In a discharge tube, most of the gas is pumped out so that electricity is conducted at?

Analyze the question: The question asks about a discharge tube from which most of the gas has been pumped out. It asks under what condition electricity is conducted.

Concept: Conduction of electricity through gases happens at low pressure.

• At normal (atmospheric) pressure, gas molecules are packed closely together. Electrons and ions collide with molecules almost immediately and cannot gain enough energy to ionise other atoms. Gases are poor conductors, and a very high voltage is needed to force a spark.
• When most of the gas is pumped out, the pressure falls and the molecules are far apart. Charged particles then travel longer distances (a longer mean free path) between collisions. They pick up enough kinetic energy from the electric field to ionise the gas molecules they hit, which produces more ions and electrons and a sustained current. This is why the gas glows (as in neon signs and fluorescent tubes).
• At very low pressure, in cathode ray tubes, cathode rays (electrons) travel in straight lines across the tube.

Solution:

1. Pumping out the gas means the pressure is reduced.
2. Reduced pressure means a longer mean free path, and so easier ionisation.
3. Therefore conduction occurs at low pressure.

Checking the options:

• A. Low voltage: Incorrect. Discharge tubes usually need a high voltage across the electrodes.
• B. Steady voltage: Incorrect. Steadiness of the voltage is not what the pumping achieves.
• C. High pressure: Incorrect. This is the opposite of what pumping out the gas does.
• D. Low pressure: Correct.

Exam tip: Remember "Pump out gas, pressure drops, current flows." Whenever you see a discharge tube, think low pressure and high voltage.