1-3: Charging Objects

Charging by Friction

If you rub a plastic pen with a cotton shirt, then bring that pen towards some small bits of paper on a table, the paper bits will be attracted to the pen:

Sometimes you can even get the paper bits to stick to the pen for a very long time:

It's safe to assume that the paper bits are neutral. But there's an obvious attraction here, so that means the pen must be charged somehow.

If you rub one object with another object, the two objects can become charged by friction. Friction is the force that tries to stop two things from moving. For example, if you press your hand down on a desk and try to slide your hand across the desk, friction tries to keep your hand from sliding.

Here's what happened:

Originally, the pen and the cotton were both neutral. We can show the charges inside each of the objects:

Then the cotton was rubbed on the pen, and some of the negative charges moved from the cotton to the plastic.

This made the plastic pen negatively charged, and the cotton positively charged. Since the pen was charged, it could attract the neutral paper.

But, how do we know that the plastic pen is negative? Remember, either positively or negatively charged objects could attract the neutral paper.

The Electrostatic Series

Over the years, scientists have performed a lot of tests on different materials, to see what their static electric properties are. They have come up with the electrostatic series: an ordered list showing the ability of materials to hold on to electrons when they are rubbed together.

We can use this list to determine the charges on any pair of objects, when they are charged by friction. If we find plastic and cotton on the Electrostatic Series, we can see that plastic holds on to electrons more strongly than cotton does. This means that the cotton loses electrons to the pen; when the pen gains electrons, it becomes negative and the cotton becomes positive.

Charging by Conduction

When scientists talk about conduction, they are usually referring to how well something invisible flows. If an object "conducts heat," that means that heat flows easily through it, like ceramic floor tiles (which feel cold on your feet because they conduct heat away from your feet).

Here, conduction describes how well charges flow from one object to another when they physically touch. Let's bring a positively charged object towards a neutral object:

Just like before, the neutral object on the right now has an induced charge separation. The positive object on the left will attract the neutral object. But now let's make the two objects touch, so they can conduct some charge:

The negative charges in the right object are attracted to the left object, because the left object is negative. These charges can move because the objects are touching, and they move for two reasons:

1. The negative charges are attracted to the positive object, and

2. The negative charges all repel each other, and want to get as far away as they can from each other.

Because of both of these reasons, the negative charges spread themselves out between the two objects. (Note that the positive charges do not move, ever.)

Count up the positive and negative charges for each object, now that some of the negative charges have moved. What is the charge on each object now?

When a charged object touches a neutral object, both objects end up with the same charge as the originally-charged object. This is what happens when a neutral object is charged by contact: it gets a charge just by being touched by a charged object. No friction necessary!

Grounding

With all these charged objects flying around, getting charged can be a little too easy. When you want to make an object neutral again, you "ground" that object.

The process of grounding means connecting that object to the ground -- that is, to the planet Earth. This lets Earth either give that object a few more negative charges, or take a few of its extra negative charges, to make that object neutral again. (Remember, positive charges never move.)

If something is connected to the ground, we often give it this special symbol:

The easiest way to ground an object is to connect it, using a wire, to a water faucet/tap. That faucet is connected to water pipes, which are made of metal and go into the ground. Charges can flow easily through metals, as we'll see later.

Let's say we have a negatively charged object, and we want to ground it. Here is the original object:

If we connect it by a wire to the ground, we allow those extra negative charges to flow down to the Earth. ("Grounding" is also called "earthing" sometimes. The two words means the same thing.)

Now that the negative object is connected to the ground, the extra negative charges can flow until the object is neutral again.

Now that the object is neutral, you can remove the grounding wire, and you're all done. This does give the Earth a bit of an extra negative charge, but our planet is so big, a few charges added or taken away won't make a difference.

Application: Grounding Wrist Straps

When people work with sensitive electronics like computer chips, they often wear a grounding wrist strap. This usually has a wristband, which you put on, and a wire with an alligator clip on the end that you can clip to a water faucet or other grounding object:

If you don't wear one of these grounding wrist straps, static electric charge can build up on you without you even knowing it. Then, if you touch a sensitive object like a computer processor, the charge can suddenly flow to it, causing the computer chip to be ruined. Safety first!

Practice Questions

The Basics

1. Explain the difference between charging by friction and charging by contact.

2. When it is said that something has been "grounded," what does that mean?

3. Predict the charge, after charging by friction, on each material when (a.) fur is rubbed with zinc, and (b.) ebonite is rubbed with silk.

Extensions

1. If you rub nickel with cotton, both objects will become charged by friction. The same thing will happen if you rub nickel with acetate. Try to predict how much charge would transfer, if you compared the cotton with the acetate after charging.

2. Some people put a "grounding strap" on their car. (Maybe you've seen one but just have never known what it was!) Do an internet search for this object, then try to determine (a.) how it works, (b.) why someone would use this, and (c.) why some people think it helps them.