# 4.6.6 The Magnetic Effect of a Current

### The Field around a Wire

• When there is a current in a wire, a magnetic field is created around the wire

Diagram showing the magnetic fields around a current-carrying wire

• The magnetic field lines form concentric circles around the wire
• The direction of the field is given by the right-hand grip rule:

When the thumb is pointing in the direction of the current, the fingers will curl in the direction of the field

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### Magnetic Field Strength & Direction

• The direction of a magnetic field is defined as being:
• The direction of the force on the north pole of a magnet placed at that point
• The strength and direction of the field depend on the size and direction of the current:
• If the current is increased, the field will get stronger
• If the direction of the current is changed, the direction of the field will change
• The strength of the magnetic field is related to the distance between the field lines:
As the field lines spread out, the field gets weaker
• The field lines around a wire get further apart the further they are from the wire

### The Field around a Coil

• 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

Diagram showing the magnetic field around a flat circular coil

• A solenoid is a long coil
• The magnetic field around a solenoid look the same as the magnetic field around a bar magnet:

The magnetic field around a solenoid (a long coil) is identical to the magnetic field of a bar magnet

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

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### Strength & Direction within a Solenoid

• Inside the solenoid the field lines straighten up and are very close together – they form a strong uniform field

### Solenoid Applications

• A solenoid can be used as an electromagnet by adding a soft iron core
(This increases the strength of the magnetic field significantly)
• Electromagnets are used in a wide variety of applications including:
• Door bells
• Electronic door locks
• An electromagnet is also used in a relay:

When a current passes through the coil, it attracts the switch, closing it which allows a current in the right-hand part of the circuit

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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