The conductivity of gases at low pressure can be termed as I. hot cathode emission II. thermo ionic emission III. cold cathode emission IV. Field emission

Analysis

The question asks us to identify the specific phenomena associated with the conduction of electricity through gases when the pressure is kept very low (often in discharge tubes).

Concept Explanation

When a gas is at low pressure, it becomes a conductor of electricity if a sufficiently high potential difference is applied across it. This process involves the ionization of gas molecules.

1. Cold Cathode Emission: In a gas discharge tube at low pressure, the cathode is not heated. Instead, positive ions from the ionized gas strike the cathode with high kinetic energy, knocking off electrons. This is known as secondary emission or cold cathode emission.
2. Field Emission: When a very strong electric field is applied, the field itself is strong enough to pull electrons out of the surface of the cathode. This is known as field emission (or auto-electronic emission).

Thermo-ionic emission and hot cathode emission (which are essentially the same thing) require the cathode to be heated to high temperatures to "boil off" electrons. These do not describe the standard conductivity of gases at low pressure in a cold discharge tube.

Step-by-Step Solution

• Step 1: Evaluate the options provided. We are looking for the processes that allow current to flow in a gas discharge tube without external heating.
• Step 2: Identify that "Cold cathode emission" (III) occurs because the cathode is not heated; it relies on ion bombardment.
• Step 3: Identify that "Field emission" (IV) occurs because the high potential difference creates a strong electric field that extracts electrons.
• Step 4: Compare these with the provided options. Since III and IV are the correct physical processes for this phenomenon, we look for the option containing both.

Conclusion

The conductivity of gases at low pressure is primarily driven by the extraction of electrons from the cathode via ion bombardment (cold cathode emission) and the influence of strong electric fields (field emission). Therefore, III and IV are the correct descriptions.

Exam Tip

Remember: "Cold" means no heat is applied to the cathode. If you see "low pressure gas discharge," think of processes that don't require a heating filament. If the question mentioned a "thermionic valve" or "vacuum tube," then you would look for "hot cathode" or "thermionic" emission.