Transcript P3-EMAG3-Electromagnetism3

IGS KS4 Physics
P3 (Triple Science)
Electromagnetism Revision 3
TRANSFORMERS
Slides 28
What’s in a step-down transformer
Slides 915
Explaining how a transformer works
Slide 16
Why the coils must be made of insulating wire
Slide 17
Easy transformer calculations
Slides 1824
Using & rearranging the transformer formula
Slides 25 & 26
A diagram to memorise “The Construction of a Simple Transformer”
Slide 27
Transformers & dc supplies
Slides 28 & 29
The National Grid & Transformers
Slide 1
Transformers
Transformers step voltage up or down.
A mobile phone charger is basically a step-down
transformer. It takes 230 volts from the mains supply and
reduces it to 9 ish volts for the mobile phone battery.
Slide 2
Transformers
Iron core
Slide 3
Transformers
Slide 4
Transformers
Slide 5
Transformers
Slide 6
Transformers
Slide 7
Transformers
Slide 8
Transformers
An alternating current
flowing in the primary coil
means that the coil
produces magnetic field
lines…
Slide 9
Transformers
…the iron core guides
the magnetic field lines
through the secondary
coil in a big loop.
Slide 10
Transformers
Because the supply
voltage is alternating, the
current in the primary
changes direction
…which makes the
repeatedly…
magnetic field lines
change direction
constantly…
Slide 11
Transformers
Because the supply
voltage is alternating, the
current in the primary
changes direction
…which makes the
repeatedly…
magnetic field lines
change direction
constantly…
Slide 12
Transformers
Because the supply
voltage is alternating, the
current in the primary
changes direction
…which makes the
repeatedly…
magnetic field lines
change direction
constantly…
Slide 13
Having the field lines in the secondary coil swapping
direction all the time is like pushing a bar magnet in and out
of the secondary coil …this induces a voltage across the
secondary coil.
Because the supply
voltage is alternating, the
current in the primary
changes direction
…which makes the
repeatedly…
magnetic field lines
change direction
constantly…
Slide 14
So if you’re asked to explain how a voltage applied to the
primary coil of a transformer causes a voltage across the
secondary coil, you put…
An alternating current in the primary coil…
…produces a changing magnetic field in the iron core...
…and hence in the secondary coil.
This induces an alternating potential difference across the
ends of the secondary coil.
Slide 15
The wire of the coils has to be insulated, ie it has to have a
insulating layer wrapped around it.
If the wire wasn’t insulated, the electric current would be
“short circuited” ie it wouldn’t flow round the coil, but
from one wire across to another, or through the iron core.
Slide 16
Really transformers have many more turns on their coils
than the 4 & 8 in the example given so far.
Often the calculations might have the number of turns
being twice or 10 times as big on one coil as on another &
then you might be able to easily do such calculations in
your head.
10 V
Half as many turns on the
secondary coil, so the voltage
will halve.
Vs = 10 V
(A STEP DOWN transformer)
480 V
4 times as many turns on the
secondary coil, so the voltage
will go up times 4.
Vs = 120 x 4 = 480 V
Slide 17
(A STEP UP transformer)
Sometimes the numbers won’t be so easy & you’ll need the
transformer formula…
“p.d.” stands for “potential
difference”, which is another
word for “voltage”.
This formula in symbols would be:
Vp = Np
Vs
Slide 18
Ns
Vp = Np
Vs
Ns
Vp is “voltage across the primary”
Vs is “voltage across the secondary”
Np is “number of turns on the primary”
Ns is “number of turns on the secondary”
Rearranging this formula is something only the higher tier
students need do.
Slide 19
Rearranging the Transformer Formula.
Making Vp the subject… (the “proper way”)
Higher Tier ONLY
Multiply both sides by Vs
On the left, Vs top & bottom cancel.
Vp is now the subject of the formula.
Slide 20
Rearranging the Transformer Formula.
Making Vp the subject… by “cross multiplication”:
Higher Tier ONLY
All the terms (letters) can be in one of 4 “boxes”.
Terms can be moved DIAGONALLY.
To get Vp “on its own” Vs needs to move.
This is the same formula that was obtained by
“proper” rearranging on the previous slide.
Slide 21
Move Ns to get Np on its own.
Get the one you want (Ns) “on the
top row”
Get the one you want (Vs)
“on the top row”
Move the other terms,
leaving Vs on its own.
Move the other terms to get
Ns on its own.
Higher Tier ONLY
Slide 22
Higher Tier ONLY
Higher Tier ONLY
Slide 23
Higher Tier ONLY
Eg1.
A transformer is used to step down the voltage of mains supply,
230 V, to 11.5 V. The primary coil has 1000 turns. Calculate the number
of turns required on the secondary coil.
Higher Tier ONLY
You may use the formula
Ns = Np Vs
Vp
Ns = 1000 × 11.5
230
Ns = 50 turns
Slide 24
What do you put if you’re asked “With the aid of a diagram
describe the structure
Slide 25
of a transformer”?
What do you put if you’re asked “With the aid of a diagram
describe the structure
of a transformer”?
You might see from other sources that the core is “laminated
soft iron”.
You don’t need the “laminated” or “soft”.
The core is “laminated” into layers with insulation between
so that induced currents don’t flow around within the core,
heating it up & wasting energy.
The “soft” means that the iron can be easily magnetised one
way round & then demagnetised & re-magnetised the other
way round. If a “hard” material were used it’d waste energy
magnetising & demagnetising it.
Slide 26
Transformers & batteries (or any dc supplies)
ZERO volts
The diagram shows a 1.5 V cell connected to a voltmeter
through a step-up transformer.
Will the voltmeter show a voltage bigger than 1.5 V?
No. The CHANGING magnetic field produced by alternating
current in the primary is crucial to the way transformers
work. Because the cell gives a steady dc current, the
magnetic field in the coils will be constant, so no voltage will
be induced across the secondary coil. The voltmeter will
read ZERO volts.
Slide 27
The National Grid & The Use of Transformers
Within It - 1
The national grid is system that links power stations and
houses all over the country. Before the national grid, each
large town had its own power stations.
The advantages of having a national grid are that;
• If the local power station breaks down or can’t supply as
much power as is required then power can be supplied from
other power stations.
• Electricity can generated in places a long way from
customers.
• It allows for better balancing of supply & demand.
Slide 8
The National Grid & The Use of Transformers
Within It - 2
Using transformers allows us to reduce the power wasted in heating
the wires that carry electricity around the country.
Simply using high voltages in houses as well as on the power lines
would also reduce power wastage, but such high voltages would be too
dangerous to use in homes.
Transformers are used to step-up the voltage from power stations &
other transformers are used to step-down the voltage to make it safer
for use in homes.
Slide 29