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The poles of a magnet are the places where the magnetic forces are strongest. When two magnets are brought close together they exert a force on each other. Two like poles repel each other. Two unlike poles attract each other. Attraction and repulsion between two magnetic poles are examples of non-contact force.

Magnetic Field for Permanent Magnet
Magnetic Field between two magnetic poles

A permanent magnet produces its own magnetic field.

 An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly.

The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.

The force between a magnet and a magnetic material is always one of attraction.

The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet.

The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point. The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.

A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field.

Question: Describe a method using a compass to plot the magnetic field lines around a bar magnet. (4 marks)

When a current flows through a conducting wire a magnetic field is produced around the wire. The strength of the magnetic field depends on the current through the wire and the distance from the wire.

Magnetic Field in a Wire

Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.

The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases the strength of the magnetic field of a solenoid. An electromagnet is a solenoid with an iron core.

Magnetic Field for a Solenoid

A solenoid arrangement can increase the magnetic effect of the current, because each coil has its own magnetic field which is added to the other in the solenoid.

When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other. This is called the motor effect.

Fleming’s left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.

Left Hand Rule for direction of current, magnetic field and force

For a conductor at right angles to a magnetic field and carrying a current:

force = magnetic flux density × current × length

Question: The length of the wire in the magnetic field is 0.050 m
The force on the wire is 0.072 N
magnetic flux density = 360 mT
Calculate the current in the wire. (4 marks)

Question: A student recorded these measurements when investigating a wire in a magnetic field:
•        potential difference across wire = 1.50 V
•        resistance of wire = 0.60 Ω
•        length of wire in magnetic field = 0.050 m
•        force on wire = 3.75 × 10−4 N
Calculate the magnetic flux density of the magnetic field.
Give the unit. (5 marks)

A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor. Each side of the coil feels a force. The force is in opposite directions because the current in each side of the coil flows in a different direction. The two opposing forces cause a turning effect in the coil. The commutator makes sure the current always flows in the same direction round the coil.

Forces in an Electric Motor

Question: Explain why the coil of a motor rotates when there is a current in the coil. (4 marks)