Under ordinary conditions gas molecules are neutral, so a gas is a dielectric: it does not conduct current. If an ionising agent — heating (a flame), X-rays or ultraviolet rays, radioactive radiation — acts on the gas, an electron is detached from a neutral molecule and a positive ion and a free electron are formed; this is ionisation. Electrons can also attach to neutral molecules and form negative ions. The reverse process — a positive ion joining an electron and becoming a neutral atom again — is recombination. If the ionising agent stops, the number of ions falls through recombination and the gas becomes a dielectric again. The current in gases is carried by electrons and ions of both signs. Staying near high-voltage equipment, or working with open flames or X-ray sources, is dangerous, so only specialists do it.
Worked examples
An ionising agent produces 4·10⁹ ion pairs every second and recombination removes the same number of pairs: the number of ions does not change — equilibrium.
The gas between the plates of a charged capacitor starts to conduct if a flame is near it; when the flame is removed, the current falls to zero.
Class activity
Think: why can air near a flame conduct current? Write a few sentences in your notebook. Never bring a flame near electrical devices.
Practice
1
Define ionisation and recombination.
Ionisation is the detaching of an electron from a neutral molecule; recombination is an ion and an electron joining to form a neutral atom.
2
When does a gas start to conduct current?
When an ionising agent acts and ions and free electrons form.
3
If 8·10⁹ ion pairs form each second and 3·10⁹ pairs recombine, by how many 10⁹ pairs per second does the ion count grow?
5
4
Why does a gas stop conducting when the ionising agent stops?
The ions disappear through recombination and the gas again consists of neutral molecules.