| Statement |
Response |
Comment |
| 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 |
