Magnetic field and current carrying wires

When there is a current in a wire, a circular magnetic field is produced around the wire. This magnetic field can deflect the needle of a magnetic compass. The strength of the magnetic field is greater:

  • closer to the wire
  • if the current is increased
Wire runs vertically, current flowing bottom to top. Magnetic field rotates anticlockwise. Second wire runs vertically, current flowing top to bottom. Magnetic field rotates in a clockwise direction.Magnetic fields around a wire carrying an electric current

Solenoids

A solenoid is a long coil of wire. When a direct electric current is passed through it, the shape of the magnetic field is very similar to the field of a bar magnet. The field inside a solenoid is strong and uniform. The small magnetic fields caused by the current in each turn of the coil add together to make a stronger overall magnetic field.

Current flows towards you in the top five wires, away from you in bottom five. Magnetic fields flow anti-clockwise around top five wires, clockwise around bottom. North pole right. South pole left.A solenoid, shown here in cross section, has a stronger electromagnetic field than a single straight wire

The strength of the magnetic field around a solenoid can be increased by:

  • increasing the number of turns on the coil
  • increasing the current
  • placing an iron core inside the solenoid

The use of an iron core makes the solenoid into an electromagnet which can be switched on and off.