Why doesn't a magnetic field do work on a charge?

The Lorentz force qv×B is always perpendicular to v, so F·dl = 0 along any path.

Why are field lines closed for B but not for E?

Magnetic monopoles do not exist (∇·B = 0), so B-field lines have no beginnings or endings; E-field lines start on positive and end on negative charges.

How does a cyclotron accelerate particles?

An alternating electric field across the gap accelerates particles each half-cycle; the magnetic field inside the dees curves their path back to the gap at the right time.

What unit is the tesla?

1 T = 1 N/(A·m) = 1 V·s/m². Earth's field is roughly 50 μT.

Why do solenoids resemble bar magnets?

Both produce dipole-like external fields. A solenoid's interior field can be made strong and uniform, mimicking a magnet with adjustable strength.

An electron moves at 2×10⁶ m/s perpendicular to B = 0.5 T. Force on it is:

1.6×10⁻¹³ N. F = qvB = 1.6×10⁻¹⁹·2×10⁶·0.5 = 1.6×10⁻¹³ N.

Magnetic field at the centre of a 0.1 m radius loop carrying 2 A is:

2π×10⁻⁵ T. B = μ₀I/(2R) = 4π×10⁻⁷·2/(0.2) = 4π×10⁻⁶ T = 1.257×10⁻⁵ T ≈ 2π×10⁻⁶ — recheck. B = μ₀I/(2R) = (4π×10⁻⁷)(2)/(2·0.1) = 4π×10⁻⁶ T.

Period of circular motion of a charge in a magnetic field depends on:

Mass and charge and field. T = 2πm/(qB) — independent of speed and radius.

Two parallel wires 10 cm apart carry 5 A each in opposite directions. They:

Repel with 5×10⁻⁵ N/m. F/L = μ₀I₁I₂/(2πd) = 2×10⁻⁷·25/0.1 = 5×10⁻⁵ N/m; opposite currents repel.

Magnetic force does no work on a moving charge because:

F is perpendicular to v. F·v = 0 when F ⊥ v, so dW/dt = 0.

Electromagnetism MDCAT 2026 — Notes, Key Concepts & MCQs (Physics)

Electromagnetism

Electromagnetism averages 2-3 MCQs per paper — magnetic force, Ampere's law, and motion of charges in fields are perennial.

Electromagnetism is a Physics chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 36-MCQ Physics section. Mastering the core concepts below typically secures the full chapter weightage.

Magnetic force on a moving charge

A charge q moving with velocity v in magnetic field B experiences F = qv×B, magnitude qvB sin θ, perpendicular to both v and B (right-hand rule for positive q, left for negative). Magnetic force does no work because F ⊥ v. A charged particle moving perpendicular to B follows a circle of radius r = mv/(qB), period T = 2πm/(qB), independent of speed. This is the basis of the cyclotron and mass spectrometer.

Force on a current-carrying conductor

A wire of length L carrying current I in field B experiences F = IL×B = BIL sin θ. Two parallel wires carrying currents I₁ and I₂ separated by d attract (currents same direction) or repel (opposite) with force per unit length F/L = μ₀I₁I₂/(2πd), where μ₀ = 4π×10⁻⁷ T·m/A. This relation defines the ampere.

Magnetic field of currents

Long straight wire: B = μ₀I/(2πr). Circular loop of radius R at centre: B = μ₀I/(2R). Long solenoid: B = μ₀nI inside (n = turns per metre), nearly zero outside. Toroid: B = μ₀NI/(2πr). A solenoid with 1000 turns/m carrying 2 A has B = 4π×10⁻⁷·1000·2 ≈ 2.51×10⁻³ T. Ampere's law: ∮B·dl = μ₀I_enc.

Torque on a current loop and the magnetic dipole

A loop of area A carrying current I has magnetic moment μ = IA (vector along normal by right-hand rule). In external field B, torque τ = μ×B, magnitude μB sin θ; potential energy U = −μ·B. This drives moving-coil galvanometers and DC motors. Torque maximises at θ = 90° (loop plane parallel to B) and is zero at θ = 0° (stable equilibrium with μ ∥ B).

e/m experiment and applications

Thomson's 1897 e/m experiment combined crossed E and B fields: when E/B = v, particles pass undeflected, giving v; switching off E, the radius r in B alone gives e/m = v/(rB). The accepted value is e/m = 1.76×10¹¹ C/kg. The mass spectrometer uses this principle to separate isotopes by their mass-to-charge ratio. References: HRW Chapters 28-29, Serway 29-30, FSc Punjab Chapter 14.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Electromagnetism Is Tested

MDCAT questions on Electromagnetism are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.