Chapter 21: Magnetism

Electromagnetic Induction

The process by which a changing magnetic field induces an electric current in a conductor.

Faraday’s Law

The induced electromotive force (emf) in a circuit is proportional to the rate of change of magnetic flux through the circuit. $\epsilon =-\frac{\mathrm{d\Phi }B}{\mathrm{dt}}$

Lenz’s Law

The direction of the induced current opposes the change in magnetic flux that caused it.

Induced Currents

• An emf is induced in a closed loop when the magnetic flux through the loop changes.
• Induced current depends on:
  • Rate of change of magnetic flux.
  • Resistance of the circuit.

Applications of Electromagnetic Induction

• Electric Generators: Convert mechanical energy into electrical energy using electromagnetic induction.
• Transformers: Change voltage levels in AC circuits using electromagnetic induction.
• Induction Cooktops: Use alternating magnetic fields to heat cookware directly.

Key Formulas

Magnetic Flux (Φ_B)

$\Phi B_{}=B\cdot A\cdot cos\theta$

Induced emf

$\epsilon =-\frac{\mathrm{d\Phi }{B}_{}}{\mathrm{dt}}$

Questions for Students

1. What is electromagnetic induction, and how is it used in generators?
2. State and explain Faraday’s law of induction.
3. Describe the significance of Lenz’s law in electromagnetic induction.
4. Calculate the emf induced in a loop if the magnetic flux changes by 0.05 Wb in 2 seconds.
5. Explain how a transformer uses electromagnetic induction to change voltage levels.