1-5 Conductors and Insulators

Insulators and Static Electricity

So far, we have been mostly looking at static electricity and how charges can jump from one place to another. When you take a rubber balloon, rub it on your head or your shirt, and charge it by friction, we can look a little more closely at where those charges are (and aren't).

Here is the balloon before it is rubbed with anything: we can assume it's neutral. Now let's rub the right side of it on human hair, and use the electrostatic series to see what the charge is. Rubber is near the "strong" end, and since hair is chemically pretty similar to fur (which is close to the "weak" end). That means rubber will take electrons from the hair, and the balloon will be negative.

But, since rubber is an insulator -- a material that does not allow charge to flow easily through it -- only the side of the balloon that was rubbed will be charged. Those extra electrons are trapped there, and can't move evenly across the balloon.

If we want to stick this balloon to the wall, the charged side will stick. The negatively-charged balloon creates an induced charge separation in the wall, and the negative charges in the wall will move back a little bit:

If you try to stick the neutral side to the wall, it will not work! It will either fall down, or after you let go of it, as it's falling it might turn itself and stick to the wall further down.

We have already seen that static electricity is the kind of electricity that stays in one place for a while. So, since insulators prevent charges from moving easily, it makes sense that insulators are very useful when doing experiments in static electricity.

Conductors and Static Electricity

What would happen if we rubbed a conductor -- a material that allows charge to flow easily through itself -- with a different material, and tried an experiment with static electricity?

Let's say we rub a piece of copper metal, which is an excellent conductor, with silk. The Electrostatic Series tells us what will happen: copper will take electrons from silk, meaning the copper will have a negative charge where it is being rubbed.

But since copper is a good conductor, those extra electrons will be able to easily flow through the entire object. Remember, electrons all repel each other, and if they can, they will move away from each other as far as possible. That wasn't possible with an insulator like rubber, but with copper it happens almost as soon as it gets charged:

What this means is that, while the copper might have a negative charge, this extra charge is spread out over the entire object. Any one part of the copper isn't going to have too many more extra electrons than protons. So, any one part of the copper isn't going to be too far off neutral, which means it's basically impossible to charge-and-stick copper to a wall like you would with a balloon.

Applications of Insulators and Conductors

The general rule goes like this:

Power Lines

A great example of this is on power lines, where they are being held by a utility pole:

The actual power line itself, which carries the electric current, is made of a conductor. We want electrons to flow easily through that, so it will be made of some kind of metal, usually copper. (Here, the conductor has a protective cover on top of it, where it meets the insulator.)

But, the pole needs to hold the wires off the ground, and something needs to separate the wires from the pole. That's where the insulators come in: they are usually made of glass or ceramic. Depending on the style of utility pole, and the amount of energy in the wires, you will need a lot of insulators or a few. This pole only needs a few insulators because the wires are not carrying much energy.

For higher-energy wires, more insulators are used. Here is a high-energy power line being held from a metal tower:

As you can see, there are a lot of glass insulators stacked together, to make sure this high-energy power line does not touch the tower and get grounded. Glass is an excellent insulator, so that's why it's being used here.

Circuit Boards

Another example is on printed circuit boards (PCBs), used inside everything that has a computer-like electronic device inside it:

If you've ever opened an electronic device, you've seen one of these. The green parts of the board are the insulator, preventing current from flowing. Quite often, this insulator is a type of fibreglass, which is long glass strands which are glued together. (Again, glass is used as an insulator!)

The little golden threads are called "traces," and they act like wires, carrying electrons easily from one electronic component to another. They may look like they are gold, but they are usually made of copper.

Power Cords/Cables

If you've ever plugged a power cord into a wall, you have used both conductors and insulators. Here is a cut-away view of a power cord to show you what's inside:

You will be holding onto an insulating rubber coating, but inside that rubber coating there are probably three separate conducting wires carrying charges.

Each of the wires inside might be made up of a bunch of smaller "strands," or it might just be three solid copper wires. Wires made up of many thin strands are more flexible, so the cord can be easily bent around any object without breaking anything inside.

Here is a good video showing a summary of conductors and insulators. It's a little long but the descriptions are really clear.

The video clip below is the first in a series put together by TVO in the early 1980s, and it talks about conductors and insulators. The animation looks a little crude by today's standards, but the explanations are extremely clear and easy to understand.

Practice Questions

The Basics

1. Explain the main difference between conductors and insulators.

2. What is the general rule about when to use a conductor, and when to use an insulator?

Extensions

1. Generally speaking, are conductors metals or non-metals? How about insulators?

2. There is a form of the element carbon called "graphite" which has very unusual electrical properties. Do a bit of research to find out why graphite is strange in this way.