Paper 2 Magnetism Flashcards
What is a magnetic field
The region around a magnet where it has a magnetic effect
What happens when iron filings or plotting compasses are placed near the magnet
They point along curved lines known as lines of flux and they show the direction of the magnetic field at each point
What direction do the filings always point in
From north pole to south pole
What happens when filings are placed near a current carrying wire
They point in circles around the wire and when the current is reversed they point in the opposite direction
What’s a wire in a coil shape know as
A solenoid
What is the shape of the field line like around a solenoid
It’s the same as around the bar magnet
3 key points about field lines
Never touch or cross
The closer together the lines are the stronger the field
Direction of field line shows direction of force that would act on a tiny N-pole placed at that point
What are molecular magnets
They are tiny magnets with N and S poles
What does saturated mean
Is when all molecular magnets are aligned
Properties of soft magnetic materials
It’s easy to magnetise and will demagnetise quickly, useful in electromagnets which can turn off magnets quickly e.g. iron
Properties of hard magnetic materials
Harder to magnetise and harder to demagnetise. perfect for permanent magnets e.g needle in compass e.g steel
What’s an electromagnet
A solenoid (coil of wire) with an iron core. It’s a temporary magnet (can be turned on and off)
How do electromagnets work
When a current is passed along the wire, a magnetic field is created an the iron core is magnetised
when the current is switched off the iron bar loses its magnetism
How can the strength of the magnetic field be increased
Increasing voltage so current increases
Increasing the number of turns in the coil
Inserting an iron core
What is 1 tesla equal to
One Newton per amp per metre
Force =
Flux density x length of conductor in field x current
What does a simple electric motor consist of
A rectangular coil wire, which is placed inside a magnetic field and can rotate between the poles of a magnet, a split ring commutator and a brush contact
How does a simple electric motor work
When the coil turns 90* (so the coil is vertical) the current stops flowing because gaps in the commutator break the current but still turns because of its own movement
When brushes make contact again both the commutator and coil have turned over look in book
How does Flemming’s left hand rule work
Forefinger- direction of field
Middle finger- current
Thumb- movement
What are the two ways of reversing the direction of the forces on the coil in the electric motor
Reverse the direction of the current
Reverse the poles
Electromagnetic induction (the generator effect)
When a wire (conductor) cuts through magnetic field lines a voltage is induced across ends of conductor this is electromagnetic induction as the electrons in the wire have been given a push
What happens if the wire is stationary (the generator effect)
No field lines are cut so no voltage is induced
What happens if the wire moves parallel to the field lines (the generator effect)
No field lines are cut so no voltage is induced
What happens if you move the wire faster or use a stronger magnet (the generator effect)
The voltage used inceases
What happens if you move the wire in the opposite direction or reverse the magnetic field (the generator effect)
Then the voltage induced will be reversed
What’s a bicycle dynamo and how does it work
Consists of a magnet that rotates near a coil of wire (wound on a soft iron core) so that the lines of ‘flux’ cut the wire
A generator converts…energy into…energy
Kinetic energy into electrical energy
Why can a generator be though of as a ‘motor operating in reverse’
Because a motor converts electrical into kinetic which is opposite to a generator
How does a microphone work
The diaphragm of a microphone is attached to the coil. When the diaphragm vibrates in response to incoming sound waves, the coil moves backwards and forwards past the magnet. This induces a current in the coil alternating direction when the diaphragm does
How is a transformer constructed
Consists of primary and secondary coil wrapped around the iron core. 2 coils aren’t connected electrically. This is achieved by using an insulated wire so no current flows through the iron between the 2 coils. 2 coils only connected by magnetic field
How do transformers work
When current flows in primary coil it becomes an electromagnet so iron core magnetised which is linked to 2dary coil and as A.C is used a voltage is induced in the 2dary coil
How do we know an A.C is used in a transformer
The magnetic field in the 2dary coil keeps changing (size and direction) and so a voltage is induced in the 2dary coil. Since the magnetic field changes direction, the voltage induced will also change so must be A.C
Voltage across primary (Vp)/voltage across secondary (Vs)=
Number of turns on primary (Np)/Number of turns on secondary (Ns)
