GCSE Questions: Energy Changes
Q4. The photo shows a lift (elevator) inside a building.
(a) The motor in the lift does 120 000 J of work in 8.0 seconds.
Calculate the power output of the motor in the lift.
Power = work done/ time taken
P = 120 000/8.0
P = 15 000 W
P = 15 kW
[3 marks]
(b) The power input to the motor is greater than the power output.
Tick two reasons why.
| Reason |
Tick |
Energy is transferred in heating the surroundings.
|
|
| Friction causes energy to be transferred in non-useful ways. |
|
| The motor is connected to the mains electricity supply. |
|
| The motor is more than 100% efficient. |
|
| There are only four people in the lift. |
|
[2 marks]
(c) The diagram shows part of the circuit that operates the lift motor.
The lift can be operated using either of the two switches. Explain why.
The switches are in parallel. Closing either switch completes the circuit.
[2 marks]
(d) The lift goes up 14 m. The total mass of the people in the lift is 280 kg.
(i) Write down the equation that links gravitational field strength, gravitational potential energy, height and mass.
Change in gravitational potential energy = mass x gravitational field strength x change in height
ΔEGPE = mgΔh
[1 mark]
(ii) Given that gravitational field strength = 9.8 N/kg, calculate the increase in gravitational potential energy of the people in the lift.
Give your answer to 2 significant figures
ΔEGPE = mgΔh
ΔEGPE = 280 x 9.8 x 14
ΔEGPE =38416 J
ΔEGPE =38000 J (2 sig figs)
ΔEGPE =38 kJ (2 sig figs)
[3 marks]
[Total 11 marks]
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