Under which of the following conditions do gases conduct electricity?

Analyze the question The question asks about the conditions under which a gas becomes a conductor. This is the topic of conduction of electricity through gases (gas discharge).

Explain the concept Gases are normally good insulators because they have very few free charge carriers. For a gas to conduct, it must be ionised, meaning its atoms or molecules are split into positive ions and electrons.

• Low pressure: The gas molecules are far apart, so an electron or ion can travel a long distance (a long mean free path) before colliding with another particle. This lets it gain enough kinetic energy from the electric field to ionise other gas molecules when it finally collides.
• High p.d.: A large potential difference gives a strong electric field, which accelerates the charged particles to high speeds. This provides the energy needed for ionisation by collision.

At high pressure, the molecules are crowded together. Charged particles collide too often and cannot build up enough energy to ionise the gas, so very little conduction occurs unless the p.d. is extremely large (as in lightning).

Show the solution

• A. Low pressure, low p.d.: the particles travel far but the field is too weak to ionise the gas. ✗
• B. Low pressure, high p.d.: long free paths and a strong field mean ionisation occurs easily, and the gas conducts (e.g. a discharge tube, a neon sign, or a fluorescent tube). ✓
• C. High pressure, low p.d.: frequent collisions and a weak field mean no ionisation. ✗
• D. High pressure, high p.d.: collisions are still too frequent, so conduction is very difficult. ✗

Answer: Option B, low pressure and high p.d.

Tip Remember: "Low pressure, high voltage" is the recipe for a gas discharge tube. Low pressure gives the particles room to speed up, and high p.d. gives them the push.