3.14 - Electromagnetism

The magnetic field around a current-carrying wire

• Whenever an electric current flows through a wire, a magnetic field is created around it.
• Around a long, straight current-carrying wire the field is made up of concentric circles, lying in planes perpendicular to the wire, with the wire at their centre.
• This pattern can be shown with small plotting compasses placed around the wire: as current flows, each compass lines up along a circle, tracing the field direction.

The right-hand thumb rule

• Point the thumb of the right hand in the direction of the current and curl the fingers around the wire.
• The fingers then point in the direction of the magnetic field.
• Reversing the current reverses the field.

Field strength

The field strength depends on two things:

1. Size of the current - A larger current produces a stronger field.
2. Distance from the wire - The field is stronger closer to the wire and weaker further away.

Solenoids and electromagnets

Coiling the wire strengthens the field

• The magnetic field of a single current-carrying wire is fairly weak. It can be made much stronger by wrapping the wire into a coil.
• A coil of wire is called a solenoid.
• The field lines around each turn line up and add together, giving:
  • A strong, uniform field inside the solenoid, where the field lines are parallel and evenly spaced.
  • A field outside the coil that has the same shape as a bar magnet's.
• One end of the solenoid behaves as a north pole and the other as a south pole. Reversing the current reverses the poles.
• To find the north pole, curl the fingers of the right hand around the coil in the direction of the current. The thumb then points to the north pole (the right-hand grip rule).

Increasing the field strength

• Increasing the current through the coil.
• Increasing the number of turns of wire.
• Adding an iron core inside the coil.

The electromagnet

• Placing a soft iron core inside the solenoid greatly increases the magnetic field.
• The soft iron is easily magnetised to become an induced magnet while the current flows, and quickly loses its magnetism when the current is switched off.
• An electromagnet is a solenoid with an iron core: a magnet whose field can be switched on and off, and varied in strength by changing the current.

Plotting the field of a solenoid

1. Place a piece of card across the middle of the solenoid.
2. With a current flowing, sprinkle iron filings or use a plotting compass on the card to trace the magnetic field lines.

Uses of electromagnets

• Electromagnets can be switched on and off and their strength can be varied, making them useful in many devices.
• A permanent magnet produces a magnetic field all the time and needs no power supply, which is better when a continuous field is needed without an electrical circuit.

Cranes

• Used in scrap yards to pick up, move and drop magnetic materials like iron and steel.
• Turning the current on picks the metal up; switching it off drops it.

Relays

A relay uses a small current in one circuit to safely switch on a much larger current in a separate circuit:

1. A switch closes in the low-current circuit, turning on the electromagnet.
2. The electromagnet attracts an iron armature on a pivot (the rocker).
3. The rocker pivots and pushes the contacts together in the high-current circuit, turning it on.

Examples:

• Relays are used in car ignition circuits to protect the driver from the high currents needed by the starter motor.
• Electromagnets are also used in electric bells and security door locks.