In photoemission, what is used to cause electrons to flow in a vacuum tube?

• A. Negative plate
• B. Positive plate
• C. Neutral plate
• D. Ground plate

Answer: (B)

In the context of photoemission within a vacuum tube, a positive plate plays a crucial role in facilitating the flow of electrons. When light strikes the photosensitive material, it causes electrons to be emitted from that surface. These free electrons then need a potential difference to be drawn away from the surface and directed towards the positive plate.

The positive plate creates an electric field that attracts the emitted electrons. This attraction is essential for collecting the electrons and establishing a measurable current in the circuit. Without the positive plate applying a potential difference, the emitted electrons would simply remain near the surface where they were released, and no flow of electrons would occur.

The other options—negative, neutral, and ground plates—do not effectively contribute to this process. A negative plate would repel the electrons, a neutral plate would have no effect on the electrons due to a lack of potential difference, and a ground plate might serve a purpose in different contexts but does not actively facilitate the attraction of electrons in this specific process.