Faraday's Law, discovered in 1831 by Michael Faraday, states that the induced electromotive force in a closed circuit is equal to the time rate of change of the magnetic flux through the circuit. Under Faraday's Law, electric current is induced only if either the magnetic field is changing or the conductor is moving. Generators, for example, spin a coil of wire around a magnet to produce a steady current [sources: BBC, National Academy of Sciences, Cassidy et. al.].
There's something almost magical about magnetism. As children, we're captivated by a magnet's ability to affect metals like iron, nickel and cobalt without touching them. We learn about attraction and repulsion between magnetic poles and witness the shape of a magnetic field formed in iron filings surrounding a bar magnet. Physicists tell us that electromagnetism, the force governing both electricity and magnetism, is many times stronger than gravity. The suspension of a maglev train above its track is a striking example of this power.
As the name "electromagnetism" suggests, electricity and magnetism are very closely linked. This relationship allows them to affect each other without contact, as in the maglev train example, or through electromagnetic induction. Electromagnetic induction occurs when a circuit with an alternating current flowing through it generates current in another circuit simply by being placed nearby. An alternating current is the kind of electricity flowing through power lines and home wiring, as opposed to a direct current, which we get from batteries.
How does one circuit cause a current in another without touching it, and what does any of this have to do with magnetism? Before we get into that, we need to look at a few principles linking magnetism and electricity:
- Every electric current has a magnetic field surrounding it.
- Alternating currents have fluctuating magnetic fields.
- Fluctuating magnetic fields cause currents to flow in conductors placed within them, which is also known as Faraday's Law.
Adding these three properties together means that a changing electric current is surrounded by an associated changing magnetic field, which in turn generates a changing electrical current in a conductor placed within it, which has its own magnetic field…and so on. It is the electromagnetic equivalent of nesting Matryoshka dolls. Thus, in the case of electromagnetic induction, placing a conductor in the magnetic field surrounding the first current generates the second current.
Induction is the principle that makes electric motors, generators and transformers possible, as well as items closer to home such as rechargeable electric toothbrushes and wireless communication devices. If you own a rice cooker, chances are you already cook using induction. Now let's look at how induced current is used to turn up the heat in induction cooktops.