Question

In a cathode ray tube state:
(i) the purpose of covering cathode by thorium and carbon.
(ii) the purpose of the fluorescent screen.
(iii) how is it possible to increase the rate of emission of electrons.

Answer 1

(i) The cathode is covered with thorium and carbon as it has low work function and it needs to be heated up to 2000 K to emit electrons. This serves it as a better electron emitter than tungsten.

(ii) The fluorescent screen helps in changing the electrical signal applied on the deflecting plates into the visual pattern on the screen.

(iii) The rate of emission of the electrons can be increased by

1. Using a metal with a lower work function,
2. Increasing the temperature of the surface of the metal, or
3. Increasing the surface area of the metal emitting the electrons.