Vidaara.orgClass 12 · Physics
CodeVID-P12-03-CH-01
Electromagnetic Induction and Alternating Current — Full Chapter Test
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- This is a full-length test covering the whole chapter — every topic is included.
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all steps in numericals and draw neat labelled diagrams wherever asked. For full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
6 × 1 = 6 marks
1.
The SI unit of magnetic flux is the:
- A.tesla
- B.weber
- C.henry
- D.volt
2.
The negative sign in Faraday's law expresses:
- A.Ohm's law
- B.Lenz's law
- C.Gauss's law
- D.Coulomb's law
3.
$I_{rms}$ equals:
- A.$I_0$
- B.$I_0/\sqrt2$
- C.$I_0\sqrt2$
- D.$2I_0$
4.
At series resonance the impedance equals:
- A.zero
- B.R
- C.$X_L$
- D.infinity
5.
For a transformer $V_s/V_p$ equals:
- A.$N_p/N_s$
- B.$N_s/N_p$
- C.$I_s/I_p$
- D.1
6.
LC oscillation frequency is:
- A.$2\pi\sqrt{LC}$
- B.$1/(2\pi\sqrt{LC})$
- C.$1/\sqrt{LC}$
- D.$\sqrt{LC}$
Section B — Short Answer (2 marks)
4 × 2 = 8 marks
7.
Define magnetic flux and state Faraday's law.
8.
A rod of length 0.5 m moves at 6 m/s perpendicular to a 0.4 T field. Find the EMF.
9.
Write the expressions for $X_L$ and $X_C$.
10.
State the principle of a transformer.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
11.
A series LCR circuit has $R = 12\,\Omega$, $X_L = 25\,\Omega$, $X_C = 9\,\Omega$. Find the impedance.
12.
Explain why power is transmitted at high voltage and the role of transformers.
Section D — Long Answer (5 marks)
2 × 5 = 10 marks
13.
Explain series resonance in an LCR circuit with a phasor diagram, derive $\omega_0 = 1/\sqrt{LC}$, and find $f_0$ for $L = 1\,\text{H}$, $C = 1\,\mu\text{F}$.
14.
Describe the construction and working of a transformer with a labelled diagram, derive the turns-ratio relation and list its energy losses.
Answer Key
Section A — Multiple Choice Questions
- (B) weber
- (B) Lenz's law
- (B) $I_0/\sqrt2$
- (B) R
- (B) $N_s/N_p$
- (B) $1/(2\pi\sqrt{LC})$
Section B — Short Answer (2 marks)
- $\Phi = BA\cos\theta$; induced EMF $\varepsilon = -N\,d\Phi/dt$ equals the rate of change of flux.
- $\varepsilon = Blv = 0.4 \times 0.5 \times 6 = 1.2\,\text{V}$.
- $X_L = \omega L$ and $X_C = 1/\omega C$.
- Mutual induction — a changing flux in the primary induces an EMF in the secondary.
Section C — Short Answer (3 marks)
- $Z = \sqrt{12^2 + 16^2} = \sqrt{400} = 20\,\Omega$.
- High voltage means low current, reducing $I^2R$ losses; step-up at the station and step-down near homes.
Section D — Long Answer (5 marks)
- At resonance $X_L = X_C$, $Z = R$ minimum, current maximum; $\omega_0 = 1/\sqrt{LC} = 1000\,\text{rad/s}$; $f_0 \approx 159\,\text{Hz}$.
- Two coils on a laminated core; mutual induction gives $V_s/V_p = N_s/N_p$; losses: copper, eddy-current, hysteresis, flux leakage.
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