(a) Explain how a gas can be made to conduct electricity. (b) Name the electric charge carriers in gases.

Answer Details

(a) How a gas can be made to conduct electricity

Under normal conditions a gas is a very good insulator because it contains almost no free charge carriers; its molecules are neutral. A gas is made to conduct by ionising it, that is, by knocking electrons out of some of its molecules so that free electrons and positive ions are produced. This is achieved by:

• Applying a very high potential difference (strong electric field) across the gas so that stray electrons are accelerated hard enough to knock electrons out of neutral molecules on collision, causing further ionisation (an ionisation avalanche).
• Reducing the gas pressure (as in a discharge tube). At low pressure the molecules are far apart, so accelerated electrons travel a long free path and gain enough energy between collisions to ionise the gas.
• Ionising the gas by other agents such as X-rays, ultraviolet light, or radiation from radioactive sources, which free electrons from the molecules.

Once ions and free electrons exist, they drift under the applied field and constitute a current, so the gas conducts.

(b) The electric charge carriers in gases

• Negatively charged free electrons (and negative ions).
• Positively charged positive ions (molecules that have lost electrons).