What makes a material a good conductor? What about a good insulator?

• A. A good conductor transfers heat slowly; a good insulator transfers heat quickly.
• B. A good conductor transfers heat easily due to free-moving particles; a good insulator slows heat transfer because it contains trapped air or has poor conductivity.
• C. A good conductor transfers heat only in solids; a good insulator only in liquids.
• D. A good conductor is always a metal; a good insulator is always plastic.

Answer: (B)

Heat moves most easily through a material when the particles inside can transfer energy readily. In good conductors, there are free-moving particles—especially free electrons in metals—that quickly carry thermal energy from a hot region to a cooler region. That’s why metals feel cold to the touch when they’re not at the same temperature as your hand and heat up your hand quickly when in contact with something hot.

A good insulator slows heat transfer because it has few free carriers to move energy and often includes trapped air, which itself is a very poor conductor. The low conductivity of air and the presence of air gaps interrupt the flow of energy, so heat moves slowly through the material.

That’s why you can keep a pot handle cool with plastic (a poor conductor) and keep heat inside a house with double-pane windows that trap air between panes. Metals conduct well, plastics and many everyday insulators do not.