Electromagnetism

A magnetic field is created when an electric current flows through a wire. The direction of the magnetic field depends on the direction of the current flowing through the wire.

  • If the current is flowing upwards through the wire, the magnetic field will be going anticlockwise.
  • If the current is flowing downwards through the wire, the magnetic field will be going clockwise.

The right-hand rule (or right-hand thumb rule) helps us determine the direction of the magnetic field based on the current’s direction.

The arrows in the diagram above show the direction of the magnetic field. This representation is based on a straight wire; however, in practice, it can also be a coiled wire.

As the distance from the wire increases, the circles (magnetic field lines) get further apart.

Solenoids

When current is flowing through a coil of wire, the magnetic field will look similar to the magnetic field of a bar magnet. If we place a compass near the wire, we will see that it traces the direction of the magnetic field. We can represent the magnetic field with magnetic field lines.

Here we have a solenoid, which is a coil of insulated wire. It produces a magnetic field that flows out of the north and into the south. The field lines are concentric circles around the wire. They are closer together near the wires because that is where the magnetic field is strongest.

  • The two open ends of the coil are attached to a power supply

Electromagnets

To understand electromagnetism, we can split the word “electromagnet” into two parts. ‘Electro‘ relates to electricity andmagnetrelates to magnetism. An electromagnet is a solenoid wrapped around an iron core.

The diagram below shows a simple electromagnet, with the coil wrapped around an iron nail.

Once the power supply is turned on and current flows through the wire, a magnetic field is produced. When you turn off an electromagnet, it loses its magnetism. This makes it useful for moving magnetic objects, by picking them up and dropping them off in different places.

Increasing the magnetic field strength around a solenoid

We may want to increase the strength of the magnetic field around the solenoid, and there are three primary ways to do this:

1. Wrapping the coil around an iron core

2. Increasing the flow of current through the coil

  • To increase the current flow, increase the potential difference of the power supply

3. Increasing the number of turns of the coil.

Example of an electromagnetic device: An electric bell

1. When the button is pressed, current flows through the electromagnet.

2. The electromagnet attracts the iron armature, which causes the hammer to hit the bell.

3. The armature moving breaks the circuit, which stops the current and destroys the magnetic field.

4. As the current stops, the armature moves back to its original position, reconnecting the circuit.

5. The electromagnet is reactivated, and the entire process repeats.

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