10.3 Conduction and Induction

Ok, today we're going to look at how we can create charge on an object and also how to move charge from an object through induction or conduction. So we have three different methods. The first one is friction, which is how we can actually create a charge. Friction basically means when two objects rub against each other. So what we're going to do is rub two objects together, and what will happen is the electrons from the weaker substance, whatever has weaker bonds, will move to the substance that has stronger bonds. We'll use balloons as an example. When we rub a rubber balloon on our hair, what happens is our hair gives up electrons to the rubber balloon, giving the balloon a negative charge and leaving our hair with a positive charge. There are two common examples that are often used in physics and you need to know how they work because they show up on diplomas. The first one is if we rub a chunk of ebonite, which is just hard black plastic rubber-like material, with some sort of hair or fur. What happens is the electrons move, just like with the balloon and our hair. The hair particles or the fur particles give up electrons, and those electrons go to the ebonite. So what happens is the fur is left positive because it has lost electrons, and the ebonite will be negative because it has gained electrons. If we do the same experiment with a glass rod and a piece of silk, what happens is the opposite. The glass ends up giving up electrons to the silk. So in this case, the glass would be left positive and the silk would be left negative. So you need to remember that ebonite will typically be a negatively charged rod when we rub it, and a glass rod will typically be a positive charge when we rub it. A good way to remember and compare these is by using this chart: 1. 2. 3. 4. Copper Ebonite Glass Silk Anything higher on the list will gain electrons, and anything lower on the list will lose electrons, so it will be positive. So the objects at the bottom of the list will be positive, and the objects at the top will be negative. By rubbing any two of these objects together, you can determine which one will be positive and which one will be negative. Now let's look at this chart to figure out what would happen if we rub copper with silk. You can see that copper is above silk, so copper would be the negatively charged one and silk would be left positive. This gives you an idea of how you can use friction to create a charge separation on an object and leave it as positive or negative. Moving on to charge by conduction, think of conduction as being in contact. We're talking about touching objects together to transfer charge from one to the other. We discussed this in class when we talked about things sparking. For example, when you rub your feet on the carpet and then get close to a doorknob, you'll see a little arc or spark, and you'll feel a shock. We consider that conduction, even though you technically didn't touch it. If the arc can jump the Small distance, we still consider that the same, the transfer of electrons from one object to the other. So let's go through a scenario to see how it actually works. Let's suppose we have a negatively charged rod, for example, a chunk of ebonite. It will be negatively charged. We're going to move it close to something that's neutrally charged. In this case, without a rod, a round metal sphere that's uncharged. What happens is as we get closer, the negatively charged rod pushes the electrons away from the neutrally charged object, creating a charge separation. Once we touch them, the charges want to balance out. So think of it as a simple math problem. Originally, our first object had a charge of negative six, and our other object was neutral, so it had a charge of zero. Once we touch them, three of the electrons move over, resulting in the charges being negative three for both the rod and the object. When we move them away, the charges stay as negative three and negative three. Conduction basically tries to balance out the differences in charges. If we start with a negative item and touch it to a neutral object, they both become negative. Similarly, if we started with a positive rod, the positive charge doesn't move to the other object. Instead, electrons from the other object move to balance out the charges. Now, let's consider another scenario. If we had a positively charged rod with a charge of positive four and we touch it to a neutral object, once they touch, we'll be left with a positive two charge on each. Again, it's not the positives that move, but rather the electrons that move in the opposite direction to balance out the charges. So, starting with a positive charge, we take away electrons from the neutral object. Now, let's look at the situation where our original charge is negative seven and we touch it to a neutral object. In this case, mathematically, we might think that each object would have a charge of negative three and a half after they touch. However, since we can't have half an electron, the charges will be balanced in the best way possible. So, we'd likely end up with a negative four charge on one object and a negative three charge on the other. Three electrons would move to try to balance out the charges, leaving one object with an extra electron. In summary, the transfer of electrons through conduction involves balancing out the charges between objects. The only thing that moves is the electrons, not the protons. Neutral material was an insulator, so let's go back to the same diagram. So if we had our negative 7 and we touch it against the rubber ball instead of a metal ball, if that rubber is an insulator, what's gonna happen? Nothing. We touch them together and move it away. It's probably gonna still stay at negative seven. Zero, okay? So, conduction is pretty easy. Just basically remember you're gonna balance the charges from before to after. Remember our conservation of charge still exists. Total is still gonna be the same, they're just gonna get shifted from one object to the other. Our last one we're going to look at quickly is induction. And this one is pretty cool. What it is, is we can actually charge something without actually physically doing it by conduction. The only thing is, to do induction, you have to have a ground. So what that means is we've got a ground wire attached to the object and what that's going to do is it's going to give us an unlimited supply of electrons to either give or to take. So if we need electrons, it'll take some from the ground. If we have too many, it'll push them down to the ground. And that's what a ground wire does. It just sort of tries to balance things up to make sure things don't get charged up. So the first thing is we're going to do the exact same sort of scenario. We got a metal ball again, but now we're going to attach it to the ground. So you can see in the diagram, it just shows it by a little wire that's touched. And usually, to show that it's a ground wire, we just do those two lines at the bottom. And that indicates then it's a ground. Okay, so we've got our grounded object. So once again, we're going to bring a negative six charged object towards it. So our rod that's negatively charged, we're going to leave it closed. So when we did conduction before, it touches. What happened with all the negatives? Would we basically try to move away, right? So if we have negatives in the rod, our negatives would get as far away as possible, leaving the side closest to it just positive. Yeah, but if they can't go anywhere, that's all you would do. When you move the negative away, it would balance things out again. But in this case, because it's connected to the ground, those negatives, those extra negative charges that are trying to run away, are actually going to go down the wire. So we're going to chase them down the wire. So then, basically, however many there are, they're gonna be moving down the wire. They're gone. So then, all we have to do, without touching it, right? We're just getting close. That'll push the negatives away. We're gonna cut the ground wire. So we're gonna make a break in the ground, where it's either just disconnected or physically cut it, whatever the case is. So if we stop the ground, that means all those electrons that were chasing it on the ground wire can't get back. So now, when we pull our rod away, we've lost those electrons. You can see here, in this case, some of the electrons have gone down into the ground. But once we cut the wire, those are stuck there. They can't get back up. So our sphere has a lack of electrons, so that means it'll be positively charged. So one thing to remember with conduction is if we bring a negatively charged object to the neutral and we touch, they're both going to be negative. If we bring a positively charged rod and we touch it, they're both going to be positive. But what induction works is, when we bring a negatively charged one close, that chases away the electrons. So once we break that, it's going to be left over as positive. And the reverse would happen if we brought a positively charged rod close. It'll take electrons up the ground wire this time. So then, when we move the positively, we have an excess of negative, so we'd be left with a negative charge. Induction is pretty cool, and I'll try to demonstrate it if I can find the equipment when we're in class. The last thing we're gonna look at is an electroscope. It's a simple, easy design that was invented many years ago. And what they did was basically... Used to help detect a small amount of charge because one electron doesn't have very much charge, it is pretty tough to detect them. You can't feel it, you can't see it, so you have to have a way to actually show that it's there. An electroscope is just a simple thing. We have this metal rod and it goes inside of a box, usually a round object, at the window. The idea is that it prevents any kind of wind or any excess outside energy from affecting it. It's insulated, so there's a little rubber stopper that stops the charge of the metal container from affecting anything. At the very bottom of it, there are two leaves. Quite often, they use gold because gold is a good conductor and they are really light and thin. What happens is, if we bring something close to the top, let's suppose we bring a negatively charged object towards the top, what's gonna happen is that it will detect if there's a charge or not by those leaves opening up. Normally, they just hang straight down, and as you bring something close to it, the charge will be detected, and you will see the bottom leaves repel each other, causing them to open up. That's like the bottom diagrams here, to sort of look at the different scenarios. If we bring a negatively charged object close to it, that's gonna chase the electrons away. So, the electrons will go down to the bottom to the leaves, and because they're both negative on both sides, they're gonna repel, so the leaves would open up. And if we brought a positively charged rod, the same thing would happen. We'd bring it close, and in this case, it's gonna attract all the electrons to the top. So, that would leave the leaves at the bottom positive as well, so those would also repel. So, an electroscope is good to show the charge, whether it's there or not, but we can't tell whether it's positive or negative. But what we can do is we can also do conduction and induction on it. So, if we actually touch the negatively charged rod to the metal, it would balance things out, and if we moved it away, the leaves would stay open instead of closing again because they've now been negatively charged. So, we can do this with positive or negative charge, and we can actually do it by induction. It's a little bit trickier, but you have to get close, like in one of the first diagrams. If you get it close to chase the electrons away, then you have to touch it with another object, another metal rod, while you're still holding it close to pull from the ground or the two to try to balance things out. Then, when you move the rod away, it should still pull up a bit of charge and stay apart, kind of like it did the last time. Conduction and induction can be used with electroscopes. It's a little trickier, but it will show the effect as well. And that's all.