magnetism and electromagnetism Flashcards
permanent magnet vs induced magnet
permanent: produces its own magnetic field
induce: becomes a magnet when in a magnetic field, always attracted
what materials can be made into a magnet
cobalt
nickel
iron
steel (alloy of iron)
what is a magnetic field
the region around a magnet where a force acts on another magnetic material
what do magnetic field sketches show
- arrows show which was force would act on a north pole in field
- where lines are closer, field is stronger (at poles)
- closer to magnet, field is stronger
how can a compass plot magnetic field
- tiny bar magnet in needle
- move compass around bar magnet and trace the needles position
- add arrow (north to south) to show magnetic field line
- repeat at diff points around north
what happens when current flows through a wire, in terms of magnetism
magnetic field is produced around wire. can be seen using compass
how can strength of field change in current
- larger current = stronger field
- closer to wire = stronger field
- change direction of current =
change direction of field - solenoid
how can you find out direction of magnetic field produced by wire
do a thumbs up with right hand with thumb in direction of current
- fingers will point in direction of field
what is a solenoid and how does it increase magnetic field
coil of wire
field lines around each coil line up, meaning there is a strong and uniform field inside the coil
how can you find out direction of magnetic field produced by solenoid
do a thumbs up and curl fingers in direction of current
- thumb will point to north pole
how can strength of field change in solenoid
- increase current = stronger field
- increase in coils = stronger field
- put iron in coils = stronger field
what is an electromagnet and why are they useful
solenoid with an iron core
- strength can be varied
- can be quickly turned on and off
what is a relay
when high voltage circuits are too dangerous to use a switch
switch is replaced by two metal contacts, one attached to a spring and an iron block
2nd lower voltage circuit containing an electromagnet is added.
how does a relay work
off: no current -> no field. contacts kept apart by spring
on: current flows. EM creates field & attracts iron block, allowing contacts to close and complete circuit
how do doorbells work
on: current flows. EM creates field, attracts iron contact which is attached to a clapper, allowing clapper to hit the bell.
as iron contact has moved, circuit is broken. no current -> no field, contact returns to original position, completing circuit. process can repeat
what happens when you put a current-carrying wire is put perpendicular between two magnets
field around wire interacts with field between magnets. force is exerted on wire, causing it to move up or down (the motor effect)
when a wire is put perpendicular between two magnets, what can affect the size of the force?
- magnetic flux density (strength of field)
- current
- length of wire
when a wire is put perpendicular between two magnets, how can we find out the direction of the force?
make three right angles with thumb, first, and second fingers, and point:
first finger: north to south
second fingers: direction of current
- thumb will show direction of force
how does an electric motor work
when a loop of wire carrying a current is placed between two magnets, the loop will rotate as the motor effect creates force in different directions for each side of the loop.
a split-ring commutator allows electric current to pass onto ring at every 90° turn to keep the motor rotating in the same direction
how does a loudspeaker work
- an AC supply is connected to a coil of wire wrapped around base of a cone
- one pole of permanent magnet inside coil, other pole surrounds it
- ac pases through coil, field generates
- fields from coil & magnet interact, causing a force that causes cone to move
- when AC changes direction, force on the cone changes direction
- cone moves in & out, creating sound waves
- frequency of sound wave can be changed by changing frequency of AC
what is the generator effect
the induction of a potential difference (or current, if circuit is complete) in a wire which is experiencing a change in magnetic field
creating a potential difference in a wire that is passing through a magnetic field
explain how potential difference/current can be induced
pd/current “PD”
- moving the wire across field induces PD across ends of the wire
- moving wire in other directions for each, PD reverses
- this is called induced PD
what affects size of induced pd/current
(IP)
- stronger field = larger IP
- move wire faster = larger IP
- coil wire/more coils = larger IP
how does induced current react to the magnetic field inducing it
magnet moves in & out coil, causing induced current (generator effect)
- induced current creates its own field
- this field opposes movement of magnet:
(north pole inserted into coil - that end of coil also becomes north pole, repelling magnet meaning its harder to pull in, and vice versa)
how does an alternator generate alternating current
- coil is rotated in a field
- current is induces as it spins
- coil is attached to two commutators, which allow current to pass out of coil every half turn
- alternating current generated
how does pd look in oscilloscope graph for alternators and why
up and down, crossing x axis
- peaks when wire directly crosses field lines
- 0 when coil is vertical (does not cross)
- troughs when wire directly crosses field lines, however pd has reversed as coil has swapped direction
factors that affect size of alternating current in electric motor
increases with:
- strength of field
- number of coils
- rotation speed (increases size and frequency)
how does a dynamo generate direct current
- coil is rotated in a field
- current is induces as it spins
- coil is attached to split ring commutator, which swaps the connection at every half turn, allowing current to stay in same direction
- direct current generates
how does pd look on oscilloscope graph for dynamos
stays above the axis, two peaks for one full rotation as each side of coil passes field twice to fully rotate
how does a microphone work
- an AC supply is connected to a coil of wire attached to a flexible diaphragm
- one pole of permanent magnet inside coil, other pole surrounds it
- sound waves hit diaphragm, causing it to vibrate
- this causes wire to move in & out through field
- this causes generator effect, meaning pd induced across the ends of the wire
- induced pd passes through amplifier, into a loudspeaker
what does a transformer do and how does a it look
changes pd for ACs
two separate coils of wire, primary and secondary, wrapped around iron core.
primary is connected to an AC
how does a transformer work
- as ac flows through primary coil, iron core magnetises and demagnetises
- this changing field induces a pd in secondary coil, or current if complete circuit
structure of step up transformer vs step down transformer and why
ratio between coils’ pd = ratio of number of coils, so:
more coils = greater PD
less coils = smaller PD
step up: increases PD - more coils on secondary coil
step down: decreases PD - less coils on secondary coil
how and why are transformers used in national grid
- electricity from power station passes through step up, which increases pd to 400 000v
- cables pass electricity to step down, decreases pd to 230v
power = current x voltage, so transformers allow cables to carry a high voltage and a low current. high currents causes more power to be wasted as heat, so transformers reduce power wastage
