T11-Electromagnetic Induction Flashcards
A magnetic field is produced when a current moves through a wire .
-If we plce this wire in another magnetic field .
-The magnetic field around the wire now interas with the magnetic field between the magnes .
-NOw the wire increases a force , in his case force is in the upward direciton . #
-This causeses the wire to move upwads . Scientists clal this the MOTOR EFECT .
We calculate force using ?
Force=BIL
CHECK SHEET .
-This equation applies to a wire which is at right angles to the magnetic ifeld .
-Magnetic flux dnesity - measures the strength of the magnetic ifeld .
Factors effecting the size of the force
-Magnetic fllux density .
-Length of the conductor .
-Current
To work out the direction of a force
Flemings Left Hnd Rule .
Motor effect
-If the conductor is prrelel witht he magnetic field - IT WILL NOT EXPEREINCE A FOORCE .
-Needs to be ar right angles.
On sheet —>
The wire below a magneric field .
-If we move the wire up through the magnetic field , than a potential difference is induced across the energy of the wire .
-However , if the wire stops moving ,t hen the podential difference stops .
-If we move the wire BACK DOWN , thorugh the magnetic ield we get POTENTIAL DIFEFERENCE AGAIN .
-However the POTENITAL DIFFERENCE IS IN REVERSED IRECTION - scientists call this potnentioal difference the INDUCED POTENTIL .
–OIf we have a complete circuit ,we hace induced CURRENT , Scientists call this the generator .
KEYY - the DIRECTIONO F THE CURENT SWITCHES ,whn the DIRECTION OF MOVEMENT SWITCHES
-Mo ement stops - current stops .
-We also get an induced potential and also induced current if we keep the wire still , but MAKE THE MAGNETIC FIELD .
key - we ONLy see the generator effect ,if the wire passes through the magneti ield .
-If the wire moves ALONG , the magnetic field , we don;t get an induced p.d or current ,
The size of the induced p.d or current are LARGER IF …
-We use a stronger magnetic field .
-If we move the wire MORE RAPIDLY .
-If we shape the wire into acoil , (the greater the number of turns ont he coil , the greater the induced potential , and difference in current >) effect sme as before movement wise .
you switch the poles of the magnet
you are switching the poles of the masgnet.
When we move a magnet into a coil of wire , a current is induced int he wire .
-This induced current creates its own MAGNETIC FIELD .
-This magnetic ifeld OPPOSES THE MOVEMENT of the magnet .
WHEN WE INSERT THE NORTH POLE INTO THE COIL ,,,
That end of the coil ALSO BECOMES THE NORTH POLE .
-This REPELS the magnet , making it HARDER , TO PUSH THE MAGNET IN .
WHEN WE PULL THE NORTH POLE OUT
THE COIl becomes the SOUTH POLE , This ttracts the magnet making it harder toO PULL OUT .
*bECAUSE THE INDUCED CURRENT MAKES IT harder to move hte magnet 0 WE ARE DOING WORK - WE ARE TRANSFERRING THE ENERGY FRI TE MOVEMENT OF THE MAGNET TO THE MOVMEMENT OF THE CURRENT .
We an use the the generator effect to …
generate an alternating current .
what is an alternator
-an alternator is a cooil of wire rotating in the mgnetic field .
-The coil id vonnected to two metal rings called COMMUTATORS .
What are the two commutators
-The commutators allow the current to pass out of the coil . p.d is induced when the wire passes thourgh magnetic field .
-GRAPH ON SHEET SHOWS HOW P.D changes .
(need to describe and explian graph - have colour ccoded each sides of coil to make it easier .)
Yellow side of the wire is always connected to ring A .
-Gren side of the wirse is always connected to ring B .
…
What does the first star on ac graph show ?
-The maximum p.d is when the coil is horixontal .
-At this point , the wire is sweeping directly thorugh th emagnetic field liines t the FASTEST rate possible .
-The yellow isde is moving downwrs and th green side is movign upwards .
What does star two show on the ac grpah .
-When the coil is vertical ,t he potenital difference falls to zero .
-As the coil is now prallel to the field . At this point , the coil is not cutting through the magnetic field lines .
What does star three show on the ac grpah ?
-As the coil continues moving round , we get a p.d again ,hwoever in this case hte p.d has reversed again .
(As the two sides of the coil are now moving in a different direction to before . )
(Gren side is moving up and yellow side is moving down.)
What does star four show on the grpah ?
When the coil is vertical , it is moving parllel to the magnetici ield , so p.d = p .
What is the keyy thing about alternating current .
-As the two sied of the coila re atached to TWO DIFFERENT THIGNS , an alterntor produces an alternating potential differnece and lternating current . Scientists callt his AC .
One way to increase the strength of the magnetic field in ac ?
-If we increase the isde of the magnetic ifeld .
Another way to increase the strength of the magnetic field in ac ?
Can also increase the size of the alternatign current if we increse the number of turns in the coil , or if we increase the area of thie coil .
Final way to increase the strength of the magnetic field in ac ?
If we increase the rotation speed of the coil , then we have inceased the size and frequency of the alrentitng current .
A dynamo
-A dynamo produces direct current .
-Key feautre of dynamo is that it has a split-ring commutator .
-The split ring commutator has TWO SIDES spearated by a GAP .
-Call the sides A+B .
(side of the coil that is movign down is connected to PART a of the pslit ring commuaotr .
-Side of the coil moving up is connected to side B of the split ring commutator .)
What does star two on the dynamo graph show ?-
As the coil is cuttign thruogh the magnetic field lines ,a potnetial difference +current is induced.
WHat does star three on the dnmo graph show ?
-When the coil is verticl , p.d is zero as it is moving parallel to the magnetic ield so p.d = 0.
What doe star three show on the dynamo graph ?
-Now the coil moved around - the yellow isde is up and the green side is movingn down .
HOWEVER - the side of the ocil mvoign doen is still connected to PART A of the split ring commutator .
-The side movign up is sitl connected o prt B .
-BECAUSE OF THIS - the direcion of the p.d and the current DOES NOT REVERSE , when the coil roates , In other words there is a (DC) .
how the generator effect is used in a moving coil microphone
-Inside of a moving coil microphone is simialr to a moving coil loud speaker.
part one of the microhpone
coiol of wire is attached to a thin sheet of plastic .
-Scientists call this thin sheet of plastic diaphragm .
-The end o the coil o wire sits oevr a permanent magnet ..
P2 of a microphone
When soundwaves hit the diaphragm , they cause it to vibrate .
-Now the coil of the wire moves in and out through the magnetic field .
-This induces a potential difference across the ends of the wire .
microphone graph
-The graph of the potential difference of the microphone , shows that ithe p.d switches directiona s the coil moves fowards and backwards thourh the magnetic field .
-The fequency f the changing p.d is the same as the frequency of the osundaves .
The changing patter of p.d is now passed thorugh an amplifier and then into moving coil - loudpseaker . Massively increasing the volume of the osund .
Remind yourself of the motor effect 0 the mtoor effect is also used in a loudpspeaker and headphones
on sheet , shows a moving coil aapeaker and head also found a simialr set up in headphones .
star one of the loudspeaker
-we have a cone which ahs a coikl of wire wrapped aorund one end
star two of the loudspeaker
-the coil of wire is conected to an alternating current electricl supply .
str three of the lodspeaker
we also have a permanent magnet which goes inside the coil of wire.
Stage one of loudpeaker
as the current passes thorugh the coil , it generates a magentic ifeld .
stage two of loudspeakert
the magnetic field fromt he coil , now interacts witht he magnetic field rom the permanent magent .
-AKA these magnetic fields either attract or repel eachother .
stage three of loudspeakers
this produces a resultant force , which causes hte cone to mvoe .
stage four
when the current switched direction , the direction of the force on the cone is reversed .
-This causes the cone to move in and out generating osundaves .
By changing the frequency of the AC supply , we can chnage the frequency that the cone vibrates.
A HIGHER frequency will porduce a HIGHER PTICH sound .
-Lower frequency will produce a LOWER PITCH SOUND .
if we increase he siize of the current
then we increase the AMPLITUDE of the vibrtion .
-Increasing then the volume of the osund
A transfromer consists o two coils
-ont he elft we have priamry coil check sheet , on the right we have secondary coil .
-these coils are COMPELTELY SEPEARE .
-Thee is NO WAY one electric current can pass directlyf rom one coil to another .
-these coils are wrapped aound an IRON CORE , we use iron as i is EASILY MAGNETISED .
A transfromer consists o two coils
-ont he elft we have priamry coil check sheet , on the right we have secondary coil .
-these coils are COMPELTELY SEPEARE .
-Thee is NO WAY one electric current can pass directlyf rom one coil to another .
-these coils are wrapped aound an IRON CORE , we use iron as i is EASILY MAGNETISED .
sTAGE ONE OF TRansformer
-the primary coil is connected to an alternating current .
tage two of transformer
-as the current flows through primary coil , it creates a chanig magnetic field .
stage three of a transformer
-this magnwtic field is transmitted along the iron core and passes through the secondary coil .
stage four of a transformer
when the changing magnetic ifeld passes through the secondary coil , it induced a potential difference .
KEY - the iron core INCREASES THE STRENGTH
OF THE MAGNETIC FIELD .
One KEY thing about transformers
They only work with ALTERNATING CURRENT (AC) .
-As we need a changug magnetic field to increase a potential difference .
A DIRECT CURRENT pordcued a consistnetn MAGNETIC FIELD and therefore does work in transformers .
…
If a transformer if the priamry and secondary coils both have the SAME NUMBER of turns
-It means the potneital difference int he secondaryc oil is the same as the potential difference of the praryc oil .
-however this is hte onlt case if the transforemers had 100% ENEGRY efficency 0 no energy is wased in the transformer .
However , in practice - transfromers are not 100%
-usually more turns in secondaryc oil than in primry coil
Menaing there is a greater p.d induced in the secondaru coil , thn the p.d of the primaryc oil . This sthype of trnsfoerm is STEP UP transfomrer 0 as it STEPS UP THE potential difference )
if you have twice the number of coils in secondary coil than primry coil , so the potnentl difference int he seocndary coil is x2 the potenital difference of priamry coil .
…
What about the coils in a STEP DOWN TRANSFORMER
-A STEP DOWN transformer , ahs fewer coils in the secondary coils compared to the primary coils .
-ins tep down transfoemr the p.d in the secondary coil is LESS than the p.d in the primary coil ( inthis exmpale secodnary coil has hald the number of coils as the priamry coil , so the p.d is hlf the priamry coils .)
If we know the p.d in the primary coil and the number of coils in the primary and secondary coils . We can calculate the p.d int he secondary coil .
check sheet for equationa nd do questio .
how can we tell fromt he answer thart it is a step down transformer .
as 192V has stepped down to 45
One very important idea in transformers is power must be conserved .
power of primary coil = power of secondary coil .
Check sheet for equation to calcualtre power which is by multiplying p.d and current .
…
-Only true if the appliacne is 100 efficent .
Do question
…
How transformers are used in power transmission s1
-Electrical power is transmitted from powersations to homes through HIGHVOLTAGE CABLES .
power=current*p.d
-So if we want to transmit a large amount of POWER we can either use a LARGE current or a LARGE voltge .
Problem with power transmission
Power is WASTED in the transmission cables as heat .
-The amount of power wasted as heat depends of the SQAURE OF THE CURRENT .
-If we use a LARGE current in the transmission cables ; a huge amount of power would be wasted as heat .
SOLUTION for power transmission
instead of using a large current we use a large p.d
How transformers are used in power transmission s2
The electricity from the powerstation is first passed thrugh a step up transfroemer .
-This steps up the p.d to around 400,000 V.
-The electrical power is then transmitted down HIGH VOLTAGE CABLES .
How transformers are used in power transmission s3
It is then passed thorugh step-down transformers .
-The step down transformers reduce the p.d to around 230
Summary
By using transformers , we massively reduce power wastage in the national grid .
