Magnetism & Electromagnetism — Physics

Q.1

WBCS Prelims 2022

Which quantity is increased in step-down transformer?

A. Voltage

B. Current

C. Wattage

D. Frequency

Explanation

Why Correct: Step-down transformers decrease voltage while increasing current proportionally to maintain power conservation (neglecting losses).
Distractor Analysis: Voltage decreases in step-down transformers. Wattage remains nearly constant (ignoring losses). Frequency stays unchanged in ideal transformers.
Takeaway: Transformer ratio: Vs/Vp = Ns/Np = Ip/Is, where V=voltage, N=turns, I=current, s=secondary, p=primary.

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Q.2

WBCS Prelims 2019

At magnetic poles, the angle of dip is

A. 45°

B. 30°

C. Zero

D. 90°

Explanation

Why Correct: At magnetic poles, Earth's magnetic field lines point vertically downward or upward, making the angle between the magnetic field and horizontal plane exactly 90°.
Distractor Analysis: 45° and 30° represent dip angles at mid-latitudes where field lines have both horizontal and vertical components. Zero occurs at the magnetic equator where field lines are purely horizontal.
Takeaway: Dip angle measurements help determine magnetic latitude, with 0° at equator and 90° at poles.

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Q.3

WBCS Prelims 2013

Instruments can be shielded from the effects of external magnetic fields by surrounding them with —

A. A glass shield

B. A rubber shield

C. A brass shield

D. An iron shield

Explanation

Why Correct: Iron is a ferromagnetic material that provides magnetic shielding by redirecting magnetic field lines around the protected instrument.
Distractor Analysis: Glass is an insulator with no magnetic properties. Rubber is a non-magnetic insulator. Brass is a non-ferromagnetic alloy of copper and zinc.
Takeaway: Mu-metal, an alloy of nickel and iron, provides even better magnetic shielding than pure iron for sensitive instruments.

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Q.4

WBCS Prelims 2004

If a magnet is moved fast toward a coil, the induced electromotive force does not depend on

A. the number of turns of the coil

B. pole strength of the magnet

C. the speed of the magnet

D. the resistance of the coil

Explanation

Why Correct: Faraday's law states induced EMF depends on rate of change of magnetic flux (ε = -N dΦ/dt), which depends on number of turns, magnetic field strength (related to pole strength), and rate of motion (speed), but not on coil resistance.
Distractor Analysis: Number of turns directly multiplies the induced EMF. Pole strength determines magnetic field magnitude affecting flux change. Speed determines rate of flux change through the coil.
Takeaway: While resistance doesn't affect induced EMF magnitude, it does affect induced current magnitude through Ohm's law (I = ε/R).

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Q.5

WBCS Prelims 2004

The magnetic length of a bar magnet means

A. its total length

B. The distance between its two poles

C. its width

D. the distance of a pole from the centre

Explanation

Why Correct: Magnetic length is the distance between the north and south poles of a bar magnet. This distance is always slightly less than the physical length because poles are located slightly inside the ends.
Distractor Analysis: Total length is the physical measurement from one end to the other. Width is the lateral dimension perpendicular to the magnetic axis. Distance of a pole from the centre equals half the magnetic length.
Takeaway: Magnetic length is typically about 0.84 times the physical length for a uniformly magnetized bar magnet.

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Q.6

WBCS Prelims 2003

143. What is increased in the step-down transformer?

A. Voltage

B. Current

C. Wattage

D. Current density

Explanation

Why Correct: A step-down transformer decreases voltage while increasing current proportionally to maintain power conservation (ignoring losses).
Distractor Analysis: Voltage decreases in a step-down transformer. Wattage (power) remains approximately constant in an ideal transformer. Current density depends on conductor cross-section and is not a transformer output parameter.
Takeaway: For an ideal transformer, Vp/Vs = Np/Ns = Is/Ip, where V is voltage, N is turns, I is current, and p/s denote primary/secondary.

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