Lessons › 5 Electricity and Magnetism › 5.4 Electromagnetic InductionPrevious lessonNext lesson

5.4.2 Faraday's Law

By the end of this lesson you can:

  • State Faraday's law of induction
  • Calculate induced emf from the rate of change of flux
  • Explain how current is induced in a coil
Find your course: NSW Year 11NSW Year 12: Module 6, IQ3QLD Unit 3: ElectromagnetismVIC Unit 3: Electricity generationAP Physics 2: Unit 12IB: D.4 Induction
🎥 Watch📝 Practise🕹️ Explore🎯 Exam ready🍎 Teachers
🎥Watch
Learn the idea: a short explanation, then a check.

In our previous lesson, we introduced magnetic flux (Φ) as a measure of the total magnetic field passing through a given area.

In 1831, Michael Faraday made the groundbreaking discovery that an electromotive force (EMF) is induced in a conductor whenever it experiences a change in magnetic flux over time. If the conductor forms a closed circuit, this induced EMF drives an induced electric current.

Faraday's Law of Induction

Faraday's Law states that the magnitude of the induced EMF (ε) in a circuit is directly proportional to the rate of change of magnetic flux through the circuit:

ε = -N · ΔΦ⁄Δt

Where:

  • ε is the induced electromotive force in Volts (V)
  • N is the number of turns in the coil
  • ΔΦ is the change in magnetic flux in Webers (Wb)
  • Δt is the time interval in seconds (s)

Note: Although historically named an electromotive "force," EMF is not a force in Newtons—it is a measure of electric potential difference (voltage, or energy per unit charge). The negative sign represents Lenz's Law, indicating that the induced EMF generates a current whose magnetic field opposes the change in flux that created it.

Interactive Simulation

Before diving into deeper theory, explore the PhET simulation below. Observe how moving the magnet faster (increasing ΔΦ⁄Δt) or using a coil with more turns (N) affects the magnitude of the induced EMF and bulb brightness.

Theory
We will now look at the theory behind ​Faraday's Law.
First look at this video. In one of my first ever videos, I review flux and then discuss how that lead into a EMF being induced


Short on time?
Need a quick review?
​This video provides a a quick summary
otherwise, continue on...
This second video discusses Faraday's  Law as well, but here I use a demonstration with two coils to do so
📝Practise
Worked examples first, then problems to try, then worksheets.
Picture

Check Your Understanding: Rotating Loop & Induced EMF

1. A conductive wire loop rotates at a constant speed in a uniform magnetic field. Its plane is initially parallel to the magnetic field lines. How does doubling the rotation speed (frequency) affect the maximum induced EMF (εmax) generated in the loop?