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 of **heat** required to raise 1 gram of pure water from 14.5 to 15.5° Celsius at **standard atmospheric pressure**. 1 calorie is equal to 4.1855 joules. The abreviation for calorie is **cal**. A kilocalorie, abbreviated **kcal**, is equal to a 1000 calories. 1 kilocalorie is equal to 4185 joules.

**Btu** – also called **British thermal unit** is the amount of energy required to raise the temperature of one pound of water one degree**Fahrenheit**.

**Watt** (**W/m**^{2} or **Wm**^{-2}) – a metric unit of measurement of the intensity of **radiation** in watts over a square meter surface. One watt is equal to one **joule** of work per second. A kilowatt (**kW**) is the same as 1000 watts.