GCSE Physics P7: Magnetism and Electromagnetism

Magnets

• north and south poles

• same poles repel

• opposite poles attract

permanent magnets

• material that creates its own magnetic field

induced magnets

• material that becomes a temporary magnet when placed inside a magnetic field

• iron, nickel, cobalt

magnetic fields

• field lines point from north to south

• strength decreases with distance from the magnet

• direction always points to south pole and away from north pole, at any point

• use plotting compasses which are small compasses to show the direction of the magnetic field at a certain point

• the region around a magnet where another magnetic object feels a force

Earth’s Core

• core is magnetic, creating large magnetic field around the Earth

• we know this as freely suspended magnetic compass will align itself with the earth’s field lines and point North

• compass is effectively a suspended bar magnet with its own north pole lining up with the Earth’s ‘North Pole’ which cannot be right as like poles repel so Earth’s magnetic pole above Canada is South Pole and south pole is close to magnetic north pole

Current

• produces a magnetic field around the wire

• right hand grip rule shows the direction of curent

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

strength of magnetic field

• greater current, stronger magnetic field

• greater distance from wire, weaker field

Solenoid

• magnetic field shape is similar to bar magnet

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

• having an iron core in the centre increases the strength as its easier for magnetic field lines to pass through than air

factors that affect the strength of magnetic field

• size of current

• length

• cross sectional area

• number of turns (coils)

• using a soft iron core

The Motor Effect

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

• so a magnetic and a wire will also exert a force as the two magnetic fields (generated by the magnet and the current in the wire) will also interact. magnetic field around wire is circular but 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)

Flemming’s Left hand rule

• each direction is at 90 degrees to eachother

• current - convectional current which moves in opposite direction to the electrons

• thumb: force (F)

• index: magnetic field (B)

• middle: Current (I)

force equation with current, magnetic flux density, length

Force = magnetic flux density x current x length

F = BIL

magnetic flux density is measured in Tesla which is the number of flux lines per metre squared

How Electric Motors Work

• permanent magnets lie in fixed positions

• in between a coil of current- carrying wire lies on the axis

• force on one side moves that side up

• force on the other side (where current is flowing in opposite direction) moves down

• therefore it rotates

Electromagnetic Induction

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

• this happens if magnetic field changes as well

• a current flows if the conductor forms a complete circus

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

How Electric Generators (dynamos) work

• same setup as otor with a coil of wire able to rotate between two permanent magnets

• a turbine spins turning the coil of wire

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

• it experiences a change in magnetic field

• creating a potential difference

• if the coil of wire is connected to a complete circut, an alternatic current will flow - this is a basic alternator

• Direct current is produced if the ends (A and D) at the front are connected to a split ring commutator

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

AC and DC produced by alternator and Dynamo

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 was DC, magnetic field it produces will be constant, not including anything in the secondary coil

• more coils on secondary: step up transformer, as voltage will be increased, as changing field will cut through more of a secondary wire inducing a large p.d.

• fewer coils on secondary: step down transformer, as smaller p.d. forms on secondary

equation with transformers

number of coils on primary/ number of coils on secondary = p.d of primary/p.d. of secondary

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

How Dynamic Microphones Work

• they produce a current which is proportional to the sound signal

• fixed magnet is at the centre, and the coil of 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)

• this current is then sent to a loudspeaker

How Loudspeakers Work

• setup is identical to dynamic microphone but 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 which make a sound