# Work, The principle of Conservation of Energy, and Efficiency

## Work, The principle of Conservation of Energy, and Efficiency

#### Define work

Energy can be quite difficult to understand, but can be thought of as 'the capacity of a body to do work'.

What is work? Well, work is what something does whenever it transfers energy from one form to another. So a light bulb does work as it transfers energy from electrical energy to light and heat energy. And of course you are doing work even when you sit still doing nothing, because your body is transferring energy from chemical potential to heat and kinetic (in your heart). Be careful who you try this particular excuse on.

#### The Principle of Conservation of Energy

Nothing to do with turning off your stereo when you leave a room. This fundamental piece of science says that you cannot destroy or create energy. (If you find a way - keep it quiet until you have taken out a patent - and you'll be a rich person!).

People often say that you use up energy. That is misleading because it suggests that once it is used you can't get it back. In fact, when you use energy, you are simply converting it into other forms of energy - so it is still out there it is just no longer in the nice, useful form that you had it in a moment before.

Quite often if you can't quite put your finger on where the energy has gone (i.e. what form you have turned it into) you have turned it into heat, which is lost to the surroundings.

#### Efficiency

Very few devices can transfer energy from one form into another without wasting some on the way. Let me explain with an example.

A light bulb is designed to turn electrical energy into light energy. But most bulbs produce a lot of heat energy too. That energy has not been lost but it has been wasted - you don't say "It's cold in here. Turn on the light!" do you.

To measure the efficiency of a device, calculate what percentage of the total energy put in, became useful output energy.

For example: a bulb is provided with 100J of electrical energy but only produces 20J of light. The fraction turned into light is 20J out of 100J = 20/100.

In percentages, that's 20/100 x 100% = 20%.

So the equation for efficiency is:

Click on the different percentages to change the efficiency.