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Magnetic field patterns

A long, straight wire carrying direct current generates a circular magnetic field around the wire.

The right hand grip rule gives the direction of the magnetic field along the wire:

  • Point the thumb of your right hand in the direction of the conventional current and curl your fingers up as shown.
  • The magnetic field lines are circles around the wire pointing in the same direction as your fingers.

The direction of the magnetic field of a long, straight, current-carrying wire can be deduced using the right hand grip rule.
The direction of the magnetic field of a long, straight, current-carrying wire can be deduced using the right hand grip rule.

Increasing the current in the wire increases the strength of the magnetic field.

Reversing the direction of the current in the wire reverses the direction of the magnetic field.

A current running through a circular loop of wire produces a magnetic field that runs straight through its centre. This can be observed by taking a cross section of the loop, which looks like two wires with opposite current flows.

The field lines through the centre of the coil are in the same direction and add, resulting in a stronger magnetic flux density through the centre of the coil. The direction of the magnetic field is given by the right hand rule.

It is similar to the right hand grip rule for wires, except that the thumb points in the direction of the magnetic field and the fingers point in the direction of the current.

Magnetic field pattern of a circular loop
Magnetic field pattern of a circular loop

A current flowing through a long solenoid generates a magnetic field.

Recall that a solenoid is a wire coiled into a series of loops.

The field outside the solenoid:

  • Is similar to that of a bar magnet with a north and south pole; and
  • Gets weaker further away from the solenoid.

The field inside the solenoid:

  • Runs straight through the centre from the south to the north pole; and
  • Is stronger than outside the solenoid.

The location of the north and south poles can be found using the right hand rule. The fingers represent the current and the thumb is the magnetic field inside the solenoid.
The location of the north and south poles can be found using the right hand rule. The fingers represent the current and the thumb is the magnetic field inside the solenoid.

The strength of the field is increased by:

  • increasing the current;
  • increasing the number of coils in the solenoid; and
  • placing an iron core within the solenoid.