In the previous section, we developed the
concept of energy. We now must be able to measure and
quantify it, using a **standard** set
of **units**. Worldwide,
two systems of units of measurement are commoly used
today: the **Metric System** (** Systeme
International**) and the

**British System**.

The units of energy described in these systems are derived from a technical definition of energy used by physicists. This definition suggests that energy can be represented by the following simple equation:

## Work = Force x Distance

Similar to the definition given in the
previous topic, physicists view energy as the ability
to do work. However, they define **work** as
a **force** applied to
some form of **matter** (object)
multiplied by the **distance** that
this object travels. Physicists commonly describe force
with a unit of measurement known as a **newton** (after
Sir Isaac Newton). A **newton** is
equal to the force needed to accelerate (move) a mass
weighting one kilogram one meter in one second in a **vacuum** with
no **friction**.
The **work** or energy
required to move an object with the **force** of
one **newton** over a **distance** of
one meter is called a **joule**.

Some other definitions for the energy measurement units that you may come across in this textbook are as follows:

Calorie- equals the amount ofheatrequired to raise 1 gram of pure water from 14.5 to 15.5° Celsius atstandard atmospheric pressure. 1 calorie is equal to 4.1855 joules. The abreviation for calorie iscal. A kilocalorie, abbreviatedkcal, is equal to a 1000 calories. 1 kilocalorie is equal to 4185 joules.

Btu- also calledBritish thermal unitis the amount of energy required to raise the temperature of one pound of water one degreeFahrenheit.

Watt(W/m^{2}orWm^{-2}) - a metric unit of measurement of the intensity ofradiationin watts over a square meter surface. One watt is equal to onejouleof work per second. A kilowatt (kW) is the same as 1000 watts.