The electrons in a cathode-ray tube are produced by?

Analyze the question: It asks how a cathode-ray tube (CRT) produces its electrons, meaning the source of the electrons, not how they are deflected or focused.

Concept: Thermionic emission In a CRT, the electron gun contains a cathode with a heated filament. When a metal is heated strongly, some of its free electrons gain enough energy to overcome the metal's surface binding energy (the work function) and escape. This process is called thermionic emission. A high positive potential at the anode then accelerates these electrons into a beam.

Checking each option:

• A. Heating a metal filament. Correct. This is thermionic emission, the actual source of electrons in a CRT.
• B. Ionization of the air. Wrong. A CRT is a highly evacuated tube, so there is almost no air. Ionizing gas is how a gas discharge tube works, not a CRT.
• C. Applying a magnetic field to the y-plates. Wrong. The Y-plates are electric deflection plates that move the beam vertically. They do not produce electrons.
• D. Applying an electric field to the x-plates. Wrong. The X-plates deflect the beam horizontally. They only steer electrons that already exist.

Summary of CRT parts:

1. Heated filament and cathode: produce electrons.
2. Grid: controls the brightness (the number of electrons).
3. Anodes: accelerate and focus the beam.
4. X- and Y-plates: deflect the beam.
5. Fluorescent screen: glows where electrons strike it.

Answer: A. Electrons in a CRT are produced by heating a metal filament (thermionic emission).

Exam tip: Remember "Produce = Heat, Steer = Plates". If a question asks where electrons come from, think of heating (thermionic emission). If it asks about deflection, think of the X- and Y-plates.