Question

The diagram below shows a cathode ray tube in which the arrow indicates a beam of charged particles approaching an electric field between the plates A and B. State the approximate voltage $V_1$ used to heat the filament.

• A. 1.5 V
• B. 3 V
• C. 6 V
• D. 12 V

Answer 1

Answer: (C)

6 V

Thermal energy given to the charge carriers overcome the work function of the metal surface and thus flow of charge carriers (electrons) takes place from the metal surface. The work function of the tungsten filament used is $4.5eV$. Thus to overcome this work function, a slightly higher potential is applied to heat the filament which is $6V$.