Magnetic Fields in Wires, Coils & Solenoids (Cambridge (CIE) A Level Physics)

Magnetic fields in wires, coils & solenoids

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

  • long straight wires

  • long solenoids

  • flat circular coils

Magnetic field around a current-carrying wire

• Magnetic field lines in a current-carrying wire are circular rings, centred on the wire

• The field lines are closer together near the wire, where the field is strongest

• The field lines become further apart with distance from the wire as the field becomes weaker

• Reversing the current reverses the direction of the 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

• 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 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 around 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 magnetic flux density 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 around a flat circular coil

• A flat 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 depends on the direction of the current

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

  • It is easier to find the direction of the magnetic field on the straight part of the circular coil to determine which direction the field lines are passing through

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

Factors affecting magnetic field strength

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

  • adding a core made from a ferrous (iron-rich) material e.g. an iron rod

  • adding more turns in the coil

• When current flows through the solenoid with an iron core, it becomes magnetised, creating an even stronger field

  • The addition of an iron core can strengthen the magnetic field up to a several hundred times more

• When more turns are added in the coil, this concentrates the magnetic field lines, causing the magnetic field strength to increase