AQA
MAGNETISM AND ELECTROMAGNETISM
Magnets
Permanent and Induced Magnetism
Magnetic | Materials attracted by magnets | Uses non-contact force to attract magnetic materials. |
North seeking pole | End of magnet pointing north | Compass needle is a bar magnet and points north. |
South seeking pole | End of magnet pointing south | Like poles (N – N) repel, unlike poles (N – S) attract. |
Magnetic field | Region of force around magnet | Strong field, force big. Weak field, force small. Field is strongest at the poles. |
Permanent | A magnet that produces its own magnetic field | Will repel or attract other magnets and magnetic materials. |
Induced | A temporary magnet | Becomes magnet when placed in a magnetic field. |
Motor effect
Induced potential, transformers and National Grid
Fleming’s left-hand rule
Increase strength of magnetic field | Use larger current |
Use more turns of wire | |
Put turns of wire closer together | |
Use iron core in middle |
Electric current flowing in a wire produces a magnetic field around it.
If current is small, magnetic field is very weak.
Current large enough, iron filings show circular magnetic field.
Further away from the wire, magnetic field is weaker.
Reverse current, magnetic field direction reverses.
Thumb | Direction of current. |
Fingers | Direction of magnetic field. |
Solenoid | A long coil of wire | Magnetic field from each loop adds to the next. |
Relay | A device using a small current to control a larger current in another circuit | Solenoid is wound around an iron core. Small current magnetises the solenoid. This attracts to electrical contacts, making a complete circuit. Current flows from battery to starter motor. |
Magnetic flux | Lines drawn to show magnetic field | Lots of lines = stronger magnets. |
Magnetic flux density | Number of lines of magnetic flux in a given area | Measures the strength of magnetic force. |
HIGHER only
Force = magnetic flux density X current X length
F = B X I X l
Thumb | Direction of movement. |
First finger | Direction of magnetic field. |
Second finger | Direction of current. |
To predict the direction a straight conductor moves in a magnetic field.
If current and magnetic field are parallel to each other , no force on wire.
Electromagnet | Lots of turns of wire increase the magnetising effect when current flows |
Turn current off, magnetism lost. |
Aluminium foil placed between two poles of a strong magnet, will move downwards when current flows through the foil.
Reverse the current , foil moves upwards.
Magnetic fields from the permanent magnet and current in the foil interact. This is called the motor effect.
Size of force acting on foil depends on magnetic flux density between poles, size of current, length of foil between poles.
Electric motor | Coil of wire rotates about an axle | Current flows through the wire causing a downward movement on one side and an upward movement on the other side. |
Magnetic field around a wire
Right hand rule
Split-ring commutator | Split ring touching two carbon brush contacts |
Force | Newton (N) |
Magnetic flux density | Tesla (T) |
Current | Amperes (A) |
Length | Metres (m) |
Power | Watts (W) |
p.d. | Voltage (V) |
National Grid | Distributes electricity generated in power stations around UK |
PHYSICS HIGHER only
Step-up transformers | Step-down transformers |
Increase voltage, decrease current | Decrease voltage, increase current |
Increases efficiency by reducing amount of heat lost from wires. | Makes safer value of voltage for houses and factories. |
Power lost = Potential difference X Current
Transformer | Two coils of wire onto an iron core | Alternating current supplied to primary coil, making magnetic field change. Iron core becomes magnetised, carries changing magnetic field to secondary coil. This induces p.d. |
Power supplied to primary coil = power supplied to secondary coil
Vp X Ip = Vs X Is
Voltage across the coil X number of coils (primary) = Voltage across the coil X number of coils (secondary)
Vp ÷ Vs = np ÷ ns
Microphones | Converts pressure variations in sound waves into variations in current in electrical circuits. |
Loud speakers | Converts variations in electrical current into sound waves . | Varying current flows through a coil that is in a magnetic field. A force on the wire moves backwards and forwards as current varies. Coil connected to a diaphragm. Diaphragm movements produce sound waves. |
Generators | Coil of wire rotating inside a magnetic field. The end of the coil is connected to slip rings. | Produces altering current. |
Induced potential | When a conducting wire moves through a magnetic field, p.d. is produced |
Generator effect | Generates electricity by inducing current or p.d. |
Uses of the generator effect | Dynamo, Microphones |