9.2 - Solenoids & Electromagnets

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.