Magnetism — Set 2

Correct Answer: B. Weakly attracted by a magnet

• **Weakly attracted by a magnet** = Paramagnetic materials have small positive susceptibility χ > 0; atomic magnetic moments partially align with applied field. • **χ_para > 0 (small); examples: aluminium, platinum, O₂** — Magnetization vanishes when field is removed. • 💡 Wrong-option analysis: Always strongly magnetized permanently: that describes ferromagnets; Completely unaffected by magnets: paramagnets are weakly attracted; Always repelled strongly by magnets: that describes diamagnets.

Correct Answer: B. Strongly attracted by a magnet

• **Strongly attracted by a magnet** = Ferromagnetic materials have very large positive χ; magnetic domains align cooperatively with applied field, giving very strong magnetization. • **χ_ferro >> 1; examples: iron, cobalt, nickel** — Can be made into permanent magnets; used in motors, transformers. • 💡 Wrong-option analysis: Always non-magnetic: opposite of ferromagnetic; Weakly attracted: that describes paramagnets; Weakly repelled: that describes diamagnets.

Correct Answer: B. Paramagnetic

• **Paramagnetic** = Above the Curie temperature, thermal agitation destroys domain alignment; the material loses ferromagnetic order and becomes paramagnetic. • **Curie T (iron) ≈ 770 °C** — Below Curie T: ferromagnetic; above: paramagnetic. Relevant in transformer core design. • 💡 Wrong-option analysis: A perfect insulator: Curie temperature has no specific relation to electrical insulation; Superconducting: superconductivity is a separate, usually low-temperature phenomenon; Diamagnetic only: the transition is to paramagnetic, not diamagnetic.

Correct Answer: C. Low retentivity and high permeability

• **Low retentivity and high permeability** = Soft iron magnetizes strongly (high permeability) and demagnetizes quickly (low retentivity), making it ideal for switching electromagnets. • **Soft iron: μ_r ~ 5000; coercivity ~ 80 A/m** — Easy to magnetize and demagnetize; loses magnetism when current is removed. • 💡 Wrong-option analysis: Very high retentivity: that is desired for permanent magnets (steel), not electromagnets; No magnetic response at all: soft iron is highly ferromagnetic; Very high electrical resistance: resistance is irrelevant to the magnetization purpose.

Correct Answer: A. High retentivity

• **High retentivity** = Steel can retain strong magnetization after the external field is removed; high retentivity keeps the magnet strong long-term. • **Steel: high retentivity + high coercivity** — Resists demagnetization; Alnico and neodymium alloys also used for strong permanent magnets. • 💡 Wrong-option analysis: No magnetic domains: all ferromagnets have domains; steel does have domains; Very low melting point: steel melts at ~1370 °C, not 'very low'; Zero coercivity: zero coercivity means easy to demagnetize, which would make a poor permanent magnet.

Correct Answer: D. Area of the B-H loop

• **Area of the B-H loop** = The area enclosed by the B-H hysteresis loop represents energy lost as heat per unit volume per magnetization cycle. • **Energy loss/cycle/volume = area of B-H loop (J/m³)** — Large loop area (hard magnets) → high loss; small loop (soft iron) → low loss. • 💡 Wrong-option analysis: Speed of light: irrelevant to magnetic hysteresis; Mass of the magnet only: mass does not determine hysteresis loss; Length of the coil only: coil geometry is not the determining factor.

Correct Answer: D. Magnetic field around a current-carrying conductor

• **Magnetic field around a current-carrying conductor** = Right-hand thumb rule: thumb points in current direction; curled fingers show the circular magnetic field direction around the wire. • **Thumb → current; fingers → B field circles** — Field forms concentric circles around the wire; closer to wire = stronger field. • 💡 Wrong-option analysis: Electric current due to a battery: battery EMF is not found by right-hand rule; Gravitational force on a mass: gravity direction is given by Newton's law, not thumb rule; Heat flow in a wire: thermal flow is not governed by this rule.

Correct Answer: D. Force on a current-carrying conductor in a magnetic field

• **Force on a current-carrying conductor in a magnetic field** = Fleming's left-hand rule (FBI rule): forefinger = B field, middle finger = current I, thumb = force F direction; used for motors. • **Left hand: F = IL × B; thumb → Force** — Electric motors work on this principle; hence 'left = motor'. • 💡 Wrong-option analysis: Induced emf in a generator: that uses Fleming's right-hand rule; Angle of dip: measured with an inclinometer, not Fleming's rule; Magnetic declination: measured with a compass, not Fleming's rule.

Correct Answer: A. Induced current in electromagnetic induction

• **Induced current in electromagnetic induction** = Fleming's right-hand rule (dynamo rule): thumb = motion of conductor, forefinger = B field, middle finger = induced current direction; used for generators. • **Right hand: motion × B → induced current; 'right = generator'** — Changing flux induces EMF; right-hand rule gives its direction. • 💡 Wrong-option analysis: Electric resistance in a wire: resistance is a material property, not found by Fleming's rule; Magnetic pole strength: pole strength is found from force measurement, not Fleming's rule; Torque on a coil: torque uses τ = nBIA sinθ formula, not Fleming's right-hand rule.

Correct Answer: C. F = q(v × B)

• **F = q(v × B)** = The Lorentz magnetic force on charge q moving with velocity v in field B is F = q(v × B); perpendicular to both v and B. • **|F| = qvB sinθ; zero when v ∥ B** — This force causes circular motion of charges in uniform B; principle of cyclotron. • 💡 Wrong-option analysis: F = ma: Newton's second law, not specifically a magnetic force formula; F = qE: electric force on charge in electric field E, not magnetic; F = mg: gravitational force.