Efficiency: where the wasted energy goes

0:00When a device changes energy from one form to another, where does it all go?

0:06An incandescent bulb makes light by heating a thin wire. Say 100 joules of electrical energy go in.

0:14Only about 5 joules come out as light, the useful energy we want.

0:19The other 95 joules become thermal energy, or heat. We did not want it, so it is wasted.

0:27In this Sankey diagram, each arrow's width shows how much energy it carries.

0:33Efficiency is useful energy out, divided by total energy in, times 100 percent.

0:40For the bulb, 5 joules divided by 100 joules, times 100 percent, is 5 percent.

0:49Is the wasted energy destroyed? No: 5 joules plus 95 joules still makes all 100 joules.

0:59The heat spreads out into the surroundings, where it is very hard to use again.

1:04Electrical resistance warms wires carrying a current, and friction warms rubbing parts, so nearly every machine wastes heat.

1:14An LED, or light emitting diode, gives the same 5 joules of light from about 25 joules.

1:23Only 20 joules are wasted: 5 divided by 25, times 100 percent, is 20 percent.

1:31A gasoline car moves using only about 20 to 25 of every 100 joules of fuel energy; the rest is wasted, mostly as heat.

1:42Efficient devices need less energy for the same job, so they save money and can cut pollution.

1:50Efficiency is the useful part of the energy; the rest is not destroyed, it spreads out as heat.