cars
and model aeroplanes.
GRAVITATIONAL POTENTIAL ENERGY
A snowdrift on top of a mountain has a huge amount of potential
energy. This is known as gravitational potential energy because it is gravity
that is constantly trying to pull the snow down the mountain to the bottom.
When an avalanche occurs, the snow gathers speed and its stored potential
energy is turned into kinetic energy (the energy of movement).
Moving objects have a type of energy called kinetic
energy. The more kinetic energy something has, the faster it moves. When
objects slow down, their kinetic energy is converted into another type of
energy, such as heat or sound. Objects at rest have no kinetic energy. Kinetic
energy is often produced when objects release their potential
energy.
A moving hammer has a lot of kinetic energy. As it strikes the
nail, it slows down and loses its kinetic energy. The energy does not
disappear, however. Some of it goes to split the wood to make way for the nail,
some passes into the wood as heat energy, and some is converted into sound.
This is the energy involved in chemical reactions, when
elements join together into compounds. This energy is stored inside the
compounds as chemical potential energy. The stored energy can be released by
further chemical reactions. The food we eat stores energy that is released by
digestion. Energy can also be released by burning the chemicals in a process
called combustion. Fuels are chemical compounds that release heat energy by
combustion.
When humans or other animals eat food, they use its stored energy
to keep warm, maintain and repair their bodies, and move about. Different types
of food store different amounts of energy. The amount of energy a food contains
is measured in kilocalories (called Calories for short).
Table 3. FOOD AS CHEMICAL ENERGY
| TYPE OF ANIMAL | DAILY CALORIES FROM FOOD |
|---|
| Elephant | 40,000 |
| Panda | 20,000 |
| Man | 2,600 (moderate activity) |
| Woman | 2,300 (moderate activity) |
| Child (7-10 yrs) | 2,000 |
| Mouse | 20 |
