Module D.4 IBDP HL only Track

Induction

Master Faraday's law, Lenz's alignment rules, magnetic flux changes, and alternating current generators.

01. Observe 02. Think 03. Apply 04. Achieve Excellence

Induction

Introduction

Electromagnetic induction is how changing magnetic conditions generate emf. The guide frames Faraday’s law and Lenz’s law as both a calculation method and a direct consequence of energy conservation.

Guide Focus

  • Use magnetic flux Phi = BA cos(theta).
  • Apply Faraday’s law epsilon = -N delta Phi / delta t.
  • Use Lenz’s law and motional emf epsilon = BvL.

Key Concepts

1. Magnetic flux

Magnetic flux measures how much magnetic field passes through an area: Phi = BA cos(theta). The angle is between the magnetic field and the normal to the area.

2. Faraday’s law

A changing magnetic flux induces an emf: epsilon = -N delta Phi / delta t. More turns, faster change, stronger field, or larger area can increase the magnitude.

3. Lenz’s law

The induced emf acts in a direction that opposes the change causing it. This is required by conservation of energy.

4. Motional and generator emf

A straight conductor moving perpendicular to a uniform magnetic field has epsilon = BvL. A coil rotating in a uniform magnetic field produces sinusoidally varying emf, and increasing rotation frequency increases the induced emf.

Common Mistakes

  • Using the angle to the plane of the coil instead of the angle to the normal.
  • Ignoring the minus sign as a direction statement from Lenz’s law.
  • Assuming a steady magnetic flux induces emf.

Exam Tips

  • Ask what quantity is changing: B, A, angle, or time.
  • For direction questions, identify the original change, then oppose it.
  • For rectangular coils entering or leaving fields, track how the area inside the field changes.

Practice Questions

Question 1 (Multiple Choice)

A coil sits in a constant uniform magnetic field with no motion. The magnetic flux is constant. The induced emf is:

A. Zero. B. Maximum. C. Equal to BA. D. Equal to BvL.

Correct Answer: A

Solution Architecture

Induced emf requires a rate of change of flux. If flux is constant, delta Phi / delta t = 0.

Question 2 (Structured Paper 2 Style)

A straight conductor of length 0.25 m moves at 4.0 m s-1 perpendicular to a 0.50 T magnetic field.

(a) Calculate the induced emf. [2 marks]

(b) State one way to double the emf. [1 mark]

Paper 2 Structured Problem

Markscheme Breakdown

Part (a) Solution:

epsilon = BvL = 0.50 x 4.0 x 0.25 = 0.50 V.

Part (b) Solution:

Double B, v, or L while keeping the motion perpendicular.