Magnetism and Electromagnetism

Poles of Magnets

North and South

Same poles repel

Opposite poles attract

Permanent Magnets

Always magnetic, always have poles

Induced magnets

Materials that are ‘magnetic’ but do not have fixed poles

These can be made into temporary magnets by ‘stroking’ them with a permanent magnet

• These align domains in material all the same direction, creating a temporary magnet

• Iron, Nickel, Cobalt

Magnetic Fields

• Field Lines: N → S

• Strength decreases with distance from magnet

• Direction always points to south pole and away from north

• Use plotting compasses: small compasses which show the direction of magnetic field at a certain point

Earth’s Core

• Magnetic

• Creates a large magnetic field around the earth

• We know this because a freely suspended magnetic compass will align itself with earth’s field lines and point North

• It doesn’t point to the Geographic North pole — it is over North Canada

• The compass is effectively a suspended bar magnet, with it’s own north pole lining up with Earth’s ‘North pole’

  • However this cannot be right as like poles repel

  • So Earth’s magnetic pole above canada is a magnetic south pole (and geographic south pole is close to magnetic north pole)

Current

• Produces a magnetic field around the wire

• Direction Dictated by the right hand grip rule

• Plotting compasses on a piece of paper through which a wire is pierced shows this

Right hand grip rule

Thumb: direction of current

Fingers: Direction of field

Solenoid

• Shape of magnetic field similar to bar magnet

• Enhances magnetic effect as coiling the wire causes the field to align and form a giant single field, rather than lots of them all perpendicular to direction of current

• Iron core in centre: increases strength as it is easier for magnetic field lines to pass through than air

• Factors that affect strength: Size of current, length, cross sectional area, number of turns (coils), using a soft iron core

The Motor Effect: Explanation

• 2 magnets will interact, feeling a magnetic force of attraction/repulsion

• So a magnet and a wire will also exert a force, as the two magnetic fields (generated by the magnet and current in the wire) will also interact

  • Magnetic field around a wire is circular, but the magnetic field between two magnets is straight

  • When the two interact, the wire is pushed away from the field between the poles (at right angles to the wire direction and the field direction)

The Motor Effect: Visualised

• Fixed permanent magnets have field lines along the x-axis, as the magnets sre at A and B and the field lines are shown

• Wire is along y axis where current is moving up from C to D

• Force felt on the wire is at right angles to both the direction of the current and magnetic field lines

• Along the z axis

Fleming’s left hand rule

THUMB: Force

FIRST: Field
SECOND: Current

Force, Magnetic Flux Density, Current, Length

F = BIL

Force (N) = Magnetic Flux Density (Tesla) x Current (A) x Length (m)

Magnetic Flux Density

• Measured in Tesla

• Number of Flux Lines per metre squared

Electric Motors

• Permanent Magnets lie in fixed positions

• In between, a coil of current-carrying wire lies on an axis

  • Force on one sides moves that side up

  • Force on other side (current flows in opposite direction) moves down

  • This can be verified using Fleming’s left hand rule

• Hence it rotates

Electromagnetic Induction

• When there is a relative movement between a conductor and a magnetic field, a potential difference is induced across the conductor

• This happens if the magnetic field changes as well

• A current flows if the conductor forms a complete circuit

• This current will produce its own magnetic field, which oppose the change inducing it

Electric Generators (dynamos)

• Same setup as a motor, with a coil of wire able to rotate between two permanent magnets

• A turbine spins turning the coil of wire

• The movement of the wire causes the wire to cut through the magnetic field

• It experiences a change in magnetic field

• This creates a potential difference

• If the coil of wire is connected to a complete circuit, and AC will flow - this is a basic alternator

• DC is produced if the ends (A and D in diagram) are connected to a split ring communicator

• This reverses the current each half-rotation so current remains positive - this is called a dynamo

AC produced by Alternator

Alternating Current

DC produced by Dynamo

Direct Current

Transformers

• AC in first coil creates a changing magnetic field

• This changing magnetic field cuts through the secondary coil

• This induces a current in the secondary coil

  • Which is also AC

  • If primary current was DC, magnetic field produced would be constant, not inducing anything on the secondary coil

• More coils on secondary: step up transformer, as voltage will be increased, as changing field will cut through more of the seconday wire inducing a larger pd

• Fewer coils on secondary: step down transformer, as smaller pd forms on secondary

number of coils on primary, secondary, pd equation

number of coils on primary / number of coils on secondary = pd of primary / pd of secondary

this only works with the current too if the transformer is 100% efficient. unless it states this, assume not and just use this to find voltage

Dynamic Microphones

• Produce a current which is proportional to the sound signal

• Fixed magnet is at the centre, and the coil of the wire around the magnet is free to move

• Pressure variations in the sound waves cause the coil to move, and as it moves current is induced in the coil (because it cuts the magnetic field)

• The current is then sent to a loudspeaker

Loudspeakers

• Set up identical to Dynamic Microphones, working in reverse

• Current flows into coil

• Magnetic field from magnet and from current interact, causing the coil to move

• The cone therefore moves

• Producing pressure variations, making sound