P15 Magnetism and electromagnets (paper 2) Flashcards
give three magnetic materials
- iron
- nickle
- cobalt
what is meant by magnetic material
metals that are attracted by magnets and are attracted to either pole of a magnet
where is the magnetic force strongest on a pernament magnet
at the poles
what are the two poles
- north
- south
what are the two types of magnets
- induced
- pernament
what are induced magnets
materials made magnetic temporarily, always attracted to a magnet and then act as magnets to other materials
what direction do magnetic fields travel
from the north pole to the south pole
what do like poles/charges do
repel
what do opposite poles/charges do
attract
what happens when a current flows through a wire
it creates a magnetic field around the wire
what way does current always flow
from positive to negative
how can you determine a magnetic field
using compasses
compasses normally point north, by putting them around a wire we can tell if a field is either clockwise or anti-clockwise
what is the right hand grip rules used to find
the direction of the magnetic field
explain the right hand grip rule
with the thumb of the right pointing in the direction of the current, the way your fingers curl is the direction of the magnetic field
what does a dot to represent the wire is like in a diagram for electromagnetic field
that the current if flowing up wards
what does a cross to represent the wire is like in a diagram for electromagnetic field
that the current is flowing down
how can the magnetic fields strength be increased from a current
increase the current
what type of force is magnetism
non-contact
how can the force on a wire be increased
increase the current and the length of the wire
what is a solenoid
a coiled up peice of wire
what does coiling a wire into a solenoid do to the magnetic field through the wire
the magnetic field inside the solenoid is strong and uniform
and the magnetic field around the solenoid is similar in the shape of a bar magnet
what does adding an iron core inside a solenoid do and why
turns it into a electromagnet because the iron core becomes induced whenever a current flows through the wire
do induced magnets attract and repel
no only attract
what is an electromagnet
a magnet whose magnetic field can be turned on and off with an electric current
what are the three uses of electromagnets
- circuit breakers
- relay circuits
- an electric bell / cranes
(pick things up - act as a switch in another circuit)
how are electromagnets used in circuit breakers
as the current increases so does the magnetic field of the electromagnet. This attracts the metal switch and breaks the circuit
how are electromagnets used in electric bells
- current enters the circuit causing the electomagnet to be magnetised
- this atttracts the arms and causes the bell to be struck
- the ‘make or break’ switch turns off
- cycle repeats
how are electromagnets used in relay circuits
- the current flows and the electromagnet attracts the iron arm
- the arm pivots and pushes switches together completing the circuit
why are electromagnets useful in relay circuits
they switch on high voltage machineray that would risk shock if using a standard switch
what is the motor effect
when a current-carrying wire is in the presence of a magnetic field experiences a force and moves as the wire interacts with the magnetic field
why does the motor effect occur
because as they current carrying wire creates it’s own magnetic field it interacts with the permanet magnetic field and the resultant magnetic field causes a force
what is used to measure the strength of a magnetic field
Magnetic flux density (Tesla (T))
what is the equation that corresponds force, field strength, current and the wire length
force = field strength x current x length of wire
F = BIL
what is F = BIL used to find
the magnitude of the motor effect
what is used to find the direction of the motor effect
Flemmings left hand rule
what is each finger in the Flemmings left hand rule
- Thumb = force (motion)
- finger 1 = field direction (n > s)
- finger 2 = current direction ( + > -)
what does a commutator do
change the direction of the current every half turn to allow a motor to continuously rotate by swapping the contacts to the DC supply
what makes a motor spin faster
- more turns on the coil
- using a stronger magnet
- using an iron core
- using a bigger current
what is the cummutator used in the motor effect
split ring commutator
how can the direction of spin of a simple DC motor be reversed
- changing direction of current
- changing direction of magnetic field
what is the other name for the generator effect
electromagnetic induction
what is the generator effect
the induction of a potential difference (and a current if there’s a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in the magnetic field
how can the size of potential difference induced from the generator effect be increased
- move magnet/coil faster
- use a stronger magnet
- have more turns on the coil of wire
how can the direction of the current in the generator effect be changed
- moving the magnet/ coil in the opposite direction
- swapping the direction of the magnet being used
when do you use Faraday’s right hand rule
to find out the direction of induced current from motion in the generator effect
what are each other the fingers used for in faraday’s right hand rule
- thumb = force (motion)
- finger 1 = magnetic field (n > s)
- finger 2 = induced current ( + > - )
how are microphones used with the generator effect
- sound waves make diaphragm vibrate
- = coil of wire vibrates around a magnet
- this induces a potential difference
- p.d has the same frequency as original sound wave
what is the other name for an AC generator
an altinator
when is potential difference greatest in an altinator
when parallel to field lines
when is the potenital difference zero in an altinator
when perpendictular to field lines
what is the other name for a DC generator
dynamo
how can you make an AC generator DC
add a split ring commutator
if we use an altinating current in a conductor what type of magnetic field will be produced
an altinating magnetic field
if there is an altinating magnetic field what does this mean for size and direction
it is constantly changing
if you double the number of coils in a transformer what will happen to the pd
it will double too
what are transformers
something which reduces current and increases voltage, made up of an iron core and an insulated wire
why are transformers needed to transport electricity
to maintain efficiency
what is the equation relating voltage and number of turns on a transformer
(voltage on primary coil / voltage on secondary coil) = (number of turns on primary coil / number of turns on secondary coil)
how does a transformer work
- AC flows in the primary coil
- this creates an alternating magnetic field in primary coil
- the magnetic field from the primary coil cuts through the secondary coil
- this induces a voltage and current in the secondary coil
- the current in the seconary coil is also AC
what is the difference between step up and step down transformers
- step up has more turns on secondary coil
- step down has more turns on primary coil
how can you work out the input power of a transformer
primary current x primary potential difference
what uses the motor effect
- electrical motors
- loudspeakers
how do electric motors work
- in a simple DC motor, force acts in opposite directiosn of a current carrying wire
- this causes a rotation
- a split ring commutator then swaps the contacts every half turn to keep the motor rotating the same direction
- THE MOTOR EFFECT
how does a loudspeaker work
- an AC is sent through a coil attatched to the base of a paper cone
- the coil surronds one pole of a permanent magnet and is surrounded by the other pole
- this causes a force on the coil
- so vibrations in the current make the cone vibrate which creates a sound wave
- the frequency of the sound wave is the same as the frequency of the AC
describe how an altinator works
- alternators rotate a coil in a magnetic field
- as the coil spins a current is induced in the coil ~ this current changed direction every half turn
- slip rings ensure that the contacts don’t swap every half turn
- an AC pd is produced
what do oscilloscopes show for alernators and dynamos
how the potential different generated changes over time
what generator is this pd time graph of
alternator
what generator is this pd time graph of
dynamo
how do microphones work
- sound waves make the diaphragm vibrate
- the diaphragm vibrating makes a coil of wire vibrate
- the coil of wire vibrates around a magnet
- this induces a pd
- the induced pd has the same frequency as the original sound wave
- GENERATOR EFFECT
what are the two types of transformer
- step-up
- step-down
what type of core is used in a transformer
iron
if there are more coils what does this do to the voltage
increase it ~ more coils = more volts
what is the equation for coils on the wire and voltage
(voltage on primary coil / voltage on secondary coil) = (number of coils on primary / number of coils on secondary)
if the secondary coil has more turns than the primary what type of transformer is it
step up
if the primary coil has more turns than the secondary what type of transformer is it
step down
how can you work out the input power of a transformer
power = primary current x pirmary voltage
