Measurement of Energy

1. Measurement of Energy Units of Energy

2. Objective for today • Recall the units associated with different types of energy.

3. So far in this topic we have listed a number of different types of energy. Can you remember some of these?

5. As part of this unit you will also need to be aware of some of the units we use to describe the quantities of energy measured or transferred.

6. Energy is defined by the work it can do so the basic unit by which it is measured is the unit for work, the Joule. • The joule is named for James Prescott Joule who experimented on the mechanical equivalent of heat. • The theory of the mechanical equivalent of heat put forward the idea that a given amount of work would always produce the same amount of heat if the output was completely converted to heat. In short, this means that mechanical energy is equivalent to heat energy.

7. In our practical work we have dealt with Joules when considering the energy in the bonds that were broken and made when we burned our alcohol fuels.

8. Another measure of energy you may be familiar with is the Watt. This is often associated with electricity. The Watt is a measurement of the transfer of energy. 1 Watt represents the transfer of 1 Joule per second.

9. Watts are used in some of the measures of sound energy. • Those of you with an interest in music may have come across speakers with a Watt rating. This is because acoustic power is measured in watts and is derived from the sound intensity multiplied by the area. • If you research the subject you will find many different ways in which sound energy is measured.

10. Radiant energy, for example light, also has many ways of being measured. http://en.wikipedia.org/wiki/Radiant_energy • Looking at the page from wikipedia you can see that the many forms of measurement of radiant energy include Watts or Joules (from which Watts are derived). • In conclusion then, there are many ways in which the various types of energy are measured. The important units are the Joule and the Watt.

11. The Joule • The Joule is the measurement of the work a form of energy can do. • A simplistic way of thinking about this is that a joule is the amount of mechanical energy required to move a weight of 1 Newton (a mass of about 100g in Earth’s gravity) through 1 metre.

12. The Watt • The Watt is the unit of transfer of energy. 1 Watt = 1 Joule of energy transferred per second. • Energy measured at 1 watt would if converted completely to mechanical energy, be able to move a weight of 1 Newton through 1 metre every second.