Magnetic Effect of a Current (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Magnetic fields around wires & solenoids

Extended tier only

• Magnetic fields are formed wherever a current flows, such as in:

  • straight wires

  • solenoids

  • circular coils

Magnetic field due to a straight wire

• The magnetic field lines around a straight wire are

  • made up of concentric circles

  • centred on the wire

• A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles

• The right-hand grip rule can be used to work out the direction of the magnetic field

The direction of the field around a current-carrying wire can be determined using the right-hand grip rule

• The field lines are clockwise or anticlockwise around the wire, depending on the direction of the current

  • Reversing the current reverses the direction of the field

• The direction of the magnetic field can be determined using the right-hand grip rule

  • This is determined by pointing the right-hand thumb in the direction of the current in the wire and curling the fingers onto the palm

  • The direction of the curled fingers represents the direction of the magnetic field lines around the wire

  • For example, if the current is travelling vertically upwards, the magnetic field lines will be directed anticlockwise, as seen from directly above the wire

• Note: the direction of the current is taken to be the conventional current i.e. from positive to negative, not the direction of electron flow

Magnetic field due to a solenoid

• As seen from a current-carrying wire, an electric current produces a magnetic field

• An electromagnet utilises this by using a coil of wire called a solenoid

  • This increases the strength of the magnetic field by adding more turns of wire into a smaller region of space

• One end of the solenoid becomes a north pole and the other becomes the south pole

The magnetic field lines around a solenoid are similar to a bar magnet

• As a result, the field lines around a solenoid are similar to a bar magnet

  • The field lines emerge from the north pole

  • The field lines return to the south pole

• The poles of the solenoid can be determined using the right-hand grip rule

  • The curled fingers represent the direction of the current flow around the coil

  • The thumb points in the direction of the field inside the coil, towards the north pole

In a solenoid, the north pole forms at the end where the current flows anti-clockwise, and the south pole at the end where the current flows clockwise

Magnetic field due to a circular coil

• A circular coil is equivalent to one of the coils of a solenoid

• The field lines emerge through one side of the circle (north pole) and enter through the other (south pole)

• As with a solenoid, the direction of the magnetic field lines depends on the direction of the current

  • This can also be determined using the right-hand grip rule

Magnetic field lines of many individual circular coils can be combined to make a solenoid

Magnetic effects of changing current

Extended tier only

Magnetic field strength around a straight wire

• The strength of the magnetic field produced around a wire can be increased by:

  • increasing the amount of current flowing through the wire

• The direction of the magnetic field produced around a wire can be changed by:

  • changing the direction of the current

• The strength of a magnetic field decreases with distance from the wire

  • The magnetic field is strongest near the wire and becomes weaker further away from the wire

  • This is shown by the magnetic field lines becoming further apart

The greater the current, the stronger the magnetic field. This is shown by more concentrated field lines

Magnetic field strength around a solenoid

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

  • increasing the amount of current flowing through the coil

  • increasing the number of turns on the coil

  • inserting an iron core into the coil

• The direction of the magnetic field produced around a solenoid can be changed by:

  • changing the direction of the current

• When a soft iron core is inserted into a solenoid, it can be used as an electromagnet

  • The iron core becomes an induced magnet when a current flows through the coils 

  • The magnetic field produced by the solenoid and the iron core will create a much stronger magnet overall

Structure of an electromagnet

An electromagnet consists of a solenoid wrapped around a soft iron core