SP12- Magnetism And The Motor Effect + SP13 Electromagnetic Induction Flashcards
What’s a permanent magnet
Eg a bar magnet - it’s always magnetic
A magnet can attract magnetic materials which include steel nickel and cobalt
What’s the magnetic field
The space around a magnet where it can attract these materials
What’s a bar like
It has 2 ends, one called a north seeking pole and one called a south seeking pole, if two magnets are placed close to each other, the North Pole on one will attract south on other, 2 north’s or 2 south’s will repel
What’s an induced magnet
When a piece of magnetic material is in a magnetic field it becomes a magnet itself
It stops being magnetic when it’s taken out of the field
Where are magnets used
Electric motors, generators, loudspeakers and other electrical devices
Used in simpler things - door latches and knife holders
How can the shape of a magnetic field be found
Using plotting compasses - use lines that show how a single North Pole would move north to South
How do plotting compasses work
The needle is a very small magnet, it always points to a position near the earths North Pole, a magnet suspended on a string will tilt relative to the horizontal by different amounts in different places
Compass needles are weighted at one end to keep them level
What do we think the earths magnetic field is like
Similar to the shape of a magnetic field of a bar magnet, field thought to be caused by electric currents in the molten outer core made from a mix of iron and nickel
What’s electromagnetism
A current flowing through a wire causes a magnetic field, electric motor and many other devices depend on the magnetic effect of electric currents
What does the direction of the magnetic field depend on In electromagnetism
Depends on the direction of the current - of Current direction changes so does the direction of the magnetic field
How does the right hand rule work
If you point right thumb in direction of the current from + to -, the magnetic field goes in the direction your fingers are pointing
The stronger the current the ______ the magnetic field
The stronger the field - field is strongest closer to the wire and gets weaker further away
What’s a solenoid
When a wire is made into a coil - the magnetic fields of all different parts of the wire form an overall magnetic field, the fields from individual coils add together form a very strong field inside the solenoid, on the outside - the fields from one side tend to cancel the fields from the other to give a weaker field outside
What’s an electromagnet
A coil of wire with a current flowing through it
The magnetic field of an electromagnet can be made stronger by putting a piece of iron (an iron core) inside the coil
The iron becomes a temporary magnet - it’s only magnetic whilst the field from the electromagnet is affecting it
What is the motor effect
A wire carrying a current experiences a force when it’s placed between 2 magnets - the current in the wire creates a magnetic field around the wire and this interacts with the magnetic field between the magnets
The force is greatest when the wire is at right angles to the magnetic field produced by the magnets and is 0 when the wire is in the same direction as magnetic field
there is an equal and opposite force on the magnets
In the motor effect when is force greatest and least
The force is greatest when the wire is at right angles to the magnetic field produced by the magnets and is 0 when the wire is in the same direction as magnetic field
What does Flemings left hand rule mean
Shows the directions of current force and magnetic field are related in the motor effect
What’s magnetic flux density
The strength of the magnetic field - measured in Newton’s per amp metre (N/A m) or Tesla T)
How is force caulcuated
F = B x I x L
How does an electric motor use the motor effect
The force on a conductor in a magnetic field is used to cause rotation in electric motors
Search up a diagram!!
What’s a electric motor like? - name some aspects
There’s a magnet - north side pushes coil up and south side down so it rotates about a pivot
Carbon brushes then make electrical contacts between the circuit and motor
There’s a metal split ring commutator - it’s split so the current changes every half turn to keep it rotating
How does the split ring commutator work
It ensures that the force on the coil always turns it in the same direction
Commutator ensures that the current is always flowing in the correct direction to make the coil continue to spin
How can a voltage be induced in magnetism
A changing magnetic field can induce a voltage or PD in a wire that causes a current to flow
A PD can also be induced if a wire is moving in a magnetic field
Wire often made into coil so there is more wire in the changing magnetic field
In induced voltage, what does the wise of the induced potential difference depend on
Depends on the number of turns in a coil of wire, strength of the magnetic field, how fast the magnetic field changes or moves Past the coil
Reversing direction of change reverses the direction of the induced potential difference
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If the PD causes a current to flow, the magnetic field of the current opposes the original change
Tell me what happens when a magnet is moved through a loop of wire
Magnet and magnetic field is moving to the right in this example
Potential difference is induced in wire which causes a current to flow
Magnetic field around wire in opposite direction to original change - don’t really know what that means lol but that’s what it says hahaahah
What does a generator consist of
Consists of a coil of wire that is rotated inside a magnetic field - as the coil turns, a voltage is induced in the wire
Slip rings - ends of the coil are connected to this
Electrical contact with an external contact is made using carbon brushes which press on slip rings
Produces alternating current so called an alternator
What current does an alternator make
Alternating current
What’s a dynamo
Many appliances neee direct current - a commutator switches connection ever half turn of the coil so produces a form of direct current
A generator with a commutator is often called a dynamo
What do microphones do
Concert the pressure variations in sound waves into variations in current in electrical circuits
Tell me the steps of what a microphone does
Sound waves cause variations in air pressure
The pressure variations make a diaphragm vibrate
The diaphragm moves a coil of wire backwards and forwards - coil is around a permanent magnet so a current is induced
How does a loud speaker work
It converts variations in an electrical current into sounds waves
Loud speakers have similar components to a microphone - varying current through a coil in a magnetic field causes a force on the coil - which moves backwards and forwards as the current varies
The coil is connected to a diaphragm and the movements of the diaphragm produces sound waves
What’s the national grid
A system of wires and cables supplying electricity
Where do wires get very hot in the national grid
Energy waster by heating inside buildings is quite small but the amount is significant for transmission lines
How are current and pd changed in the national grid by transformers
If pd is increased - current is decreased
When current is smaller, less energy is transferred by heating and efficiency is improved
Power stations produce electricity at25kV and transformers change this to 400kV before its sent round the country - voltage revised when sent to other factories and buildings
What does a step up transformer do
Increases the voltage and decreases the current at the same time
What does a step down transformer do
Makes the voltage lower and the current higher
How is a transformer made
Using 2 coils of insulated wire wound on to an iron core - there’s NO electrical connection between the two coils of wire
What type of current do transformers need
An alternating current - when the direction of pd and current changes many times each second - the alternating current in the primary coil creates a continuously changing magnetic field and the iron core of the transformer carries this magnetic field to the secondary coil
What does the changing magnetic field do the the secondary coil
Induces a changing potential difference in the secondary coil - potential difference is greater in the secondary coil if it has more turns thag the Primary
What’s the equation for working out the potential differences across the coils of a transformer can be worked out using this equation:
V(lower left p/ V(LL s) = N (LL p) / N (LL s)
N = number of turns in the coil
What’s potential difference
A measure of energy transferred by each coulomb of charge that flows through a wire - current measured the number of coulombs per second
How is the power calculated
P = I x V
What 2 things multiply to the same thing and why in transformers
Energy can’t be created or destroyed so power supplied in primary coil must be equal to the secondary if it’s 100% efficient
V(lower left P) x I(LL p) = V(LL s) x I(LL s)
