1.6 Charging by Induction
So far, we have seen rubbing process as a mechanism to charge the objects. We have another way of charging the objects, which is called charging by induction. Therefore, we can charge the objects through rubbing process, and also through induction.
Let’s consider charging by induction, through an example. Assume that we have a metal ball, it can be a copper ball or aluminum ball, sitting on an insulating base. And it is originally, electrically than neutral. Let’s rub a glass strip with a silk cloth, and therefore, charge it positively, and approach it to this metal ball.
Now we know that, since metal, is a good conductor and it has abundance of free electrons, this positively charged glass rod is going to attract these negatively charged free electrons, causing them to move to the left-hand side surface of this spherical ball. And as they move to the left-hand side of this ball, they will leave the right-hand side surface lacking that much of negative charge. Therefore, causing that region to be charged positively.
Now, the net charge of the ball is still neutral because plus q plus minus q will give us 0. In other words, they will cancel one another. But simply by holding this charged glass rod, nearby, to this metal ball, we will cause a charge separation. In other words, charge polarization.
At this point, let us take a piece of wire, and connect the far-end surface to the ground. To represent the ground we will use this symbol. Ground is basically the largest charge sink which we can deposit all the excess charge in nature into the ground. And this position, the positive charges on the right-hand side surface of the spherical-conducting region, they’re continuously repelling one another. And they don’t want to be together. And as soon as they see this available path for them, they will immediately move along this path. And eventually they are going to get dumped into the ground.
Now, if we just go ahead on and cut the wire off, and remove the glass rod, and look at the charge distribution along the conducting ball, since some of the positive charges will be dumped into the ground, now the ball will have, or end up with, some excess amount of negative charge. This charge will repel one another and redistribute itself along the surface of this metal ball. And as a result of this process, we are going to end up with sum minus q prime of charge, through the surface of this metal ball.
If we just ignore the rubbing process of glass rod with the silk cloth to charge the rod, as far as the metal ball is concerned, there is no rubbing process is involved. And this is what we call charging by induction.
If we browse over the lightning phenomenon, which is one of the spectacular shows of electrical charging process, we observe both of these processes, both rubbing and charging by induction taking place. Before we look at the lightning phenomenon, if we consider an object which has, let’s say, pointed surfaces, something like this for example. And let’s assume we have a metal medium or conducting meeting of this shape. And then we just place some excess amount of charge inside of this region. Let’s say we put some excess amount of electrons.
Well, these electrons will repel one another immediately. And they are going to move to the surface of this medium. And of course, they are continuously repelling one another. They will redistribute themselves along the surface of this medium.
But one thing that we will realize, that the number of these electrons, or the charge density, will be higher along the regions that they have a smaller radius of curvature, like these ends. In comparing to the regions that they have a larger radius of curvature. So in other words, we are going to end up with more charges being collected along the pointed regions of the object.
So keeping this mind, let’s now look at the lightning phenomenon. We all know that during the thunderstorms, as the clouds move around, they rub to one another, dust particles, the water droplets, as a result of this rubbing process, they get charged. They can get charged either positively or negatively. If we consider, let’s say, a cloud which is charged, say, negatively, in these regions, because of the rubbing process as they move during the thunderstorm.
Now as the charge density throughout the cloud increases, it is going to attract equal amount of positive charge, which is available within the ground, remember the ground was the largest charge sink that we can deposit all the excess charge. Therefore, the earth in this region is going to get charged positively.
Now, we know that air is an insulating medium. Although there is an attractive force between this negatively charged cloud and positively charged ground. Because of the insulating properties of the medium, they are not going to immediately join to one another and get neutralized. But if we have some pointed objects over here, like a tower for example, or a tall tree, the charge density will reach to very high levels at the point of regions of these formations.
Now, once the charge density reaches to very, very high value, then the attractive force between these unlike charges, positive and negative, will increase greater and will reach to greater and greater values. And after some certain point, air is not going to be able to separate these two charges from one another. And this will result with a sudden discharge. And we will observe this discharge in the form of lightning.
As the lightning occurs basically, the energy suddenly discharged into this medium, causing the surrounding air to transform into plasma state, which we see this in the form of glow and the shockwave which is generated as a result of this sudden energy transfer, travels through the air and we hear that, eventually, in the form of thunder.
Of course, the lightning is a lot more complicated than this simple explanation. But it basically tells us, this example shows us, the charging mechanism, which is taking place during the lightning or during the thunderstorms as both rubbing, and as well as induction.