A Levels Physics (9702)•9702/12/M/J/21
Explanation
Frequency from CRO waveform using cycle count and time base
Steps:
- Time per division = 1 / (20 div s^{-1}) = 0.05 s/div.
- From the waveform, count cycles in one division: 750 cycles.
- Time for 750 cycles = 0.05 s.
- Frequency = 750 cycles / 0.05 s = 15000 Hz.
Why B is correct:
- Frequency equals cycles per second; 750 cycles in 0.05 s gives exactly 15000 Hz by definition.
Why the others are wrong:
- A: 15 Hz requires 0.75 cycles/division, but waveform shows 750.
- C: 20000 Hz requires 1000 cycles/division, but waveform shows 750.
- D: 30000 Hz requires 1500 cycles/division, but waveform shows 750.
Final answer: B
Topic: Progressive waves
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