Efficiency

We use many devices, like bicycle, television, kettle, and so on. Practical devices like these help us to do something, and always need energy.

A bicycle helps to move me from my home to my work place. Ideally, it just needs the energy to accelerate me and to stop me. This energy would come from my leg muscles. In real life, my muscles must pump in a lot more energy than that. It needs energy to overcome friction in all its moving parts, and between the tyres and the road surfaces.

A television receives data from radio waves, decodes them and uses light to draw to pictures on the screen. Ideally, it just needs electrical energy to accelerate and stop the electrons in the wires, and to produce the light. In practice, a lot more energy is needed to overcome electrical resistance in the many wires.

A traditional kettle sits on a fire. The fire heats up the kettle and boils the water inside. Ideally, we just need enough energy from the fire for the water molecules to overcome their attractions to each other, and to push against the air pressure. In reality, a lot more energy is lost to the surrounding instead.

The above are pictures of practical devices from an energy point of view. Energy are always lost somehow, no matter how well a device is designed.

To help save energy, to help improve design, and to help us decide which television to buy, we often use a number called energy efficiency. This is the amount of useful energy, as a percentage of energy input. For example, the useful energy in a kettle is the energy change from water to steam. The energy input is all the energy put in by the fire.

The useful energy is always less than the energy input, since some is always lost. So the efficiency is always less than 100%. The closer it is to 100%, the less energy is wasted. In this way, the efficiency number tells us how energy efficient a device is.

For example, I would prefer to buy a television that is more energy efficient. It would waste less energy, so I can save on my electricity bill. However, it would most likely be more expensive, because more people's effort has to go in to make a better design.

Definition.. Efficiency is the fraction of energy input that is useful.

Example. The water in a kettle needs 4000 J of heat to boil. To actually boil it all, 5000 J of electricity is used. What is the efficiency of the kettle? Answer. Efficiency = 4000 / 5000 = 0.8, or 80%.


Copyright 2010 by Kai Hock. All rights reserved.
Last updated: 21 September 2010.