Video Transcript
Back in the old days, if someone's house caught on fire, volunteers would rush to the scene and form a bucket brigade to put out the flames. If you think about it, you could say that the bucket brigade was 'conducting' water from the source to the fire. But, what if the volunteers stopped passing the water?
Well, we know one thing: the house would burn to the ground because it was effectively 'insulated' from the water! The bucket brigade scenario is actually very similar to why some materials act like electrical conductors and others act like Electrical Insulators. But, to find the actors in our electrical 'bucket brigade', we have to start at the atomic level.
Not all atoms are created equal. Some atoms don't hold on to their outer electrons very tightly. These are known as free electrons because they are literally free to roam around from atom to atom. Free electrons are the members of our electrical bucket brigade passing electrical energy from one electron to another.
Free electrons are free to roam from atom to atom.A material with many free electrons allows easy transfer of electrical energy and is therefore called a conductor. If we send an energetic electron into a conductor, it will impact a free electron, knocking it down the line until it hits another free electron. This sets up a chain reaction of impacts that conducts the electrical energy through the material.
A good way to think of it is like a group of balls spread out on a billiards table. Our energetic electron is like the cue ball being shot into the group and impacting one ball, which in turn knocks into another ball and so on down the line. Before you know it, the energy from the cue ball has been 'conducted' all the way to the other end of the table. The only real difference between billiard balls and electrons is that electrons conduct electrical energy at nearly the speed of light!
Insulators have very few free electrons and do not transfer electrical energy well.On the other end of the spectrum, there are atoms that hold on to their electrons very tightly. A material that contains these types of atoms has very few, if any, free electrons and does not transfer electrical energy well, if at all. This type of material is called an insulator.
If we send an energetic electron into an insulator, it effectively bounces off the atoms, unable to transfer its energy to the tightly bound electrons. It will keep bouncing around until it either frees another electron or until it simply runs out of energy!
Going back to our billiard table analogy, this is very similar to the cue ball simply bouncing off the sides of the table. Either it will hit another ball and transfer its energy, or it will just stop rolling because of friction.
The ability of a material to conduct electrical energy is known as its conductivity. Not surprisingly, materials that are good conductors have high conductivity, while materials that are good insulators have low conductivity. The conductivity of a material is dependent on the number of free electrons available, and this varies greatly between the different types of atoms. In general, the materials with the highest conductivities, or best conductors, are metals - but this doesn't mean that other materials aren't capable of conducting electricity. If that were true, no one would ever be in danger of getting electrocuted!
Examples of materials with low conductivitiesOn the other end of the spectrum, the materials with the lowest conductivities, or best insulators, are glass, ceramic, rubber and some plastics. Not all materials are classified as insulators or conductors, because in the real world, they don't do a particularly good job of either one.
One final note on conductivity: good electrical conductors are also good heat conductors. Electricity and heat are just two different forms of energy, but both rely on free electrons to transfer through the material. Sometimes it's easier to tell if a material would be a good conductor by using our sense of touch to see how well it conducts heat.
Let's look at some everyday examples of conductors and insulators. Have you ever given much thought to the electrical cords that power just about everything in your home? An electrical cord is made of both conductors and insulators. The wire in the center is typically made of copper, and the outer jacket is made from plastic or rubber.
An electrical cord is a dramatic example of the different levels of conductivity between materials. Copper is so much more conductive than rubber that the electrons can travel through a hundred feet of copper with ease but can't pass through a tiny fraction of an inch of rubber insulation. In fact, the conductivity of copper is about one million trillion times greater than that of rubber.
High-voltage power lines are often protected by ceramic insulators.Now, what about glass or ceramic? Can you think of a place where you've seen these materials used as electrical insulators? You've probably seen them, but perhaps you didn't realize what you were looking at. Lots of high-voltage power lines are protected by these funny-looking insulators because they're so good at not conducting electricity.
Let's summarize what we've learned. Materials that are composed of atoms with free electrons are good conductors. Electrical energy is conducted through a material by the transfer of energy from one electron to the next. Materials that are made of atoms with tightly bound electrons are good insulators. These materials have very few, if any, free electrons available to transfer energy.
The ability of a material to conduct electrical energy is called conductivity. All metals have high conductivity and are good conductors, while rubber, plastic, ceramics and glass have low conductivity, making them the best insulators. Good electrical conductors are also good heat conductors because both forms of energy rely on free electrons to transfer through the material.
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