Thermionic Emission from Hot Filaments

Steps:

• Identify the process: Continuous electron flow from a metal filament occurs via thermionic emission, where heat frees electrons.
• Evaluate condition 1: Heating the filament provides thermal energy to overcome the metal's work function, enabling electron emission.
• Evaluate condition 3: A high positive potential attracts and accelerates emitted electrons, creating a sustained current.
• Dismiss others: High-pressure gas and magnetic fields are unnecessary for basic thermionic flow in vacuum tubes.

Why C is correct:

• Thermionic emission requires heating (1) to boil off electrons and positive potential (3) to draw them out, as described by the Richardson-Dushman equation: J = A T² e^(-φ/kT), where potential ensures collection.

Why the others are wrong:

• A: Magnetic field (4) influences electron paths but does not initiate emission or flow.
• B: High-pressure gas (2) enables glow discharges, not filament-based thermionic emission.
• D: Gas (2) and magnetic field (4) are irrelevant to producing electron flow from a hot metal.

Final answer: C