Why is vacuum a good insulator?

Answer 1

A vacuum is a good insulator because it prevents the transfer of heat by eliminating the medium (matter).
A perfect vacuum is the absence of matter. Heat is transferred by conduction, convection and radiation. Radiation is the only form of heat transfer that does not need a medium. Furthermore, radiation is generally the least efficient method of heat transfer.

Answer 2

Vacuum is a good insulator because it lacks molecules that can conduct heat. Without molecules to transfer heat through conduction and convection, heat transfer is limited to radiation, which is much slower. This helps to maintain the temperature of an object by reducing heat loss or gain.

Answer 3

There are four methods by which heat transfers from one area to another. They are, evaporation, convection, radiation and conduction.

With a vacuum, only radiation can transfer heat across the vacuum and because the other three forms of heat transfer are negated that makes a vacuum a good insulator.

Then again, you may be referring to electricity. For electricity, negative and positive ions in a liquid or electrons in a solid, need a medium to travel through. Electrons in a solid for example need to jump from atom to atom as they move or conduct through the solid. If you have a vacuum it makes it very difficult for electrons and or ions to jump across the vacuum. They will fly across a vacuum if the voltage is high enough though.

What's more is, you can help electrons to fly across a vacuum if you heat up the cathode (negative electrode) to a very high temperature, but still, you also need a high voltage to pull them across the space of the vacuum after they fly off the cathode via heating. And so, a vacuum is a very good insulator for electricity at room temperature and moderate voltages.