Energy - Energy and Efficiency

Filament light bulbs are only about ten percent efficient which means that they waste nearly all the energy that is fed into them. LED lighting is a good way of saving on energy as they are about 75 to 80 percent efficient. Gasoline engines are usually between twenty and thirty percent efficient, diesel engines are more efficient than gasoline engines but they still waste well over half of the energy supplied to them.

In electrical circuits, as the electrons flow through a conductor, energy is lost as they collide with the atoms of the conductor.

The conductor therefore heats up and the heat is dissipated (spreads out) into the surroundings. Electrical energy is wasted as sound and light if there are sparks in a circuit. In mechanical systems, the main waste of energy is heat. Even with the best engineered and effectively lubricated moving parts, it is impossible to eliminate friction. This creates heat and some sound which then spreads out into the surroundings. We say the energy is dispersed and this dispersed energy becomes harder and harder to reuse.

For your senior high school exams, you need to be able to carry out calculations about energy transfers, including the efficiency. Outside of your exams, calculating energy transfers and efficiency can be crucial when deciding on ideas to save energy and money. The efficiency of an energy transfer device is the ratio of the useful output energy to the input energy and is expressed as a percentage.

In senior high school exams, the examiners very often present quantitative information (information concerning numbers) about energy transfers and efficiency in the form of a Sankey diagram. This is drawn to scale and you are expected to be able to work out how much of the different energies are transferred as well as to use the numbers in calculations. In some cases, they make it easy and include the values required for any calculations. If you make a mistake with the numbers, don't worry too much - you will still receive credit for using the correct calculation methods so always show your working out if there is space.