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IGCSE Physics - Magnets and Currents

An informative presentation

Simran Fernandes

on 6 April 2013

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Transcript of IGCSE Physics - Magnets and Currents

9-03 Magnetic Effect of a Current If an electric current is passed through a wire, a weak magnetic field is produced, with these features:
the magnetic field lines are circles
the field is strongest close to the wire
increasing the current increases the strength of the field
The right-hand grip rule shows that if the the thumb is pointed in the conventional current direction, the other fingers will point in the same direction as the field lines.
A long coil of wire with many turns is a solenoid. 9-04 Electromagnets An electromagnet can be switched on and off; it consists of a copper solenoid wound around an iron core. When a current flows through the coil, it produces a magnetic field, magnetizing the core. Steel is not suitable as a core as it becomes permanently magnetized.
The strength of the magnetic field is increased by increasing the current or the number of turns in the coil. Reversing the current reverses the direction of the magnetic field.
A magnetic relay is a switch operated by an electromagnet used to make a small current operate a much larger circuit.
A circuit breaker is an automatic switch which cuts off the current in a circuit if it rises above a specified value; it can be reset after it has tripped. 9-05 Magnetic Force on a Current When a current passes through a wire in a magnetic field, there is a force on the wire. The force is increased if:
the current is increased
a stronger magnet is used
the length of wire in the field is increased
Fleming's Left-Hand Rule shows that the direction of the force, field, and current are given by the thumb, index, and middle finger respectively. If the current and field are in the same direction, there is no force.
When a coil lies between magnetic poles, the current flows in opposite directions along the two sides of the coil - so one side is pushed up and the other is pushed down.More turns on the coil or a higher current increases the turning effect; the pointer will move further with a higher current as well. by Simran Fernandes Magnets and Currents 9-01 Magnets A magnetic field is a region around a magnet where the magnetic forces from the poles act. They can be investigated by using a compass. Field lines run from the N pole to the S pole, and the field is strongest where the field lines are closest together.
The neutral point is where the combined field strength is zero as the field from one magnet cancels the field from the other. 9-02 Magnetic Fields The two poles of a magnet are known as the north pole and the south pole. Like poles repel and unlike poles attract, whereas the closer the poles, the greater the force between them.
A magnet has a magnetic field around it, has two opposite poles, will attract magnetic materials by inducing magnetism in them, and will exert little or no force on a non-magnetic material.
A magnetic material is one which can be magnetized and is attracted to magnets. Hard magnetic materials (e.g. steel) are difficult to magnetize but do not lose their magnetism easily. Soft magnetic materials (e.g. iron) are easy to magnetize but are only temporary magnets. Non-magnetic materials include some metals (e.g. zinc, tin, copper) and non-metals.
9-06 Electric Motors A simple electric motor runs on direct current, the 'one-way' current that flows from a battery.
The coil, made of insulated copper wire, is free to rotate between the poles of the magnet. The commutator (split-ring) is fixed to the coil and rotates with it. The carbon brushes are two contacts which rub against the commutator and keep the coil connected to the battery.
When the coil is horizontal, the forces are furthest apart and they have maximum leverage on the coil. With no change to the forces, the coil overshoots the vertical, the commutator changes the direction of the current. It keeps rotating in one direction, half a turn at a time. If either the battery or the poles of the magnet were the other way round, the coil would rotate in the other direction.The turning effect on the coil can be increased by:
increasing the current
using a stronger magnet
increasing the number of turns on the coil
increasing the area of the coil within the magnetic field.

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