A Levels Physics (9702)•9702/13/M/J/18
Explanation
Hydroelectric efficiency from power ratio Steps:
- Compute input power: P_in = mass flow rate × g × head = 10 kg/s × 9.8 m/s² × 280 m.
- Compute output power: P_out = potential difference × current = 980 V × 205 A.
- Divide output by input: P_out / P_in.
- Multiply by 100% to get efficiency percentage. Why D is correct:
- Matches the formula η = (P_out / P_in) × 100%, where hydroelectric efficiency reflects converted gravitational potential to electrical energy. Why the others are wrong:
- A. 8.3%: Assumes unrealistically high losses, violating energy conservation.
- B. 12%: Far below typical hydroelectric efficiencies of 80-95%.
- C. 83%: Slightly understates the precise power conversion ratio.
Final answer: D
Topic: Gravitational potential energy and kinetic energy
Practice more A Levels Physics (9702) questions on mMCQ.me