Gases conduct electricity under

Analysis

The question asks for the specific physical conditions required for a gas, which is normally an insulator, to become a conductor of electricity.

Concept Explanation

Under normal conditions (standard atmospheric pressure and temperature), gases are excellent insulators because their molecules are neutral and far apart, meaning there are no free charge carriers to conduct electricity.

To make a gas conduct electricity, you must create ions (charged particles). This is achieved through a process called ionisation.

1. Low Pressure: By reducing the pressure, the gas molecules are spread further apart. This increases the "mean free path" of electrons, allowing them to gain enough kinetic energy from an electric field to collide with gas molecules and knock off electrons (ionisation).
2. High Voltage: A high potential difference (voltage) is required to provide the strong electric field necessary to accelerate these electrons and maintain the flow of current through the gas.

Step-by-Step Solution

1. Identify the state of the gas: At high pressure, gas molecules are packed too closely, leading to frequent collisions that prevent electrons from gaining enough energy to ionise the gas. Therefore, high pressure is incorrect.
2. Evaluate the voltage: Low voltage is insufficient to overcome the ionisation energy of gas molecules. Therefore, low voltage is incorrect.
3. Combine the requirements: To facilitate a discharge (conduction), we need to reduce the density of the gas (low pressure) and apply a strong force to move the charges (high voltage).
4. Conclusion: Gases conduct electricity only when subjected to low pressure and high voltage. This is the principle behind devices like neon signs and fluorescent tubes.

Correct Option

Comparing this to your options, the correct condition is low pressure & high voltage.

Exam Tip: Think of a fluorescent tube in your classroom. It contains gas at low pressure, and the starter/ballast provides the high voltage "kick" needed to make the gas glow (conduct). This will help you remember that "Low Pressure + High Voltage = Light/Conduction."