Energy Resources and Energy Transfers
5 marks · NormalA power station uses natural gas to generate electricity. Some of the energy from burning gas is used for useful work; some is wasted as heat.
State the unit of power and give its symbol.
The power station transfers 2.4 × 10^8 J of useful electrical energy every second. Calculate its useful power output. Give your answer in MW.
Explain why the total energy output of the power station is more than just the useful energy output.
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) watt (W)
(b) 0.24 MW
(c) Some energy is always lost as heat and sound; the total output includes both useful and wasted energy due to inefficiencies in the process.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · State the correct unit (watt) and symbol (W).
M1 (b) · 1 mark · Converts electrical energy per second to power calculation.
A1 (b) · 1 mark · Correct answer: 0.24 MW with suitable unit. (follow-through)
B1 (c) · 1 mark · Explains that some energy is wasted or lost as heat/sound.
B1 (c) · 1 mark · Refers to inefficiency or total output including all energy forms (useful + wasted).
Worked solution
a) The unit of power is the watt (W).
b) Power output = energy transferred per second = 2.4 × 10^8 J/s = 240,000,000 W = 0.24 MW.
c) The total energy output is higher because some energy is always lost (e.g. as heat to the surroundings or as sound), so it includes both useful energy and wasted energy. No system is perfectly efficient.