Electromagnetism

Here we can see the magnetic field lines around a bar magnet, running from the north pole, around the outside of the magnet, and into the south pole.

Running current through a wire creates a magnetic field around it. This is how electromagnets work: a coil of wire carrying current generates a magnetic field, just like a permanent magnet would.

An electromagnet gets stronger with more turns of wire, more current, or an iron core inside the coil. A permanent magnet stays magnetic because tiny regions inside it, called domains, stay lined up; an electromagnet loses its magnetic field when the current is turned off.

Electromagnetic induction

Electromagnetic induction is the creation of a voltage, and current, in a conductor when the magnetic field through it changes. The change can come from moving a conductor through a magnetic field, moving a magnet near a coil, or changing the current in a nearby coil.

Electromagnetic induction is the principle that makes inductors, transformers, and generators work, which we will cover next.

Inductors

An inductor is an electrical device that converts electrical energy into a magnetic field, then releases that energy back into the circuit when needed. It is the electrical equivalent of a flywheel. Just as a flywheel resists changes in rotational speed, an inductor resists changes in current.

Inductors are typically made from a coil of wire, often wound around a metal core. Inductance is measured in Henries (H).

Because an inductor resists changes in current, it lets steady DC pass easily but opposes AC, and the higher the AC frequency, the more it opposes it.

Transformers

Transformers are two inductors, like the ones we just covered, that transfer energy from one coil to another through electromagnetic induction. A transformer has a primary coil and a secondary coil, wrapped around a shared core. As AC flows through the primary coil, it creates a changing magnetic field. That changing field induces a current in the secondary coil, transferring energy between the two circuits without a direct electrical connection. Transformers only work with AC, since a steady DC would not create the changing magnetic field induction requires.

A transformer that raises AC voltage is called a step-up transformer, while a transformer that lowers the voltage is called a step-down transformer. Power companies step voltage up for long-distance lines because, for the same power, higher voltage means lower current, and lower current wastes less energy as heat in the wires. Step-down transformers near homes then lower the voltage to a safe level.

The voltage ratio between the two coils matches their turns ratio:

$$\frac{V_p}{V_s}=\frac{N_p}{N_s}$$

Here $V_p$ and $N_p$ are the voltage and turns on the primary coil, and $V_s$ and $N_s$ are the voltage and turns on the secondary coil. More turns on the secondary makes a step-up transformer; fewer turns makes a step-down transformer. Power in equals power out, so when voltage goes up, current goes down, and vice versa.

Example: Step-up transformer

A transformer has 100 turns on its primary coil and 500 turns on its secondary coil. If the primary voltage is 120 V, what is the secondary voltage?

Since $\frac{V_p}{V_s}=\frac{N_p}{N_s}$, $\frac{120}{V_s}=\frac{100}{500}$, so $V_s=120\times 5=600$ V.

Answer: 600 V

Motors

A motor is an electrical device that converts electric or electromagnetic energy into rotational mechanical energy.

A basic motor works by running current through a coil of wire that sits inside a magnetic field. The interaction between the current and the magnetic field produces a force that spins the coil, converting electrical energy into rotational motion.

Generators

A generator is a device that converts rotational energy into electrical energy through electromagnetic induction. As a coil spins inside a magnetic field, or a magnet spins inside a coil, the changing field through the coil induces a current in the wire.

Motors and generators are essentially the same device used in opposite directions. A motor takes in electricity and produces rotation, while a generator takes in rotation and produces electricity. In fact, many motors can act as generators if you spin them by hand, and generators can act as motors if you run current through them.