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.

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).

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.

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.

Why Correct: According to Oersted's experiment, when a current-carrying wire is placed above a magnetic needle and the current flows from north to south, the north pole of the needle deflects eastward. This follows from the right-hand thumb rule: the magnetic field lines around the wire are circular, and below the wire (where the needle is), the field points east when current flows north to south.
Distractor Analysis: South to North would cause the north pole to deflect west. East to West and West to East are horizontal directions and do not represent the actual flow direction; current flows along the wire, not across it.
Takeaway: In Oersted's setup, remember the mnemonic: current from North to South makes North point East (NSE).

Why Correct: Wrought iron has low retentivity, meaning it loses magnetization quickly when the current is switched off, which is essential for an electromagnet that must be turned on and off.
Distractor Analysis: Option A: Permanent magnets require high retentivity, opposite of electromagnet requirement. Option B: High retentivity is for permanent magnets, not electromagnets. Option C: Magnetic strength depends on current and turns, not a unique property of wrought iron.
Takeaway: Electromagnets use soft iron (wrought iron) due to low retentivity; steel with high retentivity is for permanent magnets.