Magnetism and Electromagnetism
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Magnetic Poles and Materials
Magnets have two poles — north and south. Only ferromagnetic materials are attracted to magnets.
A common mistake is thinking all metals are magnetic — aluminium, copper and gold are not.
Magnetic Poles and Materials — Key Knowledge
- Like poles repel north-north or south-south
- Unlike poles attract north-south
- Ferromagnetic materials iron, nickel, cobalt, steel — the only metals attracted to magnets
Magnetic Fields
A magnetic field is the region around a magnet where a force acts on another magnetic material.
Field lines never cross — if they did it would mean the field pointed in two directions at once.
Magnetic Fields — Key Knowledge
- Field lines run from north to south outside the magnet
- Field strength stronger where lines are closer together
- Compass plotting a compass needle aligns with the field, showing direction
Permanent and Induced Magnets
A permanent magnet produces its own magnetic field. An induced magnet becomes magnetic only when placed in a magnetic field.
A steel paperclip becomes an induced magnet when touched to a permanent magnet — it can then attract other paperclips.
Permanent and Induced Magnets — Key Knowledge
- Permanent magnet always magnetic, cannot be turned off
- Induced magnet magnetic only while in a field, loses magnetism when removed
Electromagnets
An electromagnet is a magnet created by passing electric current through a coil of wire, usually wrapped around an iron core.
Electromagnets are used where control is needed — scrapyard cranes, circuit breakers, MRI scanners.
Electromagnets — Key Knowledge
- Can be switched on and off by controlling the current
- Strength increased by more current, more coils, adding a soft iron core
- Magnetic field around a straight wire concentric circles centred on the wire
Solenoid Fields
A solenoid is a long coil of wire. When current flows through it, the magnetic field inside is strong and uniform, and the overall field pattern is similar to a bar magnet.
Adding an iron core to a solenoid creates a stronger electromagnet.
Solenoid Fields — Key Knowledge
- Solenoid field like a bar magnet — has a north and south pole
- Inside the solenoid field lines are parallel and evenly spaced — uniform field
The Motor Effect
A current-carrying conductor placed in a magnetic field experiences a force. This is the motor effect.
F = BIl
force = magnetic flux density × current × length
The force is greatest when the wire is at right angles to the field, and zero when parallel.
The Motor Effect — Key Knowledge
- Force depends on magnetic flux density B, current I, length of wire l
- Magnetic flux density measured in tesla, T
Fleming's Left-Hand Rule
Fleming's left-hand rule predicts the direction of force on a current-carrying conductor in a magnetic field.
The mnemonic thuMb-Motion, First-Field, seCond-Current helps students remember which finger is which.
Fleming's Left-Hand Rule — Key Knowledge
- First finger direction of the magnetic Field, north to south
- Second finger direction of the Current, conventional positive to negative
- Thumb direction of the Motion/force
Electric Motors and Loudspeakers
Motors and loudspeakers are practical applications of the motor effect.
In both devices, the force comes from the interaction between the magnetic field and the current in the coil.
Electric Motors and Loudspeakers — Key Knowledge
- Electric motor current-carrying coil in a magnetic field rotates due to forces on each side acting in opposite directions; commutator reverses current every half turn to maintain rotation
- Loudspeaker varying AC current in a coil surrounded by a permanent magnet causes the coil and attached cone to vibrate, producing sound
Electromagnetic Induction (7.3 — Higher)
Moving a conductor through a magnetic field, or changing the magnetic field through a coil, induces a potential difference across the conductor.
This is the reverse of the motor effect — movement produces voltage instead of voltage producing movement.
Electromagnetic Induction (7.3 — Higher) — Key Knowledge
- Induced potential difference generated by relative movement between a conductor and a magnetic field
- Factors affecting size speed of movement, magnetic field strength, number of turns on coil
- Generators a rotating coil in a magnetic field produces alternating current
Transformers (7.3 — Higher)
A transformer changes the voltage of an alternating current supply using electromagnetic induction. It has a primary coil and a secondary coil linked by an iron core.
Vp / Vs = np / ns
primary voltage / secondary voltage = primary turns / secondary turns
The National Grid uses step-up transformers to transmit electricity at high voltage (reducing current and energy loss), then step-down transformers to reduce voltage for homes.
Transformers (7.3 — Higher) — Key Knowledge
- Step-up transformer more turns on secondary than primary — increases voltage
- Step-down transformer fewer turns on secondary than primary — decreases voltage
- AC only a changing current in the primary creates a changing magnetic field, which induces a voltage in the secondary — DC produces a constant field so no induction occurs
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Magnetism and Electromagnetism
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