KS4 Magnetism and electromagnetism: Induced potential, transformers & the National Grid
In this section we learn how Michael Faraday reversed the motor effect to discover electromagnetic induction.
Soon after Michael Faraday had discovered the Motor Effect and developed the first electric motor, he began to think....
"if a magnetic field plus a current can produce a force/movement on a conductor,
could a magnetic field plus a force/movement on a conductor produce a current ?"
Another way to look at this is to view the 3 quantities as two "equations":
This led him to discover what we now often call the Generator Effect.
You could perform a simple experiment to see if the statemement is true; all you need is:
a Magnet to make the magnetic field,
a Coil of wire as the conductor (you will move it)
and an Ammeter to measure any induced current.
The Ammeter would show a current reading!
Isn't this amazing. Just by moving a conductor (coil) towards a magnet, we can generate an electrical current, hence we call this the Generator Effect and it is almost literally the opposite of the Motor Effect.
If we stop moving the conductor, the current stops flowing.
If we move the conductor (coil) in the reverse direction, the current reverses its direction. (So if you move the conductor (coil) left and right repeatedly you will generate an alternating current.)
The faster we move the conductor (coil) the larger is the induced current.
Two more interesting observations:
1. You don't have to move the conductor (coil); instead you can move the magnet towards or away from the conductor (coil) and you will still induce a current into the circuit.
2. Moving the magnet is effectively "changing the magnetic field" near to the conductor, so another way of generating the electicity is to somehow "change the magnetic field" near to the conductor; this can be done by having an electromagnet instead of the permanent bar magnet shown in the diagram; then you would vary the current in the electromagnet to "change" its magnetic field. (You might wonder why you would want to do it this way because it is not "generating" a current by using movement, but the technique is relevant to devices called Transformers which you will learn about later, below.)
Finally, if you replace the Ammeter for a Voltmeter then instead of reading an induced current when you move the conductor relative to the magnet (or change the magnetic field), you will read an Induced Potential Difference or Induced Potential which is what we call the voltage produced across the ends of the coil.
The Surprising Effect of the Induced Current
If we have the set up as in the first diagram such that a current is induced in the coil, then we encounter a suprising but easily explained effect.
As we know, if a current flows in a coil of wire (often called a solenoid, see previous section 4.7.3) then a magnetic field is produced around the coil, resembling that of a bar magnet.
So when the induced current appears in the coil, due to moving the coil relative to a magnet, immediately a magnetic field is produced around the coil as shown in the diagram below.
Furthermore, it is found that the direction of this magnetic field is such that it opposes the change in the magnetic field that is producing the induced current!
In the above diagram the coil is moved towards the North pole of the magnet, so the current is induced in such a direction that a North pole will appear at the end nearest to the North pole of the magnet to oppose, by repelling, the approaching North pole.
If the coil is moved away from the magnet then the induced current will reverse so that a South pole appears at the end nearest to the North pole of the magnet to oppose, by attracting, the receeding North pole.
It wasn't Michael Faraday who became well known for this part of the theory of electromagnetism but another Physicist called Emil Lenz who summed up this finding in what became known as "Lenz's Law" and it states : the current induced in the conductor (eg coil) generates a magnetic field that opposes the original change (the movement of the conductor relative to a magnet or of another magnetic field).
The factors affecting the size of the induced current or potential
We have already mentioned one of these which is:
1. The speed of the relative movement of the coil/magnet or of the change in another magnetic field. The greater the speed, the greater the size of the induced potential or the induced current (if there is a complete circuit such as that formed by using an ammeter).
The other 2 are:
2. The number of turns on the coil. The greater the number of turns, the greater the induced potential or current.
3. The strength of the magnet. The stronger the magnet, the greater the induced potential or current.
So, to make a really powerful generator you need:
A coil with as many turns of wire as possible
A strong magnet
And you need to move them relative to each other very quickly.
It will be obvious that the "use" of the generator effect is in generating electricity.
But the "uses", plural, is because we can generate AC (alternating current) electricity or DC (direct current) electricity. The practical differences between these two types of generator is quite small; the fundamental features are the same.
But first two points to clear up:
1. Linear movement OR rotation
The coil and magnet, used above to explain the basics of the generator effect, is a very simple AC generator; as the magnet goes towards/into the coil, the current induced in the coil will flow one way; as it is moved out of the coil the current induced in the coil will flow in the reverse direction.
However, moving the magnet (or the coil) linearly is not an easy movement to maintain; rotation is a much easier movement to maintain, so we want to use a rotation movement.
2. Induction occurs only when field lines are "cut"
If we look again at our basic generator and add the characteristic bar magnet field lines to the diagram:
It turns out that we only get induction if the movement of the coil (or the magnet) causes the wire conductor to pass through or "cut" through the field lines. And that is what you can see happening above when the coil is moved towards or away from the magnet.
I, however, we had a really tiny coil (or a huge magnet) and positioned the coil as shown below, then you can see that a small movement of the coil left or right would not "cut" the field lines so no induction would occur.
Idea - what if we rotated the above coil? Wouldn't it then "cut" the magnet's field lines?
Yes it would, so.....
we have just designed our first rotating AC generator!! Yay. How simple is that.
Generators like this do exist in the real world; the only difference is they tend to keep the coil still and let the magnet rotate. If we rotated the coil what do you think would happen?
The wires would twist and twist and twist...so we avoid this by simply letting the magnet do the rotating.
Simple generators like this used to be used on bicycles to provide electrical power for the bikes lights. The rear wheel of the bike would rotate the magnet, and the coil would be connected to bulbs at the front and back of the bike; lighting without the need for batteries. The disadvantage with such bike lighting systems was that the lights would go out whenever the bike stopped!
Anyway, the point of this section was to learn - induction only occurs when the relative movement of a magnet and a conductor (coil) is such that the magnet's field lines are "cut".
Generators - the fundamentals
Now that we know that we want a rotating coil/magnet we are going to go straight to the most common design for any Generator. This is as shown below.
A coil (here shown as just one turn of wire; in reality there would be many turns of wire) is arranged so that it can be rotated in between two fixed magnets, one a North Pole and one a South Pole. The magnetic field lines go from N to S (left to right in this diagram).
Ok, so in the position shown the "orange" side of the coil is presently cutting upwards through the field lines whilst the blue side is presently cutting downwards; make sure you agree.
Since both sides "cut" the field lines, both sides will have a current induced into them. But which way will the current flow?
To work out the direction of the induced current we use Fleming's RIGHT hand rule which is like his Left hand rule but uses the Right hand, and the thumb points in the direction of the Wire Motion rather than the Force Felt; the "Answer" is now the direction of the induced current and is given by the Second finger.
So his Right hand rule looks like:
If we apply this rule to the orange side of the coil (Left hand: Thumb Up, First finger points left to right) we find that "The Answer", the Second Finger, points inwards. So we can add a current arrow to the orange side as shown:
If we do this for the blue side (Left hand: Thumb Down, First finger point left to right) we find that "The Answer", the Second Finger, points outwards. So we can add a current arrow to the blue side as shown:
We have now determined that the induced current in this coil will always flow in the clockwise direction shown whenever it is rotated in the direction shown; if the direction of rotation changed then so would the direction of the induced current. However, there are 3 things to note:
1. Although the induced current will flow in one direction around the coil, if you look and think carefully you will notice that when the coil rotates such that the orange sides is on the right side and the blue side is on the left which will happen every half turn, the current through each side will reverse! Look at the following diagram.
The current induced in the blue side was flowing outwards, now its flowing inwards;
The current induced in the orange side was flowing inwards, now its flowing outwards.
So the induced current in this type of Generator is fundamentally an Alternating Current.
2. Current is induced, remember, whenever the coil "cuts" through the field lines which occurs for most of the rotation, but when the coil gets close to and passes over its "vertical" position, the coil will move along the field lines; it won't "cut" them. Look at the following diagram.
So as the coil approaches and passes through the vertical position the induced current falls to zero.
It does this every half turn.
The following simplified diagram summarises the above two important points and shows how the output of the fundamental generator varies as its coil is rotated clockwise. Basically, you can see that the output is Alternating; it is a Maximum when the coil is horizontal and it is Zero when the coil is vertical:
3. The third point: all of the diagrams of what I have called a "fundamental generator" are missing a very important detail - they do not show any means of connection to the coil. Simply connecting wires to either end of the coil would not be any good because they would twist and twist when the coil rotates.
So how do we make connection to the coil whilst avoiding this twisting?
There are two way, and it is the means of connection to the coil that determines whether the "fundamental generator" becomes a viable Alternator (A.C generator) or a Dynamo (D.C generator).
The Alternator (A.C Generator)
To turn the "fundamental generator" into an Alternator or A.C Generator we make connection to the two ends of the coil using what we call slip rings.
Slip rings are simply metal conducting rings; one is attached to one end of the coil and one is attached to the other end of the coil. In the diagram below they are shown coloured the same as the coil sides. When the coil rotates, they rotate.
Finally, we make connection to the rings using carbon "brushes" (carbon - because it's a conductor and quite "slippy"). The brushes are lightly held against each ring so they make an electrical connection to the ring whilst it is rotating.
So now we have a way of making an electrical connection to a constantly rotating coil; the carbon brush slip lightly over the rotating rings. We can now connect any circuit we choose to these brushes eg a simple bulb:
So long as someone or something rotates the generator coil the bulb will be lit; the bulb doesn't care whether the current alternates (as it will do here) or whether it is steady or Direct. But some things do care and need a steady or Direct Current, so let's look at a clever alternative connection method that will turn our "fundamental generator" into a D.C Generator otherwise known as a Dynamo.
NB The output of the A.C Generator is the same as that of our "fundamental generator:
The Dynamo (D.C Generator)
To turn the "fundamental generator" into an Dynamo or D.C Generator we make connection to the two ends of the coil using a device that we have used before !
When we investigated the electric motor (which used D.C, remember) we made connection to its rotating coil using a Split Ring Commutator. Well, we can use this same connection method with the Dynamo.
The diagram below shows the fundamental generator with its coil connected to the two halves of the Split Ring Commutator.
Notice that the orange side of the coil is permanently connected to the orange half ring whilst the blue side of the coil is permanently connected to the blue half ring.
To connect to each ring we use a Carbon Brush (as for the Alternator) ; this will allow the coil and rings to rotate whilst making an electrical connection.
Then we can add any circuit we choose - let's add an L.E.D which will only light if the current goes through it in one direction (this will test whether we have truly made a Dynamo and not just another type of Alternator).
Ok, we know from our previous discussion that with the coil in the position shown and moving clockwise in the direction shown, a current will be induced so it goes "inwards" along the orange side of the coil and "outwards" along the blue side of the coil ie clockwise. This current will flow via the split ring commutator and the brushes through the LED circuit, also clockwise which is the correct direction to light the LED. See the current arrows added below:
Now let's see the coil rotated 180 degrees so its orange and blue sides change place. We know from our work on the "fundamental generator" that the current will now flow "outwards" from the orange side and "inwards" through the blue side:
So you can see, because the Split Ring Commutator turns with the coil, the current continues to flow clockwise through the external LED circuit, so it keeps lighting!
We have made a viable D.C Generator; the output current always flows in one direction unlike in the A.C Generator where the output current reversed direction every time the coil turned 180 degrees.
What happens when the coil is in the vertical position?
As noted with the "fundamental generator" (and with the A.C Generator), when the coil is in this position it does NOT cut the field lines so no current is induced. This will also co-incide with the brushes meeting the gap between the rings of the Split Ring Commutator which is another reason whey there would be zero current at this coil orientation. See the diagram below:
The output of our D.C Generator or Dynamo as the coil rotates is as shown below:
Notice, its not a perfect direct current in that the current is not constant, but it doesn't "alternate" between one direction and the other, so it is still a Direct Current. In real D.C Generators a smoothing capacitor is placed across the output which has the effect of smoothing the output so it looks more like:
Increasing the Generator output
We have already discussed the factors to increase the size of the induced current, but to repeat briefly they are:
Increase the number of turns of wire on the coil.
Increase the strength of the magnetic field.
Increase the speed of rotation of the coil within the magnetic field.
Now its time for you to have a go at a few questions.
Just as the loudspeaker followed from the "Motor Effect", the moving coil microphone follows from the "Generator Effect".
A loudspeaker used the Motor Effect to enable the conversion of electrical signals into sound waves.
A microphone uses the Genrator Effect to do the reverse, to enable the conversion of sound waves into electrical signals.
A loudspeaker is likely to be the final output of a sound system, whilst a microphone is likely to be the initial input of a sound system (people speak or sing into it).
Here is how we make a microphone; you should be able to recognise all its parts:
We start with a magnet:
We add a coil that is free to move left and right over the pole piece of the magnet:
Now, if we can make this coil move left/right (in/out) whilst we speak, then according to the Generator Effect an induced potential will appear at the ends of the coil which will then become an induced current if we attach a circuit to the ends of the coil. So let's attach a flexible plastic or paper diaphragm to the coil, something like this:
And that's all there is to it!
We now have a moving coil microphone; the name makes sense doesn't it?
Let's recap this:
Pressure variations due to the sound wave cause the diaphragm to vibrate,
causing the coil to vibrate,
causing an alternating induced potential to appear across the ends of the coil due to the Generator Effect.
An alternating induced current will appear in any attached circuit mirroring the vibration of the diaphragm and of the initial sound wave.
The microphone has successfully turned the sound wave into an electrical signal.
Finally, I hope you have noticed that the moving coil microphone is remarkably similar to the loudspeaker.
They consist of the same 3 main parts, a magnet, a moving coil and some flexible cone/diaphragm. The only difference is that we speak into the microphone to make the coil move to generate an electrical signal whereas we feed an electrical signal into the loudspeaker coil to make it move, producing a sound wave.
The two devices are so similar that you can replace one for the other!
For example, most intercom systems only have one speaker/microphone. When you talk into it you use "it" as a microphone, and when you listen to a voice from it you use "it" as a loudspeaker; it's doing both jobs!
Similarly some phones will use an identical device for its "speaker" as for its "microphone".
Transformers are incredibly useful devices for changing (or transforming) one alternating potential difference to a larger or smaller alternating potential difference eg 10V to 20V (a "step-up transformer") or 10V to 5V (a "step-down transformer").
They are also incredibly simple devices because they consist of just:
One coil of insulated wire, (we call it the primary coil),
another coil of insulated wire, (we call it the secondary coil), and
an iron core on which to wrap the two coils.
This is what they look like:
What transformers do
A transformer will either Step-Up an alternating voltage from one size to a larger size.
Or, it will Step-Down an alternating voltage from one size to a smaller size.
We are very lucky that there is a simple relationship between the sizes of the coil p.d's (Vp and Vs) and the sizes of the numbers of turns on the coils (np and ns).
In words this is:
The ratio of the potential differences across the primary and secondary coils of a transformer (Vp and Vs) depends on the ratio of the number of turns on each coil (np and ns)
As an equation this looks like:
Although you should learn the equation as it is given above, you will most often use it when it is rearranged to find one of the voltages or one of the numbers of turns.
Let's have a look at a few examples.
Transformers and Power
Back in section 126.96.36.199 "Power", part of the "Electricity" topic, we learnt that the Power of a component was easily calculated; all we needed was to know the p.d across the component and the current flowing through it, then to use:
power = p.d x current
or, P = V x I
Well, this is equally true for our transformer, assuming its coils are connected to complete circuits.
The power input (primary coil) is given by Vp x Ip
whilst the power output (secondary coil) is given by Vs x Is
In an ideal, 100% efficient, transformer the output power will equal the input power. As an equation we can write:
(NB A "real" transformer is always less than 100% efficient, but you will not have to consider such cases.)
How do transformers work?
If you look again at the diagram of the transformer above you will notice that the two coils are separated from each other and they are made from insulated wire. So, how does a p.d appear across the secondary coil when a p.d is applied to the primary coil and yet there is no electrical connection between the coils? How do they work?
Transformers work and are able to do their amazing job as a result of 2 principles that we have been studying in the previous section and in this section.
These are: Electromagnetism and Induction.
If we apply an alternating p.d to the Primary Coil (the input of the Transformer), an alternating current will flow in the coil and therefore a magnetic field will appear around the coil and it will "alternate", meaning it will vary in strength from zero to a maximum then reverse direction and decrease in strength, etc. This is simple Electromagnetism.
The following diagram shows the alternating magnetic field around the Primary Coil; notice how it spreads over the Secondary Coil; also note, the magnetic field will more readily alternate, following the changing input p.d, if the core is made from iron because iron is a "soft" magnetic material.
According to the principle of Induction, if a conductor (such as a Coil of wire) is in the vicinity of a changing/alternating magnetic field then an Induced p.d will appear across the Coil.
This is precisely what happens in the transformer; the Secondary Coil is a conductor in the vicinity of an alternating magnetic field (spreading from the Primary Coil), so a p.d will be Induced across the Secondary Coil.
We also know that the size of the Induced p.d depends on the number of turns of the coil, so we can see that, generally, a Secondary Coil with a larger number of turns than the Primary will result in a larger Induced p.d (step-up) whilst a Secondary Coil with a smaller number of turns will result in a smaller Induced p.d (step-down). This is what we find and is in accordance with the transformer equation.
Transformers and the National Grid
The National Grid is the system by which we all, in the UK, get our electricity.
It consists of:
hundreds ofpower stations (of various types),
thousands of miles of cable,
thousands of pylons to hold the cables
plus...something else which we will discover shortly.
The following diagram is a VERY simple representation of one small piece of the National Grid system. It shows a power station, some pylons, copper wire and at the end, a user.
In the real world there would of course be many pylons between a power station and users, but the point to note is the distance between the power station and typical users; it can be significant which means that the resistance of the cables becomes significant.
The role of the resistance
If you recall, from the Electricity Section, 188.8.131.52, power is dissipated in any resistance according to the equation:
power lost = current 2 x resistance
P = I2 R
So, to limit the amount of power lost or dissipated in the cables (causing them to heat up) we need to keep R as small as possible.
This is why the cables are made from copper which is the material with the lowest resistance.
But there is another factor in the equation, the current! And this factor is squared meaning it has a bigger effect on the power lost.
The role of the current
Power is transmitted from the power station at a certain p.d (or voltage) and a certain current. The power output is given by the simple equation:
P = V x I
Let's say we had a power station that was designed to produce a p.d of about 230 V which is the value users need for their mains appliances to work.
And, let's say that we wanted to put out a power of 200,000 W.
To produce the desired power output at the chosen p.d we would need to use a current of:
I = P⁄V
I = 200,000⁄230
I = 870 A
Hm! That's a very high current and would cause a significant power loss in the cables. We would lose a large amount of the power station's power output heating the cables!
Is there a better way? YES.
Instead of the power station producing 230 V let's say it produced 10000 V.
Now, to deliver the desired power output of 200,000 W it only need to use a current of 20 A (you can do the calculation.)
This is a massively reduced current which will reduce the heating effect in the cables tremendously.
The only issue is - how do we get a power station that was designed to produce a p.d of 230 V to now produce a p.d of 10000 V ? And what does the consumer do at the user end? Apart from the fact that a 10000 V supply would be VERY dangerous, all of the users appliances are designed to work with 230V.
The role of the transformer
The transformer saves the day !
At the power station end, a Step-Up transformer is used to increase the p.d from 230 V to 10000 V and at the user end, a Step-Down transformer is used to decrease the p.d from 10000 back to 230 V.
So now you know the missing "thing" from the list of parts that make up the National Grid. Transformers are absolutely vital for the efficient transfer of electricity around the country.
You also know now why mains appliances use Alternating Current, A.C rather than the Direct Current, D.C that is supplied by batteries and used by such things as mobile phones.
It is because Transformers only work with A.C.
That shows you how vital Transformers are; the whole world chose to use A.C mains current purely because it was the type that allowed Transformers to be used.
Now its time for you to have a go at a few questions.