JEE Main Physics Magnetic Effects of Current & Magnetism 2027: Fields, Forces & Galvanometer

Moving charges create magnetic fields, and magnetic fields push moving charges. That single loop of ideas powers motors, speakers, cyclotrons and galvanometers, and it powers a big chunk of JEE Main Physics too.

Magnetic force never does work on a moving charge. It only turns it.

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Chapter at a Glance

Priority reflects past-paper trends, not an official NTA weightage.

What the NTA Syllabus Covers

• Biot-Savart law applied to a circular current loop, Ampere's law for a long wire and a solenoid
• Force on a moving charge and on a current-carrying conductor, force between parallel currents, definition of ampere
• Torque on a current loop, moving coil galvanometer, conversion to ammeter and voltmeter
• Current loop as a magnetic dipole, bar magnet as a solenoid, field of a dipole, para-, dia- and ferromagnetism

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Master These Topics

1. Fields Worth Memorising

Worked example: A long wire carrying 5 A produces at 10 cm a field of 2 × 10⁻⁷ × 5 / 0.1 = 10⁻⁵ T, i.e. 10 μT.

Two parallel wires attract if currents are in the same direction and repel if opposite, with force per unit length μ₀I₁I₂ / 2πd. This defines the ampere.

2. A Charge in a Magnetic Field

Force on a moving charge is F = q(v × B). When velocity is perpendicular to B, the charge moves in a circle:

• Radius: r = mv / qB
• Period: T = 2πm / qB, independent of speed

Worked example: A proton moves at 10⁶ m/s perpendicular to B = 0.1 T. Radius = 1.67 × 10⁻²⁷ × 10⁶ / (1.6 × 10⁻¹⁹ × 0.1) ≈ 0.104 m, about 10 cm.

If velocity has a component along B, the path is a helix: circular motion in the perpendicular plane plus uniform drift along the field.

Trap: The magnetic force does no work, so it never changes the kinetic energy or speed of the charge. Only the direction changes.

3. Torque on a Loop and the Galvanometer

A loop of N turns and area A carrying current I in field B feels torque τ = NIAB sinθ, and has magnetic moment μ = NIA.

A galvanometer of resistance G and full-scale current I_g converts to:

• Ammeter (range I): shunt in parallel, S = I_g G / (I − I_g)
• Voltmeter (range V): resistance in series, R = V/I_g − G

Worked example: G = 100 Ω and I_g = 1 mA. For a 1 A ammeter, S = 0.001 × 100 / 0.999 ≈ 0.1 Ω. For a 10 V voltmeter, R = 10 / 0.001 − 100 = 9900 Ω.

4. Magnetic Materials

• Diamagnetic: weakly repelled, negative susceptibility, independent of temperature.
• Paramagnetic: weakly attracted, susceptibility falls as temperature rises (Curie's law).
• Ferromagnetic: strongly attracted, become paramagnetic above the Curie temperature.

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Common Traps to Avoid

• Saying a magnetic field can change a charge's speed. It changes direction only.
• Reversing the attraction and repulsion rule for parallel currents.
• Placing the shunt in series or the voltmeter resistance in parallel.
• Using the loop-centre formula at points off the axis.

60-Second Revision Sheet

• Wire μ₀I/2πr; loop centre μ₀I/2R; solenoid μ₀nI
• r = mv/qB, T = 2πm/qB
• τ = NIAB sinθ, μ = NIA
• Shunt S = I_gG/(I − I_g); voltmeter R = V/I_g − G

Your Study Plan

1. Day 1: fields of wires, loops and solenoids.
2. Day 2: charged-particle motion, circles and helices.
3. Day 3: forces on conductors, loops and galvanometer conversions.
4. Day 4: magnetic materials and a timed mixed set.

Practice Magnetic Effects of Current & Magnetism Questions Free → (opens in a new tab)

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Continue Your Physics Journey

• Previous chapter: Current Electricity
• Next chapter: Electromagnetic Induction & Alternating Currents
• All 20 JEE Main Physics chapters
• Complete JEE Main Syllabus 2027 guide

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