Energy Resources and Energy Transfers
5 marks · NormalA large electric motor is used to lift water from a well and also to pump the water through metal pipes. The table shows some of the energy transfers during one second of operation.
| Process | Energy transferred (J) |
|---|---|
| Electrical energy supplied | 1800 |
| GPE gained by water | 900 |
| Heat energy to pipes | 200 |
| Sound energy | 10 |
State the physical quantity which is measured in joules (J).
Calculate the efficiency of the motor in transferring electrical energy to useful energy. Give your answer as a percentage.
Describe one way that unwanted energy transfer from the pipes could be reduced.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Energy
(b) 50%
(c) Wrap the pipes with insulation (or lagging) to reduce heat loss by conduction and convection.
Common mistakes: Using wrong value for useful energy output; Not dividing by total energy supplied
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States 'energy' as the quantity measured in joules.
M1 (b) · 1 mark · Correctly identifies and uses useful energy (900 J / 1800 J).
A1 (b) · 1 mark · Gives answer as 50% (accept 49–51% for rounding).
B2 (c) · 2 marks · One mark for identifying lagging/insulation; one mark for stating it reduces loss by conduction/convection.
Worked solution
a) The joule (J) is the SI unit of energy.
b) Efficiency = (useful energy output / total energy input) × 100%
c) Heat loss from the pipes can be reduced by insulating them. Adding lagging around the pipes reduces heat loss by conduction and convection to the surroundings.