Video: Static Electricity

In this lesson, we will learn how to describe the forces experienced by electrically charged particles and objects.

14:10

Video Transcript

In this video, we’re learning about static electricity. And as we’ll see, static electricity involves the buildup and then the discharge of electric charge. The first thing to know about static electricity is that it brings us back to basics: positive and negative electric charges.

And here are the rules for how those charges interact. If we have charges of opposite type like we have here, then they’ll attract one another. That is, there’ll be an attractional force on each one of these charges from the other charge. But then, on the other hand, if we have two like charges, say these two positive charges here, then they’ll repel one another. They’ll push one another away. And the same thing happens if instead of two positive charges, we have two negative charges. Because the charges are of a similar type, they’ll repel one another.

Keeping these rules in mind, we might wonder why is it that we don’t experience this positive and negative repulsion very often in our everyday life. The answer to that question is to a large extent the objects we interact with on a daily basis — desks and chairs and floors and walls and doors — don’t have a net electric charge to them. They’re neutral. And if we bring one electrically neutral object near another electrically neutral object, there will be no electrical interaction between them and that’s the situation we often find.

One important point about these electrically neutral objects is that even though they have no overall or net charge, that doesn’t mean they don’t have any positive and negative charges within themselves. In fact, they have lots and lots of positive and negative charges. It’s just that they perfectly balance one another out. There is the same number of plus as there is minus. Overall, that’s electrically neutral. It is possible though and this is where static electricity comes from. It is possible for charge to be transferred between these objects.

To see how that happens, let’s consider the very-small-scale microscopic structure of solid objects. After all, it’s solid objects that we’re most aware of interacting with on a daily basis. Now if we started out by looking at a gas, we know that the atoms in this gas are incredibly mobile. They can move in any and all directions. If we then moved to a liquid, there is a bit more order in the molecular structure of this state of matter, but not very much more. It’s really only when we get to solid objects that we start to see a very fixed and rigid pattern for the way the atoms arrange themselves.

In a solid, the atoms are arranged in an orderly lattice structure. They look like a grid. And recall that each one of these atoms is made up of a positively charged nucleus that has protons and neutrons and then is orbited by negatively charged electrons. Compared to the fixed atomic nuclei in this lattice structure, these electrons are much more likely to move to be transferred from one place to another. So when two solid objects come in contact — maybe they bump into each other or rub together — it’s possible and even likely that some electron transfer will take place. Electrons from one side will move across the barrier to the other side.

When this happens, notice what we now have. This object to the left now has an excess of negative charge and this object to the right now has a deficit of negative charge. In other words, the object on the left now has a net negative charge and the object on the right now has a net positive charge. When this happens, we can see what will take place. Because we now have oppositely charged objects, they will attract one another.

This is exactly what happens when we have an inflated balloon and we rub that balloon against a wall. Initially, both the balloon and the wall are electrically neutral. They have the same number of positive charges as negative charges. But when they come in contact and there is friction between the two surfaces, some electrons from the wall transfer to the balloon. The balloon now has a net negative charge and the wall has a net positive charge. So they attract one another. And if you’ve ever tried this, you can see that it really does work. The balloon will stick to the wall. It won’t fall down.

At this point, we may wonder why is it when two neutral solid objects come in contact that electrons move from one of the objects to the other and not both ways. In other words, in this case why is it that electrons move from the object on the right to the one on the left and not from the left to the right. This tendency of electrons to flow in one direction when two objects interact and not the other has to do with what’s called the electronegativity of the objects.

This is a quantitative value that describes how likely an object is such as wall or rubber or steel to accept electrons. When objects of two different materials interact, say our rubber balloon and our dry wall, then whichever object is most likely to accept electrons is the one that has electrons flow to it.

That’s said, let’s consider for a moment the title of this lesson “static electricity.” Now we have to admit that this is a bit of a strange term. After all, the word static means stationary. It refers to something that’s not moving, while electricity of course is charge in motion. So which is it: the static electricity refer to something in motion or to something that’s still?

We can get a bit of a sense for it by considering again these two objects which recall were originally neutral. But now each of them has a net charge. We saw that initially these objects were separated from one another. And at that point, each of them had the same number of positive charges as negative charges. They were each electrically neutral. Then we brought them into contact. And through that contact, charge moved from the right object to the left one.

So what we now have is a buildup of electric charge. We have a buildup of negative charge on this object and a buildup of positive charge on this one. If we were to leave these objects still but still in contact with one another, this might be the way that it stays forever. The situation might be static. We have a static buildup of charge.

But let’s imagine this. Let’s say that we separate these objects once more so that they’re no longer in contact. Then say we take our negatively charged object. And using the same process with other objects not on screen, we add more negative charge to it. As we add more and more negative charge to this object, we can see by the basic charge rules we described over here that this object is increasingly attracted to positive charge.

Up to a certain point, the air that separates these two objects serves as an insulator. It keeps the charges from moving from one object to the other. But eventually, as more and more negative charge accumulates on this object, that air barrier is no longer enough to separate the charges. By that point, we have so many negative charges being drawn to the positive that the attractive force is great enough to break down that barrier. That image of breaking down a barrier is a good one because it describes how sudden and dramatic the discharge of negative charge from this one object is.

When that charge is great enough, it will jump the gap between these two objects, its electricity flowing through mid-air. This by the way is the same thing that happens when lightning strikes during a storm. There’s been massive charge build-up in the storm cloud. And eventually, that charge is great enough that it’s able to overcome this insulating layer of air between the Earth and the cloud. We see then the sense in which static electricity is both static — stationary — and electric that involves motion. It’s a charging and discharging process.

Many of us have plenty of real-life experience with static electricity. Say, for example, that you’re walking around in stocking feet on a shaggy carpet. Each time we take a step, the friction between our socks and the carpet leads to the transfer of electrons from the carpet to us. Maybe without even realizing it, as we walk around, we accumulate more and more charge. That’s the buildup of electric charge in us.

This net negative electric charge that we now have as it gets bigger and bigger is increasingly drawn to any positive charges in the vicinity. This means whenever we get near an object like a light switch or a door handle that has a relative positive charge compared to our large negative charge, all those negative charges will suddenly and dramatically flow out of our body to the relatively positive object. We’ll see perhaps a little spark of electricity. That’s the static electric discharge process and that’s where we feel that little shock.

So static electricity really does come down to the interactions between positive and negative charges, attractions and repulsion. Let’s get a bit of practice with these ideas through a couple of examples.

A polythene rod is rubbed with a cloth duster, giving it a net negative charge. A second polythene rod is rubbed with a duster and the two rods are brought close together. What will happen?

So here we have a polythene rod — that’s a plastic rod — and a cloth duster. And at the outset, we can assume that both of these objects are electrically neutral; that is, they have no net electric charge. It’s important to realize though that this doesn’t mean that the rod and the cloth don’t have positive and negative charges in them. All it means is that they balance one another out. In each object, there are the same number of positive charges as negative charges. So on a net or overall level, they’re neutral.

Okay, we’re then told that the rod and cloth are brought in contact and they’re rubbed together. And once that has done, we’re told that overall the rod now has a net negative electric charge. What is that mean? That means that while the cloth and rod were rubbed together, electrons were moving from the cloth onto the rod. Since electrons have a negative electric charge, when they build up on the rod they give it a net negative charge as well. This also means by the way that after this interaction, the cloth has a net positive charge. That’s because it’s lost some electrons to the rod.

But anyway, our focus is on this negatively charged polythene rod. We’re told that with a second originally neutral polythene rod and a second originally neutral cloth duster, a similar process occurs. The two are rubbed together. And we know that because this is the same type of charge transfer process as before, this second polythene rod will also end up with a net negative charge. We’re then told that these two rods are brought close together. And the question is what will happen.

We can see that both of these objects have a net negative electric charge. In other words, they have the same charge, in this case negative. We can recall that when it comes to electric charges interacting together, opposite charges attract and like charges repel one another. We know that both our polythene rods have a net negative electric charge. In other words, they both have like charges.

That tells us that the rods will experience a repulsive force. They’ll try to push one another apart when we try to bring them together. That then is our answer to the question of what will happen when these rods are brought close together. We can say that the two polythene rods will repel each other.

Let’s now look at one more example of static electricity.

The buildup of what type of particle gives an object an electric charge?

In this question, we’re answering what particle it is that gives an object an electric charge. Let’s say we start out with some generic object. Here’s our object. At the outset, we’ll say that this object is electrically neutral. Now, we knew that it doesn’t mean it doesn’t have any positive or negative charges in it. It just means that those charges balance one another out. There is the same number of positive and negative charges in this object.

Speaking of positive and negative charges, just what is it that creates those charges? To answer that question, let’s recall the structure of an atom. An atom consists of a positively charged nucleus made up of protons and neutrons orbited by negatively charged electrons. So really, it’s those three particles protons, neutrons, and electrons, which are our candidates for the particle that gives an object an electric charge. And we know that it’s electrons that are responsible for negative charge, protons for positive charge, and that neutrons — true to their name — have no electric charge.

So then, these are our options for this answer of what particle gives objects electric charge. And really, we can cross the neutron off the list because we know that that has no net charge to it. So then, we’re down to electrons and protons.

Getting back to our object, let’s say that this is a solid object. It’s not a liquid or a gas, but rather in a solid state of matter. That means if we were to zoom way in on a small portion of this object, we would see a very orderly atomic structure to the atoms. They’re arranged in an even grid formation. The atomic nuclei here are drawn in pink and then the orbiting electrons are drawn in blue.

Thanks to this lattice structure, the orderly arrangement of all the atoms, it’s the nuclei — the pink dots — that are fully fixed in place. They can’t move without causing a great disruption. And so, they’re very unlikely to. The electrons on the other hand — those blue dots — are much more mobile than the positively charged nuclei.

Now let’s say we were to shift our view so that now we’re looking at an up-close view of a portion of our object on the edge of the object. And just like before, we see this evenly spaced atomic structure, the atomic lattice. And let’s say that in addition to our original object, we now have a second object, also electrically neutral. If we were to bring these two objects into contact and perhaps rub them together, then what we would see is that charge transfer is likely to occur between them.

And what charge do you think it will be that will move from one object to the other? Rather than the fixed in place positive charges bound to the nucleus, it will be the more mobile electrons that will transfer between objects. It’s the exchange of those particles — negatively charged electrons — that give objects their electric charge. And that then is our answer. Even though both electrons and protons have the capacity to give an object charge, practically speaking it’s electrons that transfer between objects and build up that electric charge.

Having seen this, let’s now summarize what we’ve learned about static electricity. In this video, we’ve seen that static electricity is due to the buildup and then the discharge of electric charge. This can happen on a small scale. For example, it can happen to us as we reach for a metal door knob on a dry day or on a large scale, for example a bolt of lightning coming from a storm cloud.

In addition, we learned that charge buildup and discharge is caused by charge attraction and repulsion. That is, if electric charges are opposite, then they attract one another. And if they’re like or they’re the same, then they repel one another.

And additionally, we saw that the structure of solids — that lattice structure — means that negative rather than positive charge is what’s transferred between objects. In other words, it’s the electrons that move from one object to another, while the protons stay in place.

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