Conservation of electric charge

0:00- [Instructor] What's common between energy, linear momentum, and angular momentum? Well, they're all conserved quantities, right? But what about electric charge? Is that a conserved quantity as well? Yes, it is. So in this video, let's talk about conservation of charge.

0:19Imagine you take a fur and rub it with a balloon. What's going to happen? Well, both the fur and balloon have equal amount of positive and negative charges to begin with. As a result, they're both electrically neutral. But when you rub fur with balloon, some of the electrons will get transferred from the fur into the balloon.

0:44Now, the balloon has some excess negative charge. It has a net negative charge, and the fur has lost some electrons. It has lost some negative charges, so it has an excess positive charge. So it has a net positive charge. This effect is called the triboelectric effect.

1:05The word tribo means to rub. And so we're getting electricity by rubbing. That's the basic idea over here. But anyways, now let's consider this balloon and the fur as part of our system. Then all other objects and everything else will be the surrounding. In this particular example, notice the interaction was only happening within the system.

1:26The charge transfer only happened within the system. And so the total charge within the system stays the same. It is constant. However, if there was some charge transfer that happened between the system and the surrounding, then the total charge of our system will not be constant because some of the charges would have leaked into the surrounding or some excess charges would have come in from the surrounding.

1:53This is what we mean by consideration of charge. It basically says if no charge is flowing in or out of our system, then the net charger, the total charge of that system will not change. Anyways, what makes this experiment interesting is that both the fur and the balloon are insulators, meaning the charges inside them cannot move easily.

2:20What does that mean over here? Well, notice that these excess negative charges, they're all getting repelled from each other, and so they tend to move away from each other. But because this is an insulator and charges cannot move easily, they won't move. And so they're pretty much stuck over there.

2:38So these charges will not spread out. And the same thing's happening over here. The fur is an insulator. And so even though the positive charge over here, some excess positive charge is going to attract the negative charges, the charges can't move much. And so the positive charge pretty much stays over here.

2:55So this side pretty much stays positive. This means this side of the balloon gets attracted to this side of the fur, and we can see that attraction. Here is the example. Instead of fur, I'm using my wife's hair. So we rub the balloon on my wife's hair, and just like before, electrons get transferred from the hair to the balloon, triboelectric effect.

3:20You may be wondering why electrons get transferred from here to here and not the other way around, but that has something to do with the material and stuff. We'll not worry too much about which material loses electrons and which materials tends to gain electrons.

3:31What's important is one of them loses electron, another one gains electron. So over here, the balloon gets negative charge, my wife's hair gets positive charged, but those charges don't move. They're pretty much stuck over there, right? Which means this side of the balloon is gonna get attracted to that hair that was in contact.

3:48And we can see that. You can see the attraction. That's pretty cool, huh? Okay, here's what's even cooler. If I bring this balloon close to tiny bits of paper that I have on the table, look, the bits of paper jump up, get attracted to the balloon. But why? The bits of paper are not charged.

4:07The balloon is charged, but the bits of paper are not charged. So why do they get attracted to the balloon? Well, let's see. So here are the bits of paper. Let's first think about the charges inside the paper before we bring the balloon close to it. This paper is electrically neutral.

4:24How do we model the charges inside the atoms of this paper? Well, we could pretty much model it this way. We could say that, hey, the positive nucleus is at the center, and let's say the negative charge is equally spread out in all directions. So we can model the average position to be at the center as well.

4:42Now watch what happens when I bring that balloon very close to it. The excess negative charge is going to pull the positive charge towards it and it's going to push the negative charge away from it. Now this is an insulator. Charges cannot move easily, but the atoms can get stretched and they do.

5:04We call this electric polarization. We say the atoms are electrically polarized because the two poles, the positive negative out are separated. Now what's the effect of this? Well, the positive charge is gonna get attracted by this excess negative charge on the balloon.

5:21And of course, the negative charge is going to get repelled by the excess negative charge of the balloon, but because the negative charges are slightly farther away compared to the positive charges, notice the repulsion is slightly weaker compared to the attraction.

5:36And so the net effect is that the attraction wins and that causes the bits of paper to get attracted and jump up from the table and get clinged to the balloon. This is how a charged object can attract a net neutral object due to electric polarization. So insulators are materials in which charges cannot move very easily.

5:59In contrast, conductors are materials in which charges can move easily. Metals, for example, are great conductors of electricity, and so we can do some fun experiment. So in this experiment, he's gonna take a rubber rod and rub it with fur, and then he will touch the tip over here.

6:18What's important is that this tip is metallic and it's connected to this thin metallic rod that you can see over here. And there's a needle that is also metallic. So the whole thing is, you can think of it as like a single metallic thing, it's just that the needle is free to rotate.

6:38So watch what happens when he does that. So he's rubbing the rod, he's touching it now, and look, the needle got deflected and it stays deflected. Look, it stays deflected. So what's going on? So when he rubbed this rubber rod with the fur, some electrons got transferred from the fur to the rod, just like before, triboelectric effect, right?

7:02So let's look at that one more time. Electrons are getting transferred from the fur to that rod, and so there's some excess negative charge just like before. But now what happens when he touches that rubber rod to the tip over here? Remember, this metallic tip is connected to this metallic thin rod, which is also in contact with this metallic needle.

7:24Of course, you can also probably see this metallic rim over here, but that's not connected to this, so don't worry about that. Anyway, so what's going to happen when he touches the rubber rod to the tip? When he touches it, some charges from here will get transferred to that metal.

7:40But because in conductor's charges can easily move, these electrons, these excess electrons don't just stay there. Due to mutual repulsion, they will spread out throughout the conductor. That is something that cannot happen in an insulator because charges cannot move that easily, but here they can.

8:01And now because the whole thing is negatively charged, this part of the needle is gonna get repelled by this part. And similarly, this part of the negative needle, negatively charged needle, is gonna get repelled by this negative charge that will produce a torque and that deflects the needle.

8:20And then what happens when he removes that rod? Well, nothing else happens. The charges stay put, the excess charge stays put, so the torque stays put and as a result, the deflection stays put. So the needle stays deflected. Pretty cool, right? So if you touch this device with an object and that needle deflects, you know that object was charged.

8:43So this device can be used to detect whether an object is charged, and so this is called an electroscope. Okay, but now here's a question. What do you think is gonna happen if we were to touch the metallic tip? Why don't you pause the video and think about this?

9:00Okay. Well, what's important to know is that we, the human body, is a good conductor. And so the charges now, just like before, can transfer from here to our bodies, but our body is a much bigger conductor compared to this one, right? And so it can hold a lot more excess charge and therefore a lot of charges will get transferred.

9:22In fact, almost all the charges will get transferred to our bodies. And if our body's in contact with the earth, maybe we are standing barefoot, then the charges will go all the way to the earth because earth is an even bigger conductor than our body. This is the basic idea behind grounding.

9:38If you connect that object to the ground, then any excess charge on that object will automatically flow all the way to the ground and eventually there's no excess charge over here. Okay. Anyways, because now there is no more excess charge, the whole thing is now neutral, the torque disappears and the needle will rotate back and that's exactly what we see.

9:58Okay, I wanna show you one more cool experiment. We're gonna rub the rod just like before, but now notice, he's not gonna touch it. And still the needle deflects! No touching! Still the needle deflected. Why? Please pause again and think about what's going on.

10:16Let's see. Just like before, the rod has excess negative charge, right? But what happens to when I bring it close over here? Well, this whole thing is neutral, but remember, it does have charges. It has protons and it has electrons, but they're equal in number, that's why it's neutral.

10:34So we could say that it has equal amount of positive and negative charges to begin with. And now when I bring this very close, what do you think is gonna happen to these electrons? They're all gonna get repelled, right? And because it's a conductor, charges can easily move, all the electrons will try to move as far away from this negatively charged rubber rod as possible.

10:58And so they will all tend to move. I mean, they can't move here, they can't move this way. They have to move within the conductor, so they'll move as far as possible. They'll all get bunched up somewhere over here at the bottom. So because of the charge separation, we can see the top part of the needle is positive and the thin rod is also positive.

11:16The top part is also positive. And so they will repel. And the bottom part of the needle and the rod is negative. So again, they will repel, which means again, there will be a torque on this needle and that torque makes it deflect. Isn't this awesome? The whole thing is still electrically neutral, but charges got separated.

11:38This idea is called electrostatic induction. The whole idea is if you bring a charge object close to a conductor, even if that conductor is neutral, it has equal amount of positive negative charges, those charges can separate. That is what we call electrostatic induction.

11:58Okay, final question. What do you think is gonna happen to this deflection if we, once we take that rod away? Well, once we take that rod away, there's nothing pushing those electrons towards this end. So because of the mutual repulsion, they will all come back.

12:14Electrostatic induction disappears and we would expect the torque to disappear, and that's exactly what happens. The needle comes back. So long story short, if there's no charge flowing in or out of our system, then the net charge of our system will not change.

12:31It's very similar to conservation of energy or momentum. In insulators, charges cannot move easily. So the excess charge that you gain will pretty much stay put. They cannot spread out. Whereas in conductors, charges can move easily. So if you provide a conductor with some excess charge by just touching it, that excess charge will spread out throughout the conductor.

12:58And if you bring a charged object close to a conductor, even if it is neutral, the charges can separate. This idea is called electrostatic induction. As a result, one side of that conductor will be positive and the other side of that conductor will be negative.