Physics (GCSE) Revision (Electromagnetism)
Quiz yourself by thinking what should be in
each of the black spaces below before clicking
on it to display the answer.
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| A magnet exerts a force on any piece of material which is . . . | magnetic | Means affected by a magnet !
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| Magnetic materials include . . . | iron, steel, nickel and cobalt | There are others but they are usually mixtures of several magnetic materials
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| The region of space around a magnetic which can influence magnetic materials is called a . . . | magnetic field | Think of "lines of force"
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| 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
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| 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
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| 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
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| 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
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| 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
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| The magnetic field in an electromagnet can be made stronger by . . . | increasing the current | or by using more turns of wire
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| A strong electromagnet needs to have a core made of . . . | soft iron | which is easy to magnetise and demagnetise
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| If a steel rod is placed inside an electromagnet it will . . . | become permanently magnetised | only if the coil carries a direct current
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| 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
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| Reversing the current direction in an electromagnet . . . | reverses the poles | This is what AC does 50 times a second
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| 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
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| 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
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| The force on a coil of wire in a magnetic field is the principle of the . . . | electric motor | which spins continuously in one direction
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| 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
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| 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
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| 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
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| 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
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| A device which changes the voltage of an AC supply is called a . . . | transformer | not a 'robot'
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| Transformers are used at power stations to . . . | increase the voltage of the electricity supply | to reduce energy losses
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| Power lines carry electricity at high voltage because . . . | this reduces energy losses | We don't want to pay for what we don't get
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| 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
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| A generator consists of . . . | a coil of wire rotating in a magnetic field | It produces alternating current
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| A transformer can only work with AC because . . . | it needs a changing magnetic field | to induce a voltage in the seconday coil
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| 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
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| 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
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| If a coil rotating in a magnetic field is spun faster, the voltage induced will . . . | increase | Obviously !
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| 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
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| An AC generator consists of . . . | a coil rotating in a magnetic field | Power stations have several of these going at the same time
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| The current is taken from a generator by means of . . . | slip rings and brushes | This prevents sparks ( or tangled wires )
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| The brushes in a generator or motor are usually made from . . . | graphite ( carbon ) | It's a good conductor and it's slippery
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| 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
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| 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
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| 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
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| 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'
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| 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
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| If a transformer steps up voltage then it also . . . | steps down current | Power remains constant
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| For a given rate of transfer of energy (power) the higher the voltage . . . | the smaller the current required | This reduces heating of the wire
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Created by:
J Thomson
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