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Efficiency and wasted energy
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No machine is perfect. Efficiency measures how much of the energy put in does something useful.
The equation
efficiency = useful energy transferred ÷ total energy supplied
Multiply by 100 for a percentage.
Efficiency can never be more than 1 (100%).
Multiply by 100 for a percentage.
Efficiency can never be more than 1 (100%).
Wasted energy
Wasted energy = total energy in − useful energy out.
Most wasted energy ends up in thermal stores of the surroundings, which spreads out (dissipates).
Most wasted energy ends up in thermal stores of the surroundings, which spreads out (dissipates).
Reducing waste
Lubrication reduces friction in moving parts.
Thermal insulation reduces unwanted heating, for example loft insulation and cavity walls.
Thermal insulation reduces unwanted heating, for example loft insulation and cavity walls.
A motor is supplied with 200 J and transfers 50 J usefully. Find its efficiency.
- efficiency = useful ÷ total
- = 50 ÷ 200 = 0.25
- As a percentage: 0.25 × 100 = 25%
Answer: 0.25, or 25%
Efficiency = useful energy out ÷ total energy in. Wasted energy usually heats the surroundings. Lubrication and insulation reduce waste.
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