P2 - Effciency and Sankey Diagrams

Question 1

What is the equation for energy efficiency?

Answer 1

Useful energy transferred
Efficiency = ——————————————
Total energy transferred

Question 2

How can efficiency be increased on a motor?

Answer 2

Adding lubricant.

Question 3

How can you reduce energy loss in a building?

Answer 3

Thermal insulation e.g loft and cavity wall insulation, thick curtain
reduce conduction and convection e.g double glazed Windows
The thickness and thermal conductivity of solid walls e.g thick walls, lower conductivity

Question 4

Sankey diagrams

Answer 4

See page 22

Question 5

A motor in a remote controlled car transferred 300j of energy from the car’s energy stores. 225j is transferred to the car’s kinetic energy stores the rest is transferred to the thermal energy store. Calculate the efficiency of the motor. Give your answer as a percentage. (3 marks)

Answer 5

300/225=0.75

75%