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Wasted energy
When we use energy it often changes from one form to another. For example, a light globe will convert electrical energy into heat a light. The heat is wasted energy. Most energy converters (devices) waste some energy.
Below are some examples of devices and the useful/wasted energy conversions that take place:
Energy Diagrams
Energy diagrams are used to show how much input energy is put into a device and the different forms of energy the device will output. They are useful for illustrating useful and wasted forms of energy:
Sometimes one energy change follows another. The series of steps is called an energy chain. For example, the energy chain for a moving car has three steps.
- The stored chemical energy of the petrol is converted into heat energy when the petrol is burnt in the car’s engine.
- Some of this heat energy is converted into kinetic energy of the moving engine parts.
- This kinetic energy is then transferred through the gears to the wheels.
The energy chain is not 100% efficient, since each step in the chain involves some loss of energy. Friction between the moving parts of the engine produces heat. This heat is transferred to the air around the car. Also, as the engine parts move they produce sound energy. Therefore, not all the stored energy in the petrol is used to make the car’s wheels turn. In fact, engineers have calculated that if you start with 100 joules of chemical energy, you end up with only 25 joules of kinetic energy. The other 75 joules is wasted as heat and sound. Note that the total amount of energy you end up with is the same as the amount you started with. The 75 joules of waste heat and sound from the car is not useful, because it cannot be used again. All energy converters waste energy like this—usually as heat. The longer the energy chain, the more energy that is wasted.
The efficiency of an energy converter is the percentage of the input energy which is turned into useful energy.
Efficiency = useful energy/input energy x 100
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