IGS KS4 Physics

P3 (Triple Science)

Electromagnetism Revision 3

TRANSFORMERS

Slide 1

Slides 28 What’s in a step-down transformer

Slides 915 Explaining how a transformer works

Slide 16 Why the coils must be made of insulating wire

Slide 17 Easy transformer calculations

Slides 1824 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

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

An alternating current flowing in the primary coil means that the coil produces magnetic field lines…

Transformers

Slide 9

…the iron core guides the magnetic field lines through the secondary coil in a big loop.

Transformers

Slide 10

Because the supply voltage is alternating, the current in the primary changes direction repeatedly…

…which makes the magnetic field lines change direction constantly…

Transformers

Slide 11

Because the supply voltage is alternating, the current in the primary changes direction repeatedly…

…which makes the magnetic field lines change direction constantly…

Transformers

Slide 12

Because the supply voltage is alternating, the current in the primary changes direction repeatedly…

…which makes the magnetic field lines change direction constantly…

Transformers

Slide 13

Because the supply voltage is alternating, the current in the primary changes direction repeatedly…

…which makes the magnetic field lines change direction constantly…

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.

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.

Half as many turns on the secondary coil, so the voltage will halve.

Vs = 10 V

(A STEP DOWN transformer)

4 times as many turns on the secondary coil, so the voltage will go up times 4.

Vs = 120 x 4 = 480 V

(A STEP UP transformer)

10 V

480 V

Slide 17

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 Ns

Slide 18

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”

Vp = Np

Vs Ns

Rearranging this formula is something only the higher tier students need do.

Slide 19

Multiply both sides by Vs

On the left, Vs top & bottom cancel.

Vp is now the subject of the formula.

Rearranging the Transformer Formula.

Making Vp the subject… (the “proper way”) Higher Tier ONLY

Slide 20

Rearranging the Transformer Formula.

Making Vp the subject… by “cross multiplication”:

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.

Higher Tier ONLY

Slide 21

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.

Move Ns to get Np on its own.

Higher Tier ONLY Higher Tier ONLY Higher Tier ONLY

Slide 22

Higher Tier ONLYSlide 23

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.

You may use the formula

Ns = Np Vs

Vp

Ns = 1000 × 11.5 230

Ns = 50 turns

Higher Tier ONLY

Slide 24

What do you put if you’re asked “With the aid of a diagram

describe the structure of a transformer”?

Slide 25

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)

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.

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