This is a topic in physics that has one of the biggest impact on our life today - the physics of how electricity can be generated.
In the previous article on Electromagnetism, we have seen how electricity can produce motion. This happens because when an electric current goes through a wire in a magnetic field, the magnetic field applies a force on the wire.
In this article, we shall see how the opposite can happen - when a conducting wire moves through a magnetic field, a voltage is induce in the wire! This effect has a name - it is called "electromagnetic induction".
To understand the basic idea and method, suppose there is a coil of wire next to the the pole of a magnet. Remember the idea of magnetic field and magnetic flux density from the previous article on electromagnetism?
Imagine magnetic field lines from the pole of a magnetic going through the coil. If the loop is bigger, the more field lines go through.
Or if we move the magnet further away, fewer field lines would go go through the coil.
This is what we can imagine in our mind or draw on paper. The magnetic flux density B described in the previous article is a quantity about how strong ths magnetic field is.
With this idea, we are ready to look at a simple example on electromagnetic induction. If we move the magnet so that the number of field lines going through the coil changes, then an emf is induced. This emf is like the emf of a battery. It has the same unit of volt, and the same effect in pushing a current round the coil. It can even light up a bulb and turn a motor, if we make it powerful enough.
Remember that the field lines are just a way to help us think. It is not like there are actually invisible lines. The magnetic field is spread out continuously, not just at each line.
There is a way to make the idea of "number of field lines going through the coil" more specific. Recall that B is called magnetic flux density. This "density" does not mean grams per cubic centimetre. It is just the idea that for a stronger magnetic field, we would draw the field lines closer together on a rough drawing.
Sounds really vague, right?
So, vaguely speaking, we may picture magnetic flux density B as the number of field lines going through per unit area A of a coil. Therefore, we can imagine that the total amount of field lines going through the coil as flux density times the area.
Like this
Φ is the Greek letter, phi. In this case, it represents the magnetic flux - the amount of field lines - going through the coil.
We need to learn how to tell from experiments the following behaviours about electromagnetic induction:
(i) If we push a magnet towards a coil of wire, this will produce an e.m.f. in the coil (if you have not learnt e.m.f. yet, just take it to mean voltage first).
(ii) This e.m.f. can give a current if it is in a closed loop. This current would produce a magnetic field that repels tje magnet. Or if we move the magnet away from the coil, the current would change direction and the force would pull at the coil.
(iii) If you push or pull faster, the e.m.f. produced would be bigger.
We are now ready to start on how exactly a magnetic field can generate electricity. Lets think of a magnetic pointing at a wire loop - from a short distance.
From the above description, we can imagine magnetic field lines spreading out from a pole of the magnetic, with some of it going through the coil. The "proper" way to say this in physics is : the magnetic flux linking the coil.
If we now move the magnet closer, the amount of "lines" - the magnetic flux linking the coil - increases. When this happens, a voltage appears that can push current round the loop.
One of the tricky thing in physics is to learn and get used to the new terms, like flux, linking, induce, etc.
The next thing we need to learn is how to find the direction of the current induced in the coil. Suppose that the north pole of the magnet is moving towards the coil
We know the induced current should produce a magnetic field also. Lets call this the induced magnetic field. In this example, this induced field must be in the opposite direction to the field from the magnet. Knowing this, we can then use the right hand thumb rule to find the direction of the induced current in the coilS
But why must the induced current's magnetic field be opposite to that from the approaching magnet?
Because otherwise, the coil would attract the magnet by itself, without me having to push the magnet. This would mean electrical energy is created from nothing, which is not possible because energy must be conserved.
So if I push a magnet towards a closed copper loop, it would induce a current in the loop the repels the magnet. If I now pull the magnet away from the loop, it would generate a current that attracts the magnet. So it would always do the opposite of what I want to do - to make sure I don't get something (electricity) for nothing (my effort) !
This effect is a name - it is called the Lenz’s law.
What if I use a longer wire to make 2 turns of the coil? The each turn would have an a voltage, so the final voltage woule be doubled. This effect is described using the phrase "magnetic flux linkage", which means the effective flux linking the coil.
So if there are 3 turns of the coil, the magnetic flux linkage could 3 times that of a single coil, and final voltage would be 3 times.
It looks like we should make many many turns so that we can get a lot of electricity. This is indeed what they do in generators of electricity.
EMF induced in a coil is directly proportional to the rate of change of magnetic flux linking with the coil.
In case you do not know, EMF means the actual voltage induced. It can be different from the value we measure using a voltmeter, if the coil is connected to say a light bulb which lights up. This is because the coil itself may have resistance that would take some voltage also.
Electromagnetic induction is used in various everyday applications. One common example is in electric generators, which convert mechanical energy into electrical energy by rotating a coil within a magnetic field.
Induction also powers transformers, which adjust voltage levels in power transmission to make electricity efficient and safe for use in homes and businesses. In addition, induction cooktops use electromagnetic fields to heat pots and pans directly, making cooking faster and more energy-efficient.
Electric guitars use electromagnetic induction in their pickups to convert string vibrations into electrical signals, which are then amplified. These applications showcase how electromagnetic induction is essential in modern technology.
You can learn these concepts and more at Dr Hock's maths and physics tuition.