Efficiency

Question 1

What is the equation for the efficiency of a device?

Answer 1

Efficiency = useful energy transferred by the device / total energy supplied to the device

Question 2

What would an efficiency of 0% tell you about the energy transfer of a device?

Answer 2

The device doesn’t transfer any energy usefully

Question 3

What is dissipated energy?

Answer 3

Energy that spreads out to the surroundings/ is not transferred to useful stores.

Question 4

Explain why a device can’t have an efficiency of 1

Answer 4

An efficiency of 1 would mean that no energy is wasted, but all changes in a system cause some energy to be dissipated/wasted/stored in less useful energy stores

Question 5

Explain why energy is dissipated to the surroundings whenever work is done mechanically.

Answer 5

Frictional forces have to be overcome. Some of the energy needed to overcome these frictional forces is transferred to the thermal energy stores of the surroundings.

Question 6

A giant’s hair straighteners have an efficiency of 0.7. If his straighteners are supplied with 1000000J of energy, how much energy will be usefully transferred?

Answer 6

700,000 J

Useful energy transferred = efficiency x total energy supplied = 0.7 x 1,000,000 = 700000 J

Question 7

An electric motor usefully transfers 24J of energy. The remaining 16J of the energy supplied is wasted. What is percentage efficiency of the motor?

Answer 7

60%
Efficiency = useful energy transferred / total energy supplied
Total energy supplied = 24 + 16 =40J Useful energy transferred = 24J
Efficiency = 24 /40 = 0.60. As a percentage this is 60%