Induction and AC circuits

Written by Dr. Hamza Farooq (Physics)

ECAT · Physics · 5 questions

Name: ____________________ Date: ____________

The magnetic flux through a coil falls from 0.5 Wb to zero in 0.1 s. What is the average induced emf?
- A 0.05 V
- B 50 V
- C 0.5 V
- D 5 V
An alternating current has a peak value of 10 A. What is its rms value?
- A 7.07 A
- B 5 A
- C 10 A
- D 14.1 A
In a circuit containing only an ideal inductor, the current:
- A Is zero
- B Leads the voltage by 90 degrees
- C Is in phase with the voltage
- D Lags the voltage by 90 degrees
What is the resonant frequency of an LC circuit with L = 1 H and C = 1 microfarad? (Take pi = 3.14)
- A About 159 Hz
- B About 50 Hz
- C About 6.28 Hz
- D About 1000 Hz
What is the power factor of a purely resistive AC circuit?
- A 1
- B 0
- C Infinity
- D 0.5

Answer key

1. D, 2. A, 3. D, 4. A, 5. A

emf = change in flux / time = 0.5 / 0.1 = 5 V.
I_rms = I_peak / sqrt(2) = 10 / 1.414, about 7.07 A.
In an inductor the current reaches its peak a quarter cycle after the voltage, so it lags by 90 degrees.
f = 1 / (2 pi sqrt(LC)) = 1 / (2 x 3.14 x 0.001) which is about 159 Hz.
Current and voltage are in phase, so cos(phi) = cos 0 = 1.