Eddy Currents & AC Generator — MH-CET Physics MCQs with Solutions
Free MH-CET Physics Eddy Currents & AC Generator MCQs with step-by-step solutions (20 questions). Part of Electromagnetic Induction. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Eddy Currents & AC Generator · easy · theory
Eddy currents are:
A. Steady direct currents in a resistor
B. Currents flowing along a thin wire
C. Circulating currents induced in the body of a bulk conductor by a changing flux ✓ Correct
D. Currents produced by chemical action
Solution: They swirl in closed loops within the metal and dissipate energy as heat.
Q2 — Eddy Currents & AC Generator · medium · theory
The core of a transformer is laminated in order to:
A. Reduce copper loss in the windings
B. Reduce hysteresis loss
C. Minimise the energy lost to eddy currents ✓ Correct
D. Prevent flux leakage
Solution: Thin insulated sheets break the continuous paths available to circulating currents, greatly reducing the loss.
Q3 — Eddy Currents & AC Generator · medium · theory
Which of the following is NOT an application of eddy currents?
A. Electromagnetic braking of trains
B. Damping in a moving coil galvanometer
C. Induction furnaces for melting metals
D. The chemical action in a lead-acid battery ✓ Correct
Solution: A battery works by electrochemistry; the other three exploit currents induced in bulk conductors.
Q4 — Eddy Currents & AC Generator · easy · theory
In an induction furnace, eddy currents are used to:
A. Generate intense heat to melt a metal charge ✓ Correct
B. Measure the temperature of the metal
C. Cool the metal rapidly
D. Magnetise the metal permanently
Solution: A high-frequency alternating field induces very large currents whose resistive heating melts the metal.
Q5 — Eddy Currents & AC Generator · easy · theory
An AC generator converts:
A. Mechanical energy into electrical energy ✓ Correct
B. Electrical energy into mechanical energy
C. Heat energy into mechanical energy
D. Chemical energy into electrical energy
Solution: A coil rotated in a magnetic field has a continuously changing flux linkage, so an alternating EMF appears.
Q6 — Eddy Currents & AC Generator · medium · theory
The instantaneous EMF of an AC generator with a coil of $N$ turns and area $A$ rotating at $\omega$ in a field $B$ is:
A. $NBA\omega\cos\omega t$ at all times
B. $NBA\sin\omega t$
C. $\dfrac{NBA}{\omega}\sin\omega t$
D. $NBA\omega\sin\omega t$ ✓ Correct
Solution: Differentiating $\Phi = NBA\cos\omega t$ gives an EMF that varies sinusoidally with peak value $NBA\omega$.
Q7 — Eddy Currents & AC Generator · hard · theory
In an AC generator, the EMF is maximum when the plane of the coil is:
A. At any orientation
B. At $45^\circ$ to the field
C. Perpendicular to the magnetic field
D. Parallel to the magnetic field ✓ Correct
Solution: The flux is then momentarily zero but changing at its fastest rate, which is what the EMF depends on.
Q8 — Eddy Currents & AC Generator · medium · theory
Slip rings are used in an AC generator in order to:
A. Increase the frequency of the output
B. Convert the alternating EMF into a direct one
C. Reduce eddy current losses
D. Maintain continuous connection to the rotating coil without reversing the output ✓ Correct
Solution: A DC generator instead uses a split-ring commutator, which reverses the connection every half turn.
Q9 — Eddy Currents & AC Generator · hard · numerical
A coil of $100$ turns and area $0.04\text{ m}^2$ rotates at $50\text{ rev/s}$ in a field of $0.5\text{ T}$. The peak EMF is approximately:
A. $314\text{ V}$
B. $200\text{ V}$
C. $628\text{ V}$ ✓ Correct
D. $100\text{ V}$
Solution: $e_0 = NBA\omega = 100 \times 0.5 \times 0.04 \times 2\pi \times 50 \approx 628\text{ V}$.
Q10 — Eddy Currents & AC Generator · medium · numerical
A coil of $50$ turns and area $0.1\text{ m}^2$ rotates at $100\text{ rad/s}$ in a field of $0.2\text{ T}$. The peak EMF is:
A. $100\text{ V}$ ✓ Correct
B. $10\text{ V}$
C. $1000\text{ V}$
D. $50\text{ V}$
Solution: $e_0 = NBA\omega = 50 \times 0.2 \times 0.1 \times 100 = 100\text{ V}$.
Q11 — Eddy Currents & AC Generator · hard · numerical
A coil of $200$ turns and area $0.05\text{ m}^2$ rotates at $50\text{ rev/s}$ in a field of $0.1\text{ T}$. The peak EMF is approximately:
A. $100\text{ V}$
B. $157\text{ V}$
C. $314\text{ V}$ ✓ Correct
D. $628\text{ V}$
Solution: $e_0 = 200 \times 0.1 \times 0.05 \times 2\pi \times 50 = 1 \times 314 \approx 314\text{ V}$.
Q12 — Eddy Currents & AC Generator · easy · numerical
If the frequency of rotation of an AC generator coil is doubled, the peak EMF:
A. Remains unchanged
B. Becomes four times
C. Doubles ✓ Correct
D. Halves
Solution: $e_0 = NBA\omega \propto \omega$.
Q13 — Eddy Currents & AC Generator · easy · numerical
If the area of the coil of an AC generator is doubled, the peak EMF:
A. Becomes four times
B. Remains unchanged
C. Halves
D. Doubles ✓ Correct
Solution: $e_0 = NBA\omega \propto A$.
Q14 — Eddy Currents & AC Generator · medium · numerical
The peak EMF of an AC generator is $628\text{ V}$. Its RMS value is approximately:
A. $444\text{ V}$ ✓ Correct
B. $314\text{ V}$
C. $628\text{ V}$
D. $888\text{ V}$
Solution: $e_{rms} = \dfrac{e_0}{\sqrt{2}} = \dfrac{628}{1.414} \approx 444\text{ V}$.
Q15 — Eddy Currents & AC Generator · medium · numerical
An AC generator coil rotates at $3000\text{ rpm}$. The frequency of the output EMF is:
A. $25\text{ Hz}$
B. $100\text{ Hz}$
C. $50\text{ Hz}$ ✓ Correct
D. $3000\text{ Hz}$
Solution: $f = \dfrac{3000}{60} = 50\text{ rev/s} = 50\text{ Hz}$.
Q16 — Eddy Currents & AC Generator · medium · numerical
An AC generator produces an EMF of angular frequency $100\text{ rad/s}$. The frequency of the output is approximately:
A. $50\text{ Hz}$
B. $100\text{ Hz}$
C. $15.9\text{ Hz}$ ✓ Correct
D. $628\text{ Hz}$
Solution: $f = \dfrac{\omega}{2\pi} = \dfrac{100}{6.283} \approx 15.9\text{ Hz}$.
Q17 — Eddy Currents & AC Generator · hard · numerical
A coil of $500$ turns and area $0.01\text{ m}^2$ rotates at $200\text{ rad/s}$ in a field of $0.02\text{ T}$. The peak EMF is:
A. $200\text{ V}$
B. $2\text{ V}$
C. $100\text{ V}$
D. $20\text{ V}$ ✓ Correct
Solution: $e_0 = 500 \times 0.02 \times 0.01 \times 200 = 0.1 \times 200 = 20\text{ V}$.
Q18 — Eddy Currents & AC Generator · easy · numerical
Eddy current losses in a core can be reduced by:
A. Using thin laminations insulated from one another ✓ Correct
B. Increasing the flux density
C. Increasing the frequency of the supply
D. Using a single solid block of metal
Solution: Laminations confine the induced currents to small loops of high resistance, sharply cutting the loss.
Q19 — Eddy Currents & AC Generator · hard · numerical
In an AC generator, the EMF is zero at the instant when the plane of the coil is:
A. Never zero
B. At $45^\circ$ to the field
C. Perpendicular to the magnetic field ✓ Correct
D. Parallel to the magnetic field
Solution: The flux is then a maximum, so its rate of change — and hence the EMF — is momentarily zero.
Q20 — Eddy Currents & AC Generator · easy · numerical
Two AC generators are identical except that one has twice the number of turns. The ratio of their peak EMFs is:
A. $1 : 1$
B. $2 : 1$
C. $1 : 2$ ✓ Correct
D. $1 : 4$
Solution: $e_0 \propto N$, so the generator with more turns produces the larger EMF.