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Charge Transfer - Electroscope

Teacher/Instructor Jonathan Osbourne
Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

Charge transfer can happen through friction, induction or conduction. An electroscope is a device which detects charge.

Alright. Let's talk about electroscopes and methods that we can transfer charge. Alright. First off, we know that there is charge out there but we want to somehow be able to measure it. Well, how are we going to do that? Well, we're going to use something called an electroscope.

An electroscope is a piece of metal that consist of a knob, a long wire and then two very very very thin, usually gold foil, leaves. Okay? So these are all connected, they're all wire and that's going to mean that charge can flow freely through them. Alright. Now how does this work? Well, let's say that I bring a glass rod close to the knob of an electroscope, and this guy's been electrified. Alright? Well, what's going to happen is this positive charge, it really likes the negative charge. Remember that positives attract negatives. Now these positives can't go anywhere. So they're just kind of stuck. But the negative charges inside the conductor can all conspire to jump up to the knob. So if the negatives come up out of the leaves into the knob, then that leaves positives down here. So both of the leaves are positively charged. But just like pit balls, that means they're going to repel each other. So these guys are going to move apart. And that's how we're going to measure it. We're just going to look at the leaves and if they move apart, means we got some charge. Alright?

What about negative charge? Well, it's going to do the same thing. Look what happens. Negative right? Now, negative charge doesn't like other negative charge so the negative charge up in the knob instead of running up there is going to run down to the leaves. That's going to leave positives in the knob and negatives in the leaves. And again, they'll go apart. So using an uncharged electroscope, we're able to measure the existence of charge but we can't tell the difference between positive and negative. So it's very important to remember an uncharged electroscope can give you an idea that there is charge there but it can't tell you positive versus negative. Alright. So how can we do that? Well, let's see.

What if we charge the electroscope? So now, what if I start off with an electroscope that already has positive charge on it? So that means that these leaves are already a little bit apart. So now I'm going to bring a positively charged glass rod close to it. Well, again, that's going to attract electrons. It's going to attract negative charge. So I've got negative negative. And what does that leave behind? Well, it leaves more positives behind. So that means that these guys are going to move further apart. Okay. So that's the same thing that happened before. The uncharged electroscope. So what gives, what does that buy us? Well, what if I bring negative charge close?

Well, negative charge is going to scare the negatives down into the leaves and leave positives up in the knob. And notice what happens now. Now the leaves are less charged. So that means they're going to be drawn together, basically, by gravity. So these guys get closer together. And so now, with the charged electroscope, I can tell whether the rod that I'm bringing close to the knob has the same charge as the electroscope or the opposite charge. And that's very nice. It actually allows us to get an entire system going. Once we choose, the glass rods are positively charged, then we can just hold them up to electroscopes, hold anything that's electrified up, and just see what happens. And we can tell whether that thing is positively charged or negatively charged.

Alright. Now, how do we charge this electroscope in the first place? There's two major ways that we can charge an object that was initially uncharged. The first is conduction, and conduction is associated with taking something that's already charged and then touching it to something that's uncharged. Now, as I bring it closer, these positive charges are going to push all the positive charges over to the other side and the negative charges over there. So this is called polarized. I have polarized this material. Alright. So now when I let them touch, some of the negatives are going to jump over. And then I pull them apart and look what's left, he's positively charged. So that's charging by conduction. Charging by conduction is very very very simple. It gives you the same charge that you started off with and it involves direct contact between the two things. Alright.

The other way is charging by induction. Now, this is somewhat different. So again, I'm going to bring this rod close but not touching, to the thing that I want to charge. Alright. Again, I get negatives that congregate up here and positives that congregate down here. But now, what I'm going to do is I'm going to take another object and I'm going to bring it close and this guy's initially uncharged. And I'm going to bring it close to this guy that I want to charge. Now, that means it's close to these positives, these positives are stronger than these negatives. So that puts a bunch of negatives here. Alright. And then a bunch of positives up here. Now, when I touch these guys together, what's going to happen is the negatives are going to jump across because they want to be closer to this positive charge. So that gives me a net, negative charge here and a net positive charge here. So when I charge this object by induction, I get the opposite charge on the thing that I charged than I had on the thing that I used to charge. Induction is an indirect way of charging. Notice that at no point did I touch the initial thing that I had charged to anything. So that means that I don't have to recharge it later. I can use it again and again and again. Alright, great. So induction, indirect. Alright?

And that's electroscopes and charge transfer.