Magnetic Effect of a Current (Cambridge (CIE) IGCSE Physics): Revision Note

Magnetic fields around wires & solenoids

• 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 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

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

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 effects of changing current

Extended Tier Only

Magnetic field strength around a straight wire

• The strength of the magnetic field around a wire depends on:

  • the size of the current

  • the distance from the wire

• The strength of a magnetic field increases as the amount of current flowing through the wire increases

  • This means the field lines will become closer together

• 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

• When the direction of the current changes, the magnetic field acts in the opposite direction

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

• 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

• Changing the direction of the current also changes the direction of the magnetic field produced by the iron core

Applications of the magnetic effect of a current

• Electromagnets are used in a wide variety of applications, including:

  • relay circuits (utilised in electric bells, electronic locks, scrapyard cranes etc)

  • loudspeakers

Relay circuits

• Electromagnets are commonly used in relay circuits

• Relays are switches that open and close via the action of an electromagnet

• A relay circuit consists of:

  • an electrical circuit containing an electromagnet

  • a second circuit with a switch which is near to the electromagnet in the first circuit

Operation of a relay circuit

• When a current flows through Circuit 1:

  • a magnetic field is induced around the coil

  • the magnetic field attracts the switch, causing it to pivot and close the contacts in Circuit 2

  • this allows a current to flow in Circuit 2

• When no current flows through Circuit 1:

  • the magnetic force stops

  • the electromagnet stops attracting the switch

  • the current in Circuit 2 stops flowing

• Scrapyard cranes utilise relay circuits to function:

  • When the electromagnet is switched on, it will attract magnetic materials

  • When the electromagnet is switched off, it will drop the magnetic materials

• Electric bells also utilise relay circuits to function:

• When the button K is pressed:

  • a current passes through the electromagnet E creating a magnetic field

  • this attracts the iron armature A, causing the hammer to strike the bell B

  • the movement of the armature breaks the circuit at T

  • this stops the current, destroying the magnetic field and so the armature returns to its previous position

  • this re-establishes the circuit, and the whole process starts again

Loudspeakers

• Loudspeakers convert electrical signals into sound waves

  • They work due to the motor effect (opens in a new tab)

• A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet

Structure of a loudspeaker

• An alternating current passes through the coil of the loudspeaker

  • This creates a changing magnetic field around the coil

• As the current is constantly changing direction, the direction of the magnetic field will be constantly changing

• The magnetic field produced around the coil interacts with the field from the permanent magnet

• The interacting magnetic fields will exert a force on the coil

• As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction

  • This makes the coil oscillate

• The oscillating coil causes the speaker cone to oscillate

  • This makes the air oscillate, creating sound waves