7.2 Electromagnetism

Think of electricity and magnetism as teammates. When electricity flows through a wire, it creates an invisible magnetic field around the wire. This special link is called electromagnetism.

You can make this magnetic effect much stronger by:

• Using more power (increasing the current).
• Coiling the wire into a tight, spring-like shape (a solenoid).
• Placing a piece of iron, like a nail, inside the coil (an iron core).

Source: Wikimedia Commons

Above is a picture of a solenoid, coils of wire with current going around. This creates a field as indicated by the field lines.

The motor effect is what happens when you place a current-carrying wire inside a magnetic field. The two magnetic fields (from the wire and the magnet) interact, and the wire experiences a force (a push).

This is the principle that makes electric motors work.

We can predict the direction of the force using Fleming's Left-Hand Rule.

Source: Wikimedia Commons

We can calculate the size of the force using the equation:

Force = Magnetic Flux Density × Current × Length

F = B × I × L

Where:

• F is the force in newtons (N)
• B is the magnetic flux density (field strength) in teslas (T)
• I is the current in amperes (A)
• L is the length of the wire in the field, in metres (m)

Electromagnetic induction is the opposite of the motor effect. It is the process of creating a voltage (and potential current) by using a magnetic field.

If you move a wire through a magnetic field (or move a magnet near a wire), a potential difference (voltage) is induced across the wire. If the wire is part of a complete circuit, a current will flow.

This is the principle that makes generators and alternators work.

To increase the induced voltage, you can:

• Move the wire faster.
• Use a stronger magnet.
• Use more turns of wire (a coil).

A transformer is a device that changes the voltage of an alternating current (AC). It does not work with DC.

It has two coils, a primary and a secondary, wrapped on an iron core.

• A Step-Up Transformer: Has more turns on the secondary coil than the primary. It increases the voltage (and decreases the current).
• A Step-Down Transformer: Has fewer turns on the secondary coil than the primary. It decreases the voltage (and increases the current).

We use transformers in the National Grid. Electricity is "stepped-up" to a very high voltage (low current) for transmission across the country (this reduces energy loss as heat in the wires) and then "stepped-down" to a safe 230 V for use in homes.