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Electromagnetic Induction
Introduction
The Experiments of Faraday and Henry
Magnetic Flux
Faraday's Law of Induction
Practice: Flux and Faraday's Law
Lenz's Law and Conservation of Energy
Motional Electromotive Force
Practice: Lenz's Law and Motional EMF
Inductance
AC Generator
Practice: Inductance and the AC Generator
Chapter Review: Mixed Practice
Summary
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Electromagnetic InductionIntroduction

Reading time: ~10 min

Aman cycles home from tuition after dark. His bicycle has no battery, yet its headlamp shines, and the faster he pedals, the brighter it gets. Pressed against the front tyre is a little dynamo: inside it, a magnet spins next to a coil of wire. When Aman stops pedalling, the lamp goes dark.

For a long time electricity and magnetism were thought to be separate subjects. Then, early in the nineteenth century, Oersted, Ampère and others found that an electric current deflects a compass needle: moving charges make magnetic fields. That raised an obvious question. Does it work the other way round? Can a magnet make a current? Sort these by which way the link runs.

a bicycle dynamo lighting the headlamp
a compass needle swinging beside a wire that carries a current
a wireless pad charging a phone
the electromagnet of a scrapyard crane lifting a car
the generator at a hydro-electric dam
an electric bell ringing
the metal detector at an airport gate
A current makes a magnetic effect
A changing magnetic field makes a current

The answer is a resounding yes. Around 1830, Michael Faraday in England and Joseph Henry in the USA showed that a current is induced in a closed coil whenever the magnetic field through it changes. Making an electric current with a changing magnetic field is called electromagnetic induction, and it is the subject of this chapter.

What Is the Use of It?

When Faraday first showed that a magnet moved near a loop of wire made a small current in it, someone asked him, "What is the use of it?" He is said to have replied, "What is the use of a new-born baby?"

The baby grew up. The generators in power stations and the transformers that step voltages up and down both work by electromagnetic induction. Imagine a world without it: no electric lights, no electric trains, no telephones, no computers. Our civilisation owes a great deal of its progress to Faraday's and Henry's experiments. In this chapter you will find out

  • what exactly has to change to induce a current: the magnetic flux,
  • how large the induced emf is (Faraday's law) and which way it pushes (Lenz's law),
  • why a conductor moving through a magnetic field has an emf across it (motional emf),
  • how a coil can induce an emf in its neighbour, or in itself (inductance),
  • and how a coil turning in a magnetic field gives the alternating current in your home (the AC generator).
Sina