Electricity - Conductors and Insulators - Teacher's Notes  
  conductus, conducere
Latin words for leading, guiding. An electrical conductor does exactly that. It guides the electron flow from place to place.
insula, insulae
Latin words for separate or divided. An electrical insulator is a separation between conductors. Electrons do not flow across the division or separation formed by an insulator.
How Good is a Conductor or Insulator?
Some materials conduct electrical current very well, others conduct poorly, and others not at all. Obviously this characteristic is really a gray scale, from very good to very poor. If the material conducts electrical current well enough to not affect the operation of the circuit, then it is considered a good conductor. If a material insulates well enough that any leakage through it is negligible (does not affect circuit operation), then that material is considered a good insulator.
In recent years, we have achieved perfect conductors, via super-cooling of special alloys. We call these new materials "super-conductors".
A perfect vacuum (perhaps never achieved in the real world) would be a perfect insulator. Interestingly, pure water (again never seen in the real world) would also be an excellent insulator. (Water with any impurities is a fairly good conductor).
There are lots of materials in the world that are poor conductors or poor insulators. Sometimes we use these materials in electricity and electronics because we need poor conductivity. (Lamp filaments, and heating elements are examples of poor conductors we use in everyday life.)
Questions for your students:
Q: Is air a conductor or an insulator?
A: A conductor (not correct). Let's perform a thought experiment about batteries to see if we can reason this out. The battery terminals are not covered with plastic... they are exposed directly to the air. If air was a conductor, then current would flow between the battery terminals all of the time. The battery would soon be dead! In fact all batteries would have a very short life (they would all die within hours of when they were made). Since that doesn't happen, air must not be a conductor. Air must be an insulator!
A: An insulator (correct).
Why is that question so important? Think about how hard it would be to make circuits that work correctly if air was a conductor. Everything would have to be carefully insulated. Most types of switches and relays we use now would not work (they use air as the insulator between poles when they are in the open position). We wouldn't want to use electricity for lighting and heat if the air was a conductor... our houses would be dangerous places!

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