Magnetism — Set 1

Correct Answer: D. Like poles repel each other.

• **Like poles repel each other.** = Like poles (N-N or S-S) repel; unlike poles (N-S) attract — the fundamental law of magnetic poles. • **Like poles: repulsion; Unlike poles: attraction** — This is analogous to electric charges but magnetic poles always come in pairs. • 💡 Wrong-option analysis: Poles have no effect on each other: incorrect, poles exert forces on each other; Unlike poles repel each other: opposite is true, unlike poles attract; Like poles always attract each other: opposite is true, like poles repel.

Correct Answer: C. Tesla

• **Tesla** = Magnetic flux density B is measured in tesla (T) in SI; 1 T = 1 Wb/m² = 1 kg/(A·s²). • **1 T = 1 Wb/m²** — Earth's field ≈ 25–65 μT; MRI machines use 1.5–3 T; named after Nikola Tesla. • 💡 Wrong-option analysis: Ohm: unit of electrical resistance; Ampere: unit of electric current; Weber: unit of magnetic flux (Φ = B × A), not flux density.

Correct Answer: D. North pole to south pole

• **North pole to south pole** = By convention, outside a magnet, field lines emerge from the north pole and enter the south pole. • **Field lines: N → S outside; S → N inside** — They form closed loops; compass needle aligns with these lines. • 💡 Wrong-option analysis: Upward to downward only: direction depends on orientation of magnet, not just vertical; South pole to north pole: that is the direction inside the magnet; East to west: no fixed cardinal direction.

Correct Answer: A. Experiences torque due to a magnetic field

• **Experiences torque due to a magnetic field** = A compass needle is a small bar magnet; an external field exerts a torque τ = m × B on it, rotating it to align with the field. • **τ = mB sinθ; equilibrium when θ = 0** — The needle's north end points in the direction of field lines. • 💡 Wrong-option analysis: Turns into an electric resistor: physically impossible; Becomes non-magnetic: remains a permanent magnet; Stops all magnetic fields: a small compass cannot shield or stop external fields.

Correct Answer: A. Earth behaves like a giant magnet.

• **Earth behaves like a giant magnet.** = Earth has a magnetosphere generated mainly by convection currents of molten iron in its outer core. • **Earth's B ≈ 25–65 μT at surface** — Guides compass navigation and deflects harmful solar wind charged particles. • 💡 Wrong-option analysis: Earth's field is always uniform everywhere: field varies in strength and direction across the globe; Earth's magnetic field is produced only by the Moon: Moon has a negligible magnetic effect on Earth; Earth has no magnetic field at all: clearly false as compasses work.

Correct Answer: C. Geographic meridian and magnetic meridian

• **Geographic meridian and magnetic meridian** = Declination is the angle by which a compass needle deviates from true geographic north toward magnetic north. • **Declination varies by location and year** — Navigators must correct for it; in India it is a few degrees east or west. • 💡 Wrong-option analysis: Magnetic field and electric current: that is the basis of the right-hand rule, not declination; Magnetic field and vertical line: that defines dip (inclination), not declination; Magnetic poles and Earth's center: geometrical concept, not declination.

Correct Answer: A. Horizontal plane

• **Horizontal plane** = Dip is measured as the angle between the direction of Earth's total magnetic field and the horizontal plane at that location. • **Dip = 0° at magnetic equator; 90° at magnetic poles** — A dip needle (inclinometer) measures this angle. • 💡 Wrong-option analysis: Geographic meridian: related to declination, not dip; Magnetic meridian: the field lies in the magnetic meridian plane, but dip is measured from horizontal, not from the meridian; Vertical plane only: dip is from horizontal, not from vertical.

Correct Answer: D. Resultant magnetic field is zero

• **Resultant magnetic field is zero** = A neutral point is where the field of the magnet exactly cancels Earth's horizontal component, giving zero net field. • **|B_magnet| = |B_Earth_horizontal|; net B = 0** — A compass placed at a neutral point shows random orientation. • 💡 Wrong-option analysis: Only electric field exists: electric fields are not involved here; The magnet becomes stronger: neutral point is about cancellation, not strengthening; Only gravity acts: gravity acts everywhere, not specific to neutral points.

Correct Answer: B. Allows magnetic field to pass through it

• **Allows magnetic field to pass through it** = Permeability μ measures how well a material supports the formation of magnetic flux inside it; B = μH. • **μ = μ_r × μ₀; higher μ → stronger B for same H** — Iron has μ_r ≈ 5000; air μ_r ≈ 1. • 💡 Wrong-option analysis: Produces light from magnetism: magnetoluminescence is not what permeability measures; Stops all magnetism permanently: that would be diamagnetic shielding, but permeability quantifies the ease of magnetization; Turns current into heat only: that is resistivity's domain.

Correct Answer: D. Weakly repelled by a magnet

• **Weakly repelled by a magnet** = Diamagnetic materials have χ < 0 (small negative susceptibility); induced magnetization opposes the applied field, causing weak repulsion. • **χ_dia < 0 (small); examples: bismuth, copper, water** — Levitating frogs use diamagnetic repulsion from water in tissue. • 💡 Wrong-option analysis: Strongly attracted by a magnet: that describes ferromagnetic materials; Unable to respond to any field: diamagnets do respond, just with weak repulsion; Always permanent magnets: diamagnets cannot retain magnetization.