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magnetism & electromagnetism (transformers (change the size of the p.d…
magnetism & electromagnetism
transformers
change the size of the p.d. of an alternating current.
have 2 coils of wire, primary & secondary joined with an iron core.
when an alternating p.d. is applied across the primary coil, the iron core magnetises & demagnetises quickly, this changing magnetic field induces an alternating pd in the secondary coil.
if 2nd coil is part of complete circuit, it causes a current to be induced.
ratio between primary & secondary p.ds is same as ratio between no. of turns on both coils.
almost 100% efficient & if assumed so, input power = output power P=VI
VsIs = VpIp
step-up transformers increase the pd & have more turns on secondary coil.
step-down increase pd & have more turns on primary coil.
Vp / Vs = Np / Ns
step-up Vs > Vp (more turns on s)
low current means less energy is wasted through heating the wires & surroundings (lost as thermal energy) making national grid efficient in transmitting power.
generators & microphones
alternators generate alternating current:
generators rotate a coil in a magnetic field (or magnet in coil).
construction like a motor.
as coil or magnet spins, a current is induced in the coil - current changes direction every half turn.
ac generators have slip rings & brushes so the contacts don't swap every half turn.
they produce an alternating pd.
dynamos generate direct current:
work in same way as alternators but have a split-ring commutator which swaps the connection every half turn to keep the current flowing in the same direction.
oscilloscopes show how the pd generated in the coil changes over time - for dc - pd always +ve so stays above axis - ac goes -ve too.
height of line at given point is generated pd at that time.
increasing frequency of revolutions increases overall pd but creates more peaks.
microphones generate current from sound waves
sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet causing the coil of wire to move in the magnetic field, generates a current.
movement of coil (& so generated current) depends on properties of sound wave - louder sounds make diaphragm move further.
how microphones convert the pressure variations of a sound wave in current in an electrical circuit.
the generator effect
(electromagnetic induction)
the induction of a potential difference (& current if there's a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field.
creates a pd in a conductor & a current if conductor is part of a complete circuit.
do this by moving a magnet in a coil of wire or moving a conductor (wire) in a magnetic field - 'cutting' magnetic field lines.
shifting the magnet from side to side creates a 'blip' of current if the conductor is part of a complete circuit.
if conductor or magnet is moved in opposite direction, pd/current will be reversed --> if polarity of magnet is reversed, pd/current reversed too.
if magnet (or coil) is kept moving backwards & forwards, pd produced that keeps swapping directions --> alternating current.
get same effect by turning a magnet end to end in a coil or turning a coil inside a magnetic field - how generators work to produce ac or direct current:
as magnet turned, magnetic field through coil changes which induces a pd which can make a current through in the wire.
when magnet is turned through half a turn, direction of magnetic field through coil reverses so pd reverses so current flows in opposite direction around coil of wire.
if magnet is kept turning in same direction, pd will keep on reversing every half turn and alternating current produced.
a change in magnetic field can induce a current in a wire but when a current flows through a wire, a magnetic field is created around the wire - 2 magnetic fields.
magnetic field created by an induced current acts against the change that made it (movement of wire or change in field it's in)
induced current opposes change that made it.
to change size of induced pd, change rate that the magnetic field is changing at - induced pd (so induced current) can be increased by:
increasing speed of movement - cutting more magnetic field lines in a given time.
increasing strength of magnetic field (more field lines that can be cut).
permanent & induced magnets
all magnets have 2 poles - north + south & produce a magnetic field - a region where other magnets & magnetic materials (iron, steel, nickel, cobalt) experience a force - a non-contact force
magnetic field lines go from north to south & show which way a force would act on a north pole if it was put at that point in the field.
the closer together the lines are, the stronger the magnetic field.
the further away from a magnet, the weaker the field is.
magnetic field is strongest at the poles -- the magnetic forces are strongest at the poles.
the force between a magnet & a magnetic material is attractive, no matter the pole.
if two poles of a magnet are put near each other, they will exert a force on each other -- like poles repel & unlike poles attract.
inside a
compass
is a bar magnet - its North Pole is attracted to the South Pole of any other magnet it is near so the compass points in the direction of the magnetic field it is in.
when compasses aren't near a magnet, they always point north as the Earth generates its own magnetic field which shows that its core is magnetic.
permanent
magnets produce their own magnetic field.
the force between a permanent and induced magnet is always attractive.
induced
magnets are magnetic materials that turn into a magnet when they're put into a magnetic field.
when the magnetic field is taken away, induced magnets lose their magnetism and stop producing a magnetic field.
electromagnetism
when a current flows through a wire, a magnetic field is created around it. the field is made up of concentric circles perpendicular to the wire with the wire in the centre.
changing the
direction
of the current changes the direction of the magnetic field.
the
strength
of the magnetic field produced changes with the current and the distance from the wire - the larger the current or the closer to the wire, the stronger the field is.
the strength of the magnetic field that a wire produces can be increased by wrapping the wire into a coil - a
solenoid
.
the strength of the field increases as the field lines around each loop of wire line up with each other so lots of field lines are pointing in the same direction that are very close to each other.
the magnetic field inside a solenoid is strong and uniform (same strength and direction at every point in that region).
outside the coil, the magnetic field is like the one round a bar magnet.
the field strength can be increased further by putting a block of iron in the centre of the coil - this
iron core
becomes an induced magnet whenever current is flowing.
if the current is stopped, the magnetic field disappears.
a solenoid with an iron core is an electromagnet.
an electromagnet is a magnet whose
magnetic field can be turned on and off with an electric current
.
electromagnets used as they are quick to turn on & off or bc they can create a varying force.
electromagnets are used in some cranes to
attract and pick up
things made from magnetic materials e.g. in
scrap yards
- the electromagnet can be switched on to pick objects up & switched off to drop it.
electromagnets can be
used within other circuits to act as switches
: when the switch in circuit of electromagnet is closed, it turns on the electromagnet which attracts the iron contact on the rocker -- the rocker pivots and closes the contacts, completing circuit 2 + turning on the motor.
the motor effect
when a current-carrying wire (or other conductor) is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in causing the magnet & conductor to exert a force on each other
+ causes the wire to move.
to experience the full force, the wire has to be at 90º to the magnetic field - if the wire runs parallel to the magnetic field, it won't experience a force at all - at angles in between there will be some force.
the force always acts at right angles to the magnetic field of the magnets & the direction of the current in the wire.
the magnitude of the force increases with the strength of the magnetic field. the force also increases with the amount of current passing through the conductor.
F= BIl
force = magnetic flux density x current x length
(N) (T, tesla) (A). (m)
magnetic flux density - how many field (flux) lines there are in a region - shows the strength of the magnetic field.
used when the current is at 90º to the magnetic field.
electric motors & loudspeakers
basic dc motor
forces act on the two side arms of a coil of wire that's carrying a current - usual forces that act on a current in a magnetic field.
the coil is on a spindle & the forces act one up + one down so it rotates.
the
split-ring commutator
swaps the contacts every half turn to keep the motor rotating in the same direction. the direction of the motor can be reversed either by swapping polarity of the dc supply (reversing the current) or swapping the magnetic poles over (reversing the field)
loudspeakers & headphones (tiny loudspeakers) use electromagnets:
an alternating current (ac) is sent through a coil of wire attached to the base of a paper cone.
the coil surrounds one pole of a permanent magnet + is surrounded by the other so the current causes a force on the coil, causing the cone to move.
when the current reverses, the force acts in the opposite direction, causing the cone to move in the opposite direction too.
variations in the current make the cone vibrate which makes the air around the cone vibrate + creates variations in pressure that cause a sound wave.
the frequency of the sound wave is the same as the frequency of the ac so by controlling the frequency of the ac, the sound wave produced can be altered.
electric motors + loudspeakers
transformers
motor effect
permanent + induced magnets
electromagnetism
generator effect
