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Electromagnetism

Physics (GCSE) Revision (Electromagnetism)

StatementResponseComment
A magnet exerts a force on any piece of material which is . . . magnetic Means affected by a magnet !
Magnetic materials include . . . iron, steel, nickel and cobalt There are others but they are usually mixtures of several magnetic materials
The region of space around a magnetic which can influence magnetic materials is called a . . . magnetic field Think of "lines of force"
The magnetic field of a magnet can be made visible by using . . . iron filings or plotting compasses Sprinkle iron filings or place several small compasses around the magnet
The end of a bar magnet which turns to point North is called the . . . North-seeking pole We usually just call it the North pole - but this is a simplification
A bar magnet which is suspended by a thread will eventually . . . come to rest pointing North-South The magnet aligns itself with the Earth's magnetic field
The Earth has a magnetic field around it which resembles that of a . . . bar magnet It looks similar but this does not mean there is a bar magnet at the centre of the Earth
When a coil of wire has an electric current flowing in it, the coil acts like a . . . bar magnet You can plot the field using plotting compasses
The magnetic field in an electromagnet can be made stronger by . . . increasing the current or by using more turns of wire
A strong electromagnet needs to have a core made of . . . soft iron which is easy to magnetise and demagnetise
If a steel rod is placed inside an electromagnet it will . . . become permanently magnetised only if the coil carries a direct current
A magnet can be de-magnetised by placing it inside a coil carrying . . . alternating current This scrambles the atomic magnets and reduces the magnetic field in the magnet to zero
Reversing the current direction in an electromagnet . . . reverses the poles This is what AC does 50 times a second
Electromagnets are used in devices called . . . circuit breakers ( and relays ) When a large current flows, the electromagnet attracts an iron bar which opens a switch
When a wire carrying an electric current is placed in a magnetic field it may experience a . . . force which throws it out of the field
The force on a coil of wire in a magnetic field is the principle of the . . . electric motor which spins continuously in one direction
In a simple dc motor, the device which allows it to spin continuously in one direction is called a . . . split ring commutator It reverses the direction of current every half turn
If a magnet is moved into a coil of wire which is part of a complete circuit, a current is . . . induced Actually a voltage is induced first and then a current flows in the circuit
If a magnet is moved in and out of a coil, the direction of the induced current will . . . reverse that is, change from one direction to the opposite direction
The production of electricity by means of a coil of wire moving in a magnetic field is called . . . electromagnetic induction Discovered by Michael Faraday in 1831
A device which changes the voltage of an AC supply is called a . . . transformer not a 'robot'
Transformers are used at power stations to . . . increase the voltage of the electricity supply to reduce energy losses
Power lines carry electricity at high voltage because . . . this reduces energy losses We don't want to pay for what we don't get
Electricity supplied by the National Grid is at 400 000 volts. This is too high to use in the home so the voltage must be . . . reduced by a transformer called a 'step-down' transformer
A generator consists of . . . a coil of wire rotating in a magnetic field It produces alternating current
A transformer can only work with AC because . . . it needs a changing magnetic field to induce a voltage in the seconday coil
If a wire or coil moves through a magnetic field we say it is . . . cutting the lines of force Think of a cheese wire cutting through cheese
When a wire cuts through a magnetic field . . . a voltage is induced between its ends A current will flow if we connect the wire to a complete circuit
If a coil rotating in a magnetic field is spun faster, the voltage induced will . . . increase Obviously !
To increase the voltage induced in a coil rotating in a magnetic field we could . . . increase its area or the number of turns Or make the field stronger
An AC generator consists of . . . a coil rotating in a magnetic field Power stations have several of these going at the same time
The current is taken from a generator by means of . . . slip rings and brushes This prevents sparks ( or tangled wires )
The brushes in a generator or motor are usually made from . . . graphite ( carbon ) It's a good conductor and it's slippery
A transformer works because . . . an alternating current in the primary coil induces another alternating current in the secondary coil The coils are wound separately on an iron core
A transformer needs a soft iron core because . . . it transfers the magnetic field from the primary to the seconday coil Only AC can produce a rapidly changing magnetic field
When an alternating voltage is applied across the primary coil of a transformer . . . an alternating voltage is produced (induced) across the secondary coil Input and output are both alternating at the same frequency
The voltage across the primary coil of a transformer is related to that across the secondary by the ratio . . . Vp / Vs = Np / Ns This is called the 'turns ratio'
If there are more turns of wire on the secondary coil of a transformer than on the primary, the output voltage will be . . . higher than the input voltage This is a 'step-up' voltage transformer
If a transformer steps up voltage then it also . . . steps down current Power remains constant
For a given rate of transfer of energy (power) the higher the voltage . . . the smaller the current required This reduces heating of the wire
Created by: J Thomson on 2005-05-02



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