### Video Transcript

A rod has a net negative electric charge. Figure 1 shows this rod being brought near to a hanging balloon that has no net electric charge.

We’ll get to our question in a second. But first, let’s take a look at Figure 1. In it, we see this negatively charged rod and indeed it’s being brought close to a balloon hanging by what looks like a thread. We’re told that this hanging balloon has no net or overall electric charge to it.

Now onto our question.

One of the following figures shows how charge will be distributed on the surface of the balloon when the rod is brought near to it. Tick the box below the correct figure. Tick one box.

Here, we see these four different figures, each one showing the negatively charged rod brought near the balloon and each one is a candidate for how charge will distribute itself on the surface of the balloon when the rod is near.

The first thing we might wonder though is didn’t the question say that the balloon has no net charge, so why is there a charge on its surface? Well, keep in mind that just because an object has no net charge doesn’t mean there’s no positive or negative charges in that object. We can go so far as to say that any material object does have positive and negative charge in it since the object is made of atoms which have protons and electrons in them.

When we say then that an object has no net charge, what we mean is that all the negative charges in the object balance out with all the positives. If we add them all up with a negative charge representing minus one and a positive charge representing plus one, then once we add them we get zero.

That’s what it means to have no net charge. And that’s the condition of our balloon in these four different figures. So let’s look at these figures each one in turn.

As we look at the first figure, we see that as this negative charge is brought close, negative and positive charges seem to be evenly distributed across the whole surface of the balloon. Now, in a way, this is a good thing because as we count up the positive charges we see and the negative charges we see in this balloon, we come up with the same number for each.

That means that indeed this balloon overall does have a net charge of zero as we’re told. So that’s something right about this diagram. But there’s something wrong about it too. Think about electric charges and how they interact. If we have a positive charge and a negative charge, what do they do? We know they attract one another.

But on the other hand, if we have two like charges, say that we have another positive in addition to the first one, then these charges of course repel.

Looking at this first figure, since we’ll bring a negatively charged object close to the balloon, we would expect the same electrostatic interaction to be taking place. We’d expect positive charges to be going towards the negative of the rod and negative charges to be pushed away.

But when we look at the way charges spread out over the balloon, we don’t see that. It looks as though positive and negative charges respond the same way to this negatively charged rod. That’s not accurate. So we know we won’t choose this figure.

Now, what about this figure, the one over to the right of that? Well, in this case, as we look at our balloon, we see that only negative charges are indicated on its surface. This would tell us that overall this balloon does have a charge; it has a net negative charge. But we’re told in the problem statement that it doesn’t.

So, since this collection of charge on the balloon makes it seem as though there is a net charge to it, we know that this diagram can’t be accurate either.

Now, what about the figure right below that? Well, this one has the same issue, but with the opposite sign of charge. We now see that the balloon is surrounded by a positive charge. Once again, this makes it seem as though the balloon has an overall charge to it. But we know that’s not the case. So this isn’t our winner either.

This leads us to the final figure, the one which has positive charge we can see spread out on the right side of the balloon, the side closest to the negative rod, and negative charge on the opposite side.

If we think back to the way electric charges interact, we see that this figure shows an accurate representation of all that going on. The negatively charged rod pulls on the positive charges on the balloon. So those are closer to the negative rod. Meanwhile, the negative rod pushes on the negative charges on the balloon so those get pushed to the far side.

And at the same time, if we count up the number of negative and the number of positive charges on the balloon, we see that they cancel one another out; there’re the same number of plus as minus. It’s this figure then that shows how charge will be distributed when this negatively charged rod is brought close to the balloon.

And just as a side note, even though the balloon is made of rubber which is a fairly good insulator, this charge movement still happens. It’s still the case that on a very small scale for an insulator, positive is effectively pulled towards negative and negative pushes away from negative. So this figure then really does show how charge spreads out on the surface of the balloon near a charged rod.

Next, let’s add to this scenario by including another balloon.

After the charged rod is brought near to the balloon, a second balloon is brought near to the first balloon, on the opposite side of the balloon that is next to the charged rod, as shown in Figure 2. The second balloon has no net charge before it is brought near to the first balloon.

They say that a picture is worth 1000 words. And indeed, Figure 2 makes all this clear. We have our first balloon positioned next to our charged rod. And then, we bring a second balloon close to the first one, where both the first and second balloons have no net charge.

Now, onto our question about this scenario.

Select the statement that describes the effect of bringing the second balloon close to the first one. Explain the reason for your choice. Tick one box. The second balloon is attracted to the first balloon. The second balloon is repelled by the first balloon. The second balloon is not affected by the first balloon.

We’ll figure out which box to tick by looking again at Figure 2, where we see our charged rod and first balloon from before and now our second balloon brought near to the first. Let’s do this: let’s draw in the way that charges were distributed on the first balloon as we saw in our last question.

We saw earlier that on this first balloon, positive charges move toward the negative rod and minus charges move away from it. Now, here’s the question: what effect will these negative and positive charges on the first balloon have on the second balloon? Well, look at how they’re distributed. See that the negative charges are closer to the second balloon than the positive charges.

So in other words, they’ll have a greater effect on the charge distribution of the second balloon than the positive charges on the far side of the first. So for all the plus and minus charges that are in the material of the second balloon, the plus charges will feel a net pull to the right towards these very close minus charges on the first balloon. And the minus charges on the second balloon will feel a push to the left away from it.

Now, we might say, “Well, wait a second! Don’t the positive charges on the first balloon have the opposite effect?” And that’s true; they do. But the thing is the positive charges on the first balloon are farther away from the second balloon and the negative charges. And therefore, the effect of the negative charges dominates.

Therefore, on the second balloon, positive charges will tend to be pulled towards the right, towards the first balloon. And negative charges will be pushed away. Notice that this charge distribution on the second balloon looks just like the one on the first balloon.

But in any case, let’s consider how these two balloons with these charge distributions will interact with one another. Will the second balloon be attracted to the first, will it be repelled from the first, or will it not be affected by the first balloon?

Now that we know which type of charges on each balloon are closest to those on the other balloon, we can answer this question. We see that the negative charges on the first balloon that face the second balloon will be attracted to the positive charges on the second balloon that face the first and vice versa.

These charges — the positive on the second balloon and the negative on the first — will attract one another. And this attraction will be greater than any repulsion caused by the other kinds of charge on these balloons. For example, the negative charges on the second balloon will indeed repel the negative charges on the first balloon. And the positive charges on the first repel the positive charges on the second. But these repulsions will be happening at a greater distance than the attraction that happens between these nearby opposite charges.

The overall or net effect is that the second balloon will be attracted to the first balloon. And so we tick the first box we’re shown.

Not only do we want to select a choice, but we want to explain the reasoning behind our choice. Here’s what we could write. And we can write that the first balloon has a net charge on the side facing the second balloon — that’s the net minus charge that we highlighted over here — producing an opposite net charge on the side of the second balloon nearest to it — that’s the positive charge on the second balloon.

We have then opposite charges and the opposite charges attract one another. And since those charges are attached to the balloons, the balloons attract one another: the second moves towards the first.

Next, let’s consider what happens when balloon size is a variable; that is, it can change.

A student investigated the effect of changing the size of the balloon that a charged rod is brought near to. The student made the hypothesis that the larger the balloon is, the further it will move when a charged rod is brought near to it. Balloons of different sizes were held in place and the charged rod was brought to the same distance from each balloon. The balloons were then released. The results of the experiment are shown in Table 1.

What’s being described here is an experiment where balloons of different sizes are used. We have these differently sized balloons and a charged rod is brought to the same distance from these balloons.

Let’s just say our rod is negatively charged like before. After being held in place so that the charged rod can approach to a fixed distance from them, the balloons are then released and the amount they move in response to the charged rod is recorded.

Looking over at Table 1, we see there are two columns of data, two pieces of information that were measured: one was the radius of the balloon at its widest point; that tells us the size of the balloon and then in the next column, we see the distance moved by the balloon after the charged rod was brought near and the balloon was released. That’s the experiment and Table 1 shows us what we found.

Now, let’s consider a question related to these data.

According to the results of the experiment, how does the size of the balloon affect the distance that the balloon moves?

Looking again at Table 1, we see that the radius of the balloon at its widest point goes from 4.0 up to 8.0 centimeters. So as we move down this column, that size gets bigger. But then as we look over at the other column, distance moved by the balloon, we see that this starts out at its highest value 15 millimeters and moves down to seven millimeters.

So it’s the smallest balloon, the one with a 4.0 centimeter radius, that moves the most, 15 millimeters, and the largest balloon moves the least. Notice this is opposite what the student hypothesized. The hypothesis was that the bigger the balloon, the more it would move. But we’re seeing that the bigger the balloon, the less it moves.

We can write then that the larger the balloon, the less it is moved by the charged rod. Even though our hypothesis was opposite this conclusion, our conclusion is based on the data we collected. So we’ll stick with it.