Magnetism & Electromagnetism - 7.2.1 Electromagnetism (GCSE Physics AQA)

Electromagnetism

Electric Currents and Magnetic Fields

• Current can flow through a wire. This is simply the flow of charged particles.
• When a current flows through a wire, a magnetic field is produced. The shape of the magnetic field is like a cylinder.
• The magnetic field strength around a current-carrying wire will vary. The closer you are to the wire, the stronger the magnetic field. The closer together the field lines are, the stronger the magnetic field.
• If we know the direction of the magnetic field around a current-carrying wire, we can find out the direction of current flow through the wire. We can do this using the Right Hand Grip Rule.

Effects of Solenoids

• A solenoid is simply a piece of wire that has been shaped into a coil. Like other wires, they have the ability to carry current, which means that they are capable of producing a magnetic field.
• Each coil has a magnetic field around it. All these fields face in the same direction, creating a uniform and strong magnetic field. As there are many magnetic fields are in close proximity, the magnetic field is stronger than that around a straight wire.

Magnetic Fields in Solenoids

• Inside a solenoid the magnetic field is parallel to its axis.
• Outside the solenoid, the field is similar to that of a bar magnet.
• We can increase the magnetic field strength of a solenoid by including an iron core. This is because the iron becomes magnetic in the presence of a current. However when the current is removed, the iron loses its magnetism.

Demonstrating Effects of a Current

• We can prove the magnetic effect of currents using wires. This is clearest with electromagnets as they can be switched on/off by switching the current on/off.
• An electromagnet is very similar to a solenoid with an iron core. An electromagnet is a soft iron metal core surrounded by coils of insulating copper wire.
• An electromagnet’s magnetic field is produced by an electric current flowing through the coil surrounding the metal core.

Drawing Magnetic Field Patterns

Straight Wire

Electromagnetic devices can be represented as circuit diagrams.

AQA exams require interpretation of simple electromagnetic devices such as the one shown opposite.

The diagram below represents an electronically controlled bell.