Efficiency, power and energy resources — GCSE Physics practice questions

Question 1

The display on an exercise bike shows that the rider's power output is 250 W.
What does this value mean?

Accepted Answer

D: The rider transfers 250 J of energy every second.. Power is the rate of energy transfer. 1 W = 1 J / s, so 250 W means that 250 J of energy is transferred every second.

Question 2

A student tests four electric hand drills.
For each drill, she measures the total energy input and the useful energy output during a short test.
Which drill is the most efficient?

Accepted Answer

D: 450 · 160. Efficiency = useful output ÷ total input for each drill: 50/200 = 25 %, 270/900 = 30 %, 240/1000 = 24 % and 160/450 = 36 %. The drill with 450 J in and 160 J out has the greatest fraction, so it is the most efficient.

Question 3

Water flows onto an old water wheel at a rate of 0.25 m³ every second and falls through a vertical height of 4.0 m. The wheel drives a generator. The overall efficiency of the wheel and generator is 60 %. Ignore the speed of the water as it reaches the wheel.

The density of water is 1000 kg / m³ and g = 9.8 N / kg. What is the electrical power output?

Accepted Answer

B: 5.9 kW. Mass per second = 1000 × 0.25 = 250 kg. GPE lost per second = 250 × 9.8 × 4.0 = 9800 J, so the input power is 9.8 kW. Electrical output = 0.60 × 9.8 kW = 5.9 kW.