Percentage Efficiency

Very few devices can transfer energy from one form into another without wasting some on the way. Let us 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 the percentage of the total energy put in that has become useful output energy.

For example, a bulb is provided with 100 J of electrical energy, but only produces 20 J of light. The fraction turned into light is 20 J out of 100 J = 20/100.
In percentages, that’s 20/100 × 100% = 20%.

Definition: The efficiency of a device, machine or engine is defined as the ratio of useful power output to power input.

This is usually expressed as a percentage:

Efficiency (%) = (useful power output/power input) x 100

Let's look at another example:

A barrel being rolled up an inclined plane

If a barrel is rolled up an inclined plane as shown in the diagram, its potential energy at the top will be less than the amount of energy required to roll it up the plane.

The “lost” energy appears as heat caused by the friction between the barrel and the plane, which causes some energy to be changed into heat.

This is an example of a situation where the efficiency of the device (i.e. the inclined plane) is less than 100%.

In fact, any device changing kinetic to potential energy, or vice versa, is subject to frictional heating losses and has an efficiency of less than 100%. The more machinery that must be used to convert one form of energy into another, the more inefficient the process will be. For instance, a typical petrol engine might only have an efficiency of about 25%.

Next: Learn It - Interactive Efficiency Demonstration