KS4 Magnetism and electromagnetism: The Motor Effect
In this section we learn about electromagnetism and how understanding it led to the invention of the electric motor and the loudspeaker.
Until 1820 it was thought that Electricity and Magnetism were two completely different and unrelated phenomena. But then a Danish scientist called Hans Christian Oersted made a chance discovery.
Whilst working with electricity he happened to have a compass (a small bar magnet, you should know this) near to a wire that he had connected to a battery.
He noticed that when he connected the wire to the battery, the compass needle moved from its rest position of North.
Oersted had discovered that the current flowing in the wire had created a magnetic field strong enough to deflect the compass needle. He had accidentally discovered electromagnetism; that electricity and magnetism were inextricably interconnected. The world was about to change!
You might think that it is a bit dramatic to write "the world was about to change", but its not. Electromagnetism led to two major developments, the electric motor and the electric generator. Can you imagine our world without electric motors? They are used everywhere eg in computers/laptops to spin hard disks and cd/dvd drives, printers, plotters, cutters. They are used in escalators, lifts, conveyor belts. Without electric motors all trains would still be steam or diesel powered and the future for cars would still be petrol. In our kitchens we find electric motors in washing machines, tumble dryers, microwave ovens, hob extractor fans, fan ovens, food mixers etc. In workshops they are used in electric drills, saws, sanders and even screwdrivers. Also, every single gas fired central heating system relies on an electric motor to drive a pump to pump the water around the house/ building. And, of course without an electricity supply none of these would work and that is where the electric generator comes in; electric generators are used in almost all electricity power stations whether they are nuclear, wind or oil powered; the only type that do not use generators are the solar panels that we see on people's rooftops. It is not an over statement to say that our world would be a very different place if Oersted had not made his discovery.
From the result of Oersted's experiment we can write a simple definition of electromagnetism:
It is - the creation of a magnetic field (or magnetic effects) from an electric current.
Ok, back to 1820 and Oersted's laboratory - he was not content with his simple, chance discovery; he wanted to know more about what was happening when an electric current flowed through a wire.
He took a number of compasses and placed them around the current carrying wire; from above it would look like this:
From the side it would look like:
The compass needles were able to move/rotate freely.
So, what do you think happened when the switch was closed, allowing a current to flow through the vertical wire?
Oersted found that the compass needles arranged themselves like this if the current flowed downward through the wire:
And like this if the current flowed upward through the wire:
So, Oersted had discovered that the magnetic field around a current carrying wire was:
in the shape of a circle centred at the wire
and that the direction of the field (indicated by the direction of the compass needles) depended on the direction of the flow of current through the wire.
To work out the direction of the field we make use of a simple rule, known as the "right hand grip rule".
The idea is -
a) you imagine that your right hand grips the wire;
b) you point your thumb in the direction of the current;
c) you look at your fingers and the direction in which they are "wrapping" gives you the direction of the field.
The final thing that Oersted discovered about the magnetic field around a straight wire was, again, a simple thing.
That the strength of the magnetic field around the wire increased with the size of the current and decreased with distance from the wire (this last point is indicated on the above diagram by the fact that the field "lines" or circles get further apart with distance from the centre of the wire.
Once Oersted's new findings were published it did not take long for other physicists to push further the boundaries of this new knowledge of "electromagnetism".
Many wondered about the strength of the field produced when an electric current was passed through a wire and whether the strength of the field would be greater if more wire could be used such as in a coil of wire (a solenoid).
Electromagnetism & The Coil of Wire (or Solenoid)
A solenoid is just a coil of (insulated) wire.
Based on what we have just learnt about the basics of electromagnetism, it should not be a surprise that if we pass an electrical current through this coil of wire, we will find a magnetic field appearing around the wire.
But what is a surprise is the shape of the produced field.
Below is a diagram of a solenoid with a current flowing through it and you can see the shape of the magnetic field generated around it. Isn't it a familiar shape?
The shape of the magnetic field around the solenoid is not just "similar" to that around a bar magnet, it is the same as that around a bar magnet.
Looking at the diagram, there are 2 important observations:
1. The magnetic field inside the solenoid, like that inside a bar magnet, is uniform, meaning that the field strength is constant at all points; this is revealed to us by the fact that the field lines inside the coil are all (approx) straight and (almost) parallel at least towards the centre of the solenoid.
2. Like a bar magnet, the solenoid has N and S poles, but how do we know which end of the solenoid becomes N and which becomes S ?
The answer is, once again, by making use of a "right hand grip rule", but the meaning of the wrapped fingers and thumb are different this time.
Consider the following simplified diagram which shows a material inside the coil to make it easier to see the way the wire is wrapped.
What you do is:
a) Wrap your right hand fingers around the coil with your fingers following the direction of the current.
So, in the above diagram your right hand would go behind the coil with fingers upwards (because the current is going upwards at the back of the coil)
then you wrap your fingers towards yourself and down the front of the coil (because the current is going downwards at the front of the coil).
b) Look to see which way your thumb is pointing. It tells you the direction of the N pole. So, for the above diagram, if you do it correctly, your right thumb would point to the right, so the N pole would be at the right end of the solenoid which is what you can see on the previous diagram.
So, is there any point in wrapping wire to form a solenoid?
Uses of the electromagnetic solenoid
Without doubt the answer to the previous question is "yes". The most important reason for this answer is - the magnetic field inside and around a solenoid is much stronger at a certain distance from the solenoid compared to the strength of the field at a similar distance from a straight piece of wire.
Also the uniform nature of the field inside the solenoid makes them very suitable in applications such as electric door locks.
In the following diagram an iron bar (magnetic) is placed inside the solenoid but it is being pushed outwards by a spring such that it goes through a hole in a door, locking the door.
When the switch is closed, however, the magnetic field within the solenoid will pull the iron bar fully inside, even against the spring, such that the bar/bolt no longer goes through the hole in the door, hence the door is unlocked!
When the switch is released, the spring pushes the iron bar back into its original position and the door is locked once again. Clever, isn't it?
Electromagnetic door locks like this are used widely in hotels, office buildings and even houses are using them to replace conventional locks; the "switch", of course, is not just a push switch in these real life applications; it is key pad, key card or even phone controlled to make it secure.
Another, very common, use of the solenoid is as an electromagnet.
An electromagnet is simply a solenoid with a permanent iron core
The iron core has the effect of increasing the strength of the solenoid's magnetic field.
Electromagnets like the above have the advantage over ordinary permanent magnets in that the magnetism can be turned on or off at the flick of a switch; this makes them useful in applications such as picking up and dropping objects whether they be hugely heavy pieces of (magnetic) metal in a scrap yard, or small pieces of (magnetic) metal being lifted into position by a robot arm.
Within two year of Oersted's discovery of electromagnetism (so by 1822) the first really giant leap towards a new technology built on electromagnetism took place.
This was when the English physicist Michael Faraday produced the very first (and very primitive) demonstration of an electric motor or what would be better described as a "motor effect" since it was a far cry from a fully working, spinning motor.
His exact experimental setup is a bit archaic and complex to describe, but a simple example of the motor effect that you could see in any physics lab today is like the following.
In the above example a piece of copper wire is placed in a uniform magnetic field. When a current is allowed to flow through the wire, the wire feels a force and is seen to move!
The first time you see this it is quite amazing.
Now, notice that the wire is moving because of an interaction between the magnetic field and the electric current.
It is not moving because the wire is magnetic; it is copper wire, a none magnetic material.
The movement of the wire is perpendicular to that of the electric field and the current, so in the illustration above it would be a vertical movement.
When the current is switched off, the wire moves back to its original position.
When the current is reversed, the movement is in the opposite direction.OK, why does the wire in the magnetic field move when a current passes through it?
The Direction of the Force
In the diagram above you can see that the force arrow is pointing upwards. But how do we know if that is the correct direction for the force felt and for the resulting movement of the wire?
Well, we have John Ambrose Fleming to thank for coming up with a clever way to work out the "Answer" to the direction of the Force felt. His Left Hand Rule is very easy to use. It is shown in the diagram below.
As you can probably work out for yourself from the diagram, what you do to get "The Answer" to the direction of the force felt by the wire is:
1. Arrange your left hand fingers/thumb as shown in the diagram.
2. Point your first finger in the direction of the magnetic field (remember, this is always from a North magnetic pole to a South).
3. Point your second finger in the direction of the current as it flows through the wire or whatever conductor is being used.
4. When you have done all the above, then your thumb will give you "The Answer" by pointing in the direction of the force.
If we apply the Rule to the motor effect diagram above, then we can see that it does work:
The "Answer" is that the force felt, in this case, is upwards, which is what the original diagram showed.
Now that we have this useful Rule, we can make some interesting deductions:
1. If the direction of the magnetic field is reversed, whilst the direction of the current remains unchanged, the direction of the force felt reverses.
2. If the direction of the current is reversed, whilst the direction of the magnetic field remains unchanged, the direction of the force felt reverses.
The Size of the force felt
The size (strength) of the force felt by the conductor carrying the current when it is in the magnetic field is easily calculated using the following equation:
1. that the value L is the length of the conductor that is within the magnetic field; a piece of wire could be 1m long but if only 10mm of it is within the poles of the magnets then L is 10mm or 0.01m
2. that for this equation to be valid, the current carrying conductor must lie perpendicular to the magnetic field, as it is in the diagrams above.
Let's look at an example:
Let's look at a harder example where you have to rearrange the equation:
The principle use of the Motor Effect is in the Electric Motor, obviously!
Consider the following diagram which shows two pieces of conductor in one magnetic field; current flows in one direction through one conductor and in the reverse direction through the other.
You should be able to see, from using Fleming's left hand rule, that the leftmost piece of conductor will feel an upwards force whilst the other piece will feel a downward force.
So, if we join the conductors together into a coil of wire as shown below, can you see what would happen?
One side of the coil would go up and the other would go down, so the coil would begin to rotate around the dashed line.
This is the basic principle of the electric motor - a current carrying coil of wire placed within a magnetic field, as shown, will begin to rotate!
However there is a little more to designing a fully working electric motor.
If you consider the above diagram and imagine the coil rotating, it will rotate clockwise BUT once the left side is vertically above the right side the coil will stop moving because the 2 forces, up and down, are no longer suitable for maintaining the rotation.
So a clever little device is used to do two things:
1. It turns OFF the current before this "vertical" unmoving situation is reached but, importantly, during this time the coil will continue to rotate just a little bit due to its own momentum.
2. It reverses the current every half turn, after the "vertical" position, so that the force arrows will reverse on the two sides of the coil which will maintain the desired rotation.
The device is called a "Split ring commutator".
You can see the split ring device in the diagram above.
The + and - connections from a power supply brush lightly against the rings, maintaining a current flow but not stopping rotation.
You should be able to see that as the coil rotates (clockwise) eventually, when it reaches its vertical position, the blue + and - power supply connections will NOT contact the ring; they will meet the "split" part, so the current stops flowing but the coil will rotate a little further due to its own momentum. When the rings reconnect, due to the continuing rotation of the coil, the current will reverse direction through the coil and so the force directions will reverse; the coil will continue to rotate clockwise. We have a working electric motor!
NB Altering the size of the current alters the size of the forces produced - a larger current produces larger forces which will make the motor rotate faster.
Another, less obvious use of the Motor Effect, is in the Loudspeaker of which there are probably lots in your house/school eg in the TV, in the portable radio, in a smart speaker, in a laptop or PC, in a mobile or a landline phone and in headphones or earpieces.
Louspeakers can be very large such as those used at concerts in stadiums, or they can be tiny such as those used in earpieces.
What does a loudspeaker do? - It transfers energy from electrical to movement (kinetic) then to sound.
How does it do it?
If we think back to the electric motor - a steady current produced a steady force on the coil which resulted in a steady movement (with the help of that handy little device, the split ring commutator).
Well in a loudspeaker we have a current but it isn't steady, it alternates according to the "shape" of the music or voice, so there won't be a steady force or movement of the coil, instead the coil will move to and fro or up and down mimiking the "shape" of the alternating current which follows the "shape" of the voice/music.
Let's see a diagram of this in order to realise how the loudspeaker is much simpler than the electric motor.
We'll start with just the magnet. In a loudspeaker we use a circular magnet as shown below; you will see why in the next picture.
Then we wrap a simple coil of wire around the central pole piece of the magnet, as shown below.
As far as the motor effect is concerned that's all there is to it!
The coil will move in and out following the shape of the alternating current which is following the shape of the voice or music.
It fulfills all the "requirements" for the motor effect to occur, ie there is a conductor (the coil) in a magnetic field and there is a current flowing through the conductor.
But this isn't a loudspeaker.....yet!
The final step is to add the loudspeaker cone. This can be a paper or plastic cone shape, quite thin, which is attached to the coil, so that when the coil moves, the cone moves.
You can see this in the final diagram below.
The vibration of the cone pushes and pulls the air in front of it causing a pressure variation and hence a Sound Wave.