# Moving Charges and Magnetism, Class 12 Physics | SwaVid

Moving Charges and Magnetism, NCERT Class 12 Physics: notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers.

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# Moving Charges and Magnetism

## Formulas and definitions

## Key ideas

## Common mistakes

## Quick checks

## Keep going

## Questions parents ask

## Class 12 Physics on SwaVid

### Magnetic force and Lorentz force

### Biot-Savart law

### Ampere's circuital law

### Force between parallel currents

### Current loop as a magnetic dipole

### Moving coil galvanometer

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SwaVid › CBSE › Class 12 › Physics › Moving Charges and Magnetism

Notes written by SwaVid from the NCERT textbook.

Class 12 Physics, Chapter 4. Key ideas, formulas and common mistakes.

CBSE prints 17 marks for Units III-IV together, not for this chapter alone.

Short answer. This chapter explains how electric currents produce magnetic fields and how magnetic fields exert forces on moving charges and current elements. Students learn to calculate fields using the Biot-Savart law and Ampere's circuital law, and analyze devices like the moving coil galvanometer.

Source: NCERT Physics Part I (Reprint 2026-27) , Chapter 4, Moving Charges and Magnetism

A stationary charge experiences no force in a magnetic field, but a moving charge experiences a velocity-dependent magnetic force. The total Lorentz force on a charge q moving with velocity v in electric and magnetic fields is F = q(E + v × B).

The magnetic field dB produced by an infinitesimal current element I dl at a distance r is proportional to the current and the vector product of dl and r. It obeys the inverse-square law and points perpendicular to the plane containing dl and r.

The line integral of the magnetic field along any closed loop equals the permeability μ₀ times the total steady current passing through the surface bounded by the loop. It is used to easily find the magnetic field of symmetric configurations like straight wires and solenoids.

Two straight parallel conductors carrying currents in the same direction attract each other, while currents in opposite directions repel. This interaction defines the standard international unit of electric current, the ampere.

A planar current loop of area A and turns N acts as a magnetic dipole of moment m = N I A. In an external uniform magnetic field, the loop experiences a torque τ = m × B but no net force.

A galvanometer detects small electric currents using the torque on a coil suspended in a radial magnetic field. It can be converted into an ammeter by connecting a small shunt resistance in parallel, or into a voltmeter by connecting a large resistance in series.

Try each question first. Then tap it to see the answer.

Identify given values: I = 35 A, r = 0.20 m, μ₀ / 2π = 2 × 10⁻⁷ T m A⁻¹

Apply formula: B = (μ₀ I) / (2π r)

Calculate: B = (2 × 10⁻⁷ × 35) / 0.20 = 70 × 10⁻⁷ / 0.20 = 3.5 × 10⁻⁵ T

3.5 × 10⁻⁵ T

Identify given values: N = 100, I = 0.40 A, R = 0.08 m

Apply formula: B = (μ₀ N I) / (2 R)

Substitute values: B = (4π × 10⁻⁷ × 100 × 0.40) / (2 × 0.08)

Calculate: B = (1.6π × 10⁻⁵) / 0.16 = π × 10⁻⁴ T ≈ 3.14 × 10⁻⁴ T

3.14 × 10⁻⁴ T

Identify given values: I₁ = 10 A, I₂ = 2 A, d = 0.10 m

Apply formula: F / L = (μ₀ I₁ I₂) / (2π d)

Calculate: F / L = (2 × 10⁻⁷ × 10 × 2) / 0.10 = 4 × 10⁻⁶ / 0.10 = 4.0 × 10⁻⁵ N m⁻¹

Currents in the same direction attract each other.

4.0 × 10⁻⁵ N m⁻¹, attractive

Magnetism and Matter .

Current Electricity .

Electromagnetic Induction .

The 14 chapters of Class 12 Physics .

CBSE does not give marks to one chapter. It prints 17 marks for Units III-IV together (Magnetic Effects of Current and Magnetism; Electromagnetic Induction and Alternating Currents). Moving Charges and Magnetism is part of that group.

No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed.

Magnetism and Matter.

SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works.

- Ch 4 NCERT book
- 17 marks, Units III-IV together
- 3 quick checks
- Assuming parallel currents repel like like charges do . Remember that two parallel currents running in the same direction attract each other, and parallel currents running in opposite directions repel.
- Thinking a magnetic field does work on a free moving charge . Because the magnetic force F = q(v × B) is always perpendicular to velocity v, the instantaneous power P = F · v is zero. The magnetic force changes the direction of motion, not the speed or kinetic energy.
- Using total turns N instead of turns per unit length n in the solenoid formula . In B = μ₀ n I, n represents turns per unit length (n = N / L). Compute n before substituting into the formula.
- Confusing shunt and series resistor connections for galvanometers . An ammeter needs a low resistance in parallel (shunt) to bypass current, while a voltmeter needs a high resistance in series to limit current and drop potential.

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