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

Magnetic Field Around Wires & Solenoids

• When a current flows through a conducting wire a magnetic field is produced around the wire

  • A conducting wire is any wire that has current flowing through it

• The shape and direction of the magnetic field can be investigated using plotting compasses

  • The compasses would produce a magnetic field lines pattern that would like look the following

Magnetic Field Around a Current-Carrying Wire

The magnetic field pattern around a current-carrying wire is a series of concentric circles

• The magnetic field is made up of concentric circles

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

• As the distance from the wire increases the circles get further apart

  • This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases

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

Right-Hand Thumb Rule

The right-hand thumb rule shows the direction of current flow through a wire and the direction of the magnetic field around the wire

• Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field

Different Views of Magnetic Field Lines Around a Current-Carrying Wire

Side and top view of the current flowing through a wire and the magnetic field produced

• If there is no current flowing through the conductor there will be no magnetic field

• Increasing the amount of current flowing through the wire will increase the strength of the magnetic field

  • This means the field lines will become closer together

Magnetic Field Around a Solenoid

• When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it

Field Lines Around Loops of Wire

Diagram showing the magnetic field around a flat circular coil

• To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid

• The magnetic field around the solenoid is similar to that of a bar magnet

Magnetic Field Produced by a Solenoid

Magnetic field around and through a solenoid

• The magnetic field inside the solenoid is strong and uniform

• One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole

  • To work out the polarity of each end of the solenoid it needs to be viewed from the end

  • If the current is travelling around in a clockwise direction then it is the south pole

  • If the current is travelling around in an anticlockwise direction then it is the north pole

• If the current changes direction then the north and south poles will be reversed

• If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid

Poles of a Solenoid

If the current is travelling in a clockwise direction around the end of the solenoid, the induced pole is a south pole and vice versa

• A solenoid can be used as an electromagnet by adding a soft iron core

• The iron core will become an induced magnet when current is flowing through the coils 

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

• The magnetic field produced by the electromagnet can be switched on and off

  • When the current is flowing there will be a magnetic field produced around the electromagnet

  • When the current is switched off there will be no magnetic field produced around the electromagnet

Structure of Electromagnet

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

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

Factors Affecting Magnetic Field Strength

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

  • Increasing the size of the current which is flowing through the wire

  • Increasing the number of coils

  • Adding an iron core through the centre of the coils

• The strength of an electromagnet can be changed by:

  • Increasing the current will increase the magnetic field produced around the electromagnet

  • Decreasing the current will decrease the magnetic field produced around the electromagnet

Applications of the Magnetic Effect

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

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

  • Loudspeakers & headphones

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

 Relay Circuits

When a current passes through the coil in Circuit 1, it attracts the switch in Circuit 2, closing it enables a current to flow in Circuit 2

• 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

Electric Bell

Animation: Electric bells utilise relay circuits. As the current alternates, the metal arm strikes the bell and drops repeatedly to produce the ringing effect

• When the button K is pressed:

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

  • This attracted 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 & Headphones

• Loudspeakers and headphones convert electrical signals into sound

  • They work due to the motor effect

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

Loud Speaker

Diagram showing a cross-section 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

  • The direction of the force at any instant can be determined using Fleming’s left-hand rule

• 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