magnetism and electromagnetism Flashcards
what are the poles of a magnet
the places where the magnetic forces are strongest
what happens when two magnets are brought close together
they exert a force on each other
- two like poles repel each other
- two unlike poles attract each other
what is attraction and repulsion between two magnetic poles an example of
non-contact force
what is a permanent magnet
a magnet that produces its own magnetic field
what is an induced magnet
a material that becomes a magnet when it is placed in a magnetic field
what does induced magnetism always do
it always causes a force of attraction
what happens when you remove an induced magnet from a magnetic field
it loses most/all of its magnetism quickly
what is a magnetic field
the region around a magnet where a force acts on another magnet or on a magnetic material
list magnetic materials
- iron
- steel
- cobalt
- nickel
what is the force between a magnet and a magnetic material
it is always one of attraction
what does the strength of a magnetic field depend on
the distance from from the magnet - the field is strongest at the poles of the magnet
what is the direction of a magnetic field line
from the north(seeking) pole of a magnet to the south(seeking) pole of the magnet
what is the direction of a magnetic field at any point given by
the direction of the force that would act on another north pole placed at that point
what does a magnetic compass contain
a small bar magnet
what does the earth have and how does a compass show this
a magnetic field - the compass needle points in the direction of the earth’s magnetic field
how to plot a magnetic field of a bar magnet
- place a compass near the pile of a magnet and mark direction the compass points in
- move compass around bar magnet, marking direction of compass
- connect compass points up and add an arrow pointing from north to south pole
what happens when a current flows through a conducting wire
a magnetic field is produced around the wire
what does the strength of the magnetic field around a wire depend on
- the current through the wire; inc current = inc strength
- the distance from the wire; dec distance = inc strength
what happens if we change direction of current through a wire
we reverse the direction of the magnetic field
describe what happens when you place a compass near a wire
when the wire carries a current, the needle points in the direction of the magnetic field. when current switched off, needle of compass reverts back to original position
what does shaping a wire do
it forms a solenoid which increases the strength of magnetic field created by a current through the wire
describe magnetic field inside a solenoid
strong and uniform
what is a solenoid
a coil of current-carrying wire
what is the magnetic field around a solenoid similar to
the magnetic field has a similar shape to that of a bar magnet
what does adding an iron core to a solenoid do
it increases the strength of the magnetic field of a solenoid
what is an electromagnet
a solenoid with an iron core
how can you increase the magnetic field of a solenoid
- increase size of current
- increase number of turns on the coil
- place an iron core inside the solenoid (to form an electromagnet)
what is special about an electromagnet
we can change the strength of the magnetic field by changing the current and we can switch the magnetic field on or off by opening and closing the circuit
what is the motor effect
when a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other
why does the motor effect work
the conductor has its own magnetic field which interacts with the magnetic field of the bar magnet, leading to a force being exerted
how to calculate force experienced by a current-carrying wire in a magnetic field
F = BIL
force = magnetic flux density x current x length
force in newtons (N)
magnetic flux density in tesla (T)
current in amperes (A)
length in metres (m)
what is magnetic flux density
the strength of the magnetic field
what does the bil equation only apply to
a wire which is at right-angles to the magnetic field
how to use the right hand rule
- thumbs up with right hand
- point thumb in direction of current
- where fingers curl is direction of magnetic field
how do we represent the magnetic field of a straight, current-carrying wire
- use concentric circles
- arrows on direction magnetic field is travelling in
- draw at least 3 circles
how to use fleming’s left hand rule
- thumb = force
- pointer finger = direction of magnetic field (N to S)
- middle finger = current
what is the direction of the magnetic field always
from north to south
what is the direction of convential current always
positive to negative
what happens if a conductor is parallel to the magnetic field
it will not experience a force; it must be at right angles to the magnetic field in order to experience a force
factors that affect size of force on a conductor
F = BIl
- inc magnetic flux density = inc in size of force
- inc current = inc in size of force
- inc length of wire = inc in size of force
what is the basis of an electric motor
a coil of wire carrying a current in a magnetic field tends to rotate
what is one useful application of the motor effect
in electric motors
how does force on a conductor in a magnetic field cause rotation of coil in an electric motor
on each side of coil, current travelling in different directions. for example, left side experiences upward force and right side experiences downward force; opposite directions of force cause loop of wire to rotate due to the moment
what happens when the loop is at 90º in an electric motor
the coil stops rotating due to the forces acting on it; even if it manages to go past 90, forces will take it back to 90
how can we solve the coil stopping rotating at 90º
by switching the direction of the current when the loop passes 90; we can do this by using a split-ring commutator
what is a split-ring commutator
a device with two rings with conducting brushes on them which switch the terminals (and thus current) every 90º to ensure the coil is always rotating in the right direction
what do loudspeakers and headphones use the motor effect for
to convert variations in current in electrical circuits to the pressure variations in sound waves
components of a moving-coil loudspeaker
- coil connected to AC
- cone with coil of wire wrapped around one end (forms solenoid)
- permanent magnet
how does a moving-coil loudspeaker work
- current passes through coil generating a changing magnetic field
- magnetic field from coil interacts with magfield from perm magnet, causing coil to experience result force either towards magnet or away from magnet
what happens when the AC current switches direction in a moving-coil loudspeaker
the direction of force on cone reverses, causing paper cone to vibrate in and out, creating waves of varying air pressure which makes a sound of a particular frequency; same freq as AC supply
what happens if we increase the frequency of the current in a moving-coil loudspeaker
we inc pitch of sound waves; therefore inc in size of current = inc in amplitude of vibration
what is a wire
an electrical conductor
what happens if an electrical conductor moves relative to a magnetic field
a potential difference is induced across the ends of the conductor
what happens if there is a change in the magnetic field around a conductor
a potential difference is induced across the ends of the conductor
what happens if a wire stops moving through a magnetic field
the potential difference induced is now lost
what happens if we reverse the direction of motion of a conductor through a magnetic field
we reverse the potential difference that we are inducing
what is the generator effect
if the conductor is part of a complete a circuit, a current is induced in the conductor
define induced potential
the potential difference that we induce by moving a conductor through a magnetic field
difference between motor and generator effect
MOTOR; use current and magnetic field to bring about movement (fleming’s left hand rule)
GENERATOR; use magnetic field and force (movement) to induce a current
when does the direction of current switch
when the direction of movement of a conductor through a magnetic field switches
what happens if there is no movement in the conductor through a magnetic field
there is no induced current
what does an induced current do
it generates a magnetic field that opposes the original change; either the movement of conductor or the change in magnetic field
what is another method we can use to get an induced potential and current
keep conductor still and move magnetic field instead
when does the generator effect only apply
when the conductor is cutting through the magnetic field lines and passing through the magnetic field
what happens if a conductor moves parallel along the magnetic field
no induced potential difference and no induced current
how to increase induced potential difference/induced current
- use stronger magnetic field
- move conductor through the magnetic field more quickly
- increase number of turns on wire to form a coil
when does the generator effect also apply
when moving a bar magnet in a current-carrying solenoid; direction of current changes when direction of movement changes; we can switch direction of induced current if we switch poles of magnet
what happens when we induce a current by the generator effect
the conductor that now has current in it produces its own magnetic field, which always opposes movement of the magnet
what happens when we move a magnet into the solenoid during the generator effect
the current generates magnetic field that repels magnet; when we move magnet out of solenoid, current generates magnetic field that attracts magnet. this means it is harder to move magnet, meaning we are doing work; transfer of energy to movement of current
why is the generator effect used in an alternator
to generate ac
what is an alternator
a coil of wire rotating in a magnetic field
difference between alternator and electric motor
- in an alternator, we manually move wire to generate current.
- in elec motor, we use magnetic field produced by already established current to create movement
how does alternator generate ac using generator effect
- manually rotate coil to produce alternating current
- wire rotates and cuts through magnetic field which induces potential difference
- wires each connected to commutator to allow current to pass out of coil
- rotation of coil in alternator causes alternating potential difference
when is the maximum potential difference in an alternator and why
when the coil is horizontal, because at this point, the wire is (for a split second) moving directly through the magnetic field
when is the potential difference 0 in an alternator and why
when the coil is vertical, because the wire is moving parallel to the magnetic field, so it does not cut through the magnetic field
in an alternator why are the two sides of the coil attached to two different rings
so that the two sides of the coil move in different directions, which produces an alternating potential difference and an alternating current (AC)
how can we increase size of alternating current in an alternator
- increase strength of magnetic field
- increase number of turns on coil
- increase area of coil
- increase speed of rotation of coil (also increases frequency of AC)
why is the generator effect used in a dynamo
to generate dc
difference between dynamo and alternator in terms of rings
instead of having two split ring commutators like an alternator, a dynamo has 1
how does a dynamo generate dc
as we rotate the coil, the split-ring commutator switches direction of current every half turn so that any negative current when the two sides of coil are moving in opposite directions is counteracted by the split ring
this produces direct current and direct potential difference
how do microphones use the generator effect
to convert the pressure variations in sound waves into variations in current in electrical circuits
components of a moving-coil microphone
- diaphragm attached to coil
- induced current
- magnet
- coil of wire wrapped around tube from diaphragm
how does a moving-coil microphone work
- soundwaves hit the diaphragm causing it to vibrate
- causes coil to go in and out of perm magnet
- movement causes p.d. and current to be induced due to generator effect
- alternating potential difference induced
in a moving-coil microphone when does potential difference switch direction
as the coil moves backwards and forwards through the magnetic field
what is the frequency of the induced potential difference in a moving-coil microphone the same as
the frequency of the sound waves so changing pattern of p.d. can be passed into amplifier and into moving-coil loudspeaker; increases volume
what happens if the sound is louder in a moving-coil microphone
the higher the amplitude of the waves and the higher the induced potential difference
what are transformers used for
increasing or decreasing p.d. to reduce energy loss in national grid and make safe transit between power stations and homes
what does a basic transformer consist of
- primary coil
- secondary coil
both wound on an iron core
why is iron used in a transformer
because it is easily magnetised
what potential difference is applied across the primary coil of a transformer
alternating; causes an alternating current to flow through primary coil of wire
what happens as current flows through primary coil of a transformer
magnetic field forms around coil; since current is alternating, magnetic field will also alternate and switch direction
what does the magnetic field around the primary coil of a transformer induce
an alternating magnetic field in the iron core (so iron core becomes induced magnet). this can be done bc iron is easily magnetised
when an alternating magnetic field is induced in the iron core of a transformer what happens
the iron core now produces its own alternating magnetic field which induces p.d. in the secondary coil and causes a current to flow
what is the purpose of a transformer
to transfer the current from the primary to secondary coil by inducing alternating magnetic currents
why can we not simply transfer current from primary to secondary coil electrically in a transformer
because the coils are insulated in plastic
as well as the iron being easily magnetised, what other purpose does it serve in a transformer
it increases the strength of the magnetic field
why do transformers only work with alternating current
because we need a changing magnetic field to induce p.d. in secondary coil - dc would produce constant magfield
what do transformers allow us to change
the size of the p.d. between the two coils
property of a step-up transformer
has more turns in secondary coil than in primary coil so increase potential difference
property of a step-down transformer
has more turns in primary coil than in secondary coil so decrease potential difference
why do transformers not have the same number of turns on the primary and secondary coils
this would mean p.d. across primary would = p.d. induced across secondary assuming transformer is 100% efficient which isn’t usually the case
what would happen if transformers were 100% efficient
electrical power output would equal electrical power input
what is potential difference in a transformer equal to and why
number of turns on coil; if we had twice number of turns in secondary coil than in primary coil, potential difference would double
transformer equation
p.d. across primary coil / p.d. across secondary coil
is equal to
number of turns on primary coil / number of turns on secondary coil
how is power conserved in transformers
power of primary coil = power of secondary coil
power transformer equation
(p.d. across secondary coil) x (current in secondary coil)
is equal to
(p.d. across primary coil) x (current in primary coil)
properties of a transformer
- power conserved
- p.d. and current vary between primary and secondary coil (if step-up or step-down transformer)
advantage of power transmission at high p.d.
due to resistance in wires, current heats wire meaning power is lost as heat which reduces efficiency of transformer. lower current = less power and energy loss in the cables which makes transfer of electrical energy through wires more efficient
