Match the statement in List-I with its reason in List-II.
| List-I | List-II |
|---|---|
| (A) Vacuum tubes are also called valves. | (I) The charge carriers are supplied and move within the solid itself. |
| (B) A vacuum tube needs vacuum between its electrodes. | (II) Electrons can flow only from the cathode to the anode. |
| (C) A semiconductor device needs no heated cathode and no evacuated space. | (III) Light, heat or a small applied voltage can change the number of its mobile charges. |
| (D) The flow of charge through a semiconductor can be controlled. | (IV) Moving electrons would lose energy in collisions with air molecules. |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A)-(II), (B)-(IV), (C)-(I), (D)-(III)
Before transistors, most electronic devices were vacuum tubes. In a tube, a heated cathode gives out electrons, and they move through vacuum to the anode. In a semiconductor device, the charge carriers are produced and move inside the solid.
(A) Vacuum tubes are also called valves. — This matches (II). Current can pass only one way, from the cathode to the anode. A device that lets something flow in one direction only is called a valve.
(B) A vacuum tube needs vacuum between its electrodes. — This matches (IV). With air inside, the moving electrons would hit air molecules and lose their energy on the way.
(C) A semiconductor device needs no heated cathode and no evacuated space. — This matches (I). The carriers are already present in the solid and move through it. So no heating is needed to release them, and no vacuum is needed for their path.
(D) The flow of charge through a semiconductor can be controlled. — This matches (III). A small change in light, heat or voltage changes the number of mobile charges, and so the current changes.
NCERT (2026-27):
Introduction – “… In these devices the electrons can flow only from the cathode to the anode (i.e., only in one direction). Therefore, such devices are generally referred to as valves. …”
Introduction – “… Vacuum is required in the inter-electrode space; otherwise the moving electrons may lose their energy on collision with the air molecules in their path. …”
Introduction – “… Note that the supply and flow of charge carriers in the semiconductor devices are within the solid itself, while in the earlier vacuum tubes/valves, the mobile electrons were obtained from a heated cathode and they were made to flow in an evacuated space or vacuum. No external heating or large evacuated space is required by the semiconductor devices. …”
Introduction – “… Simple excitations like light, heat or small applied voltage can change the number of mobile charges in a semiconductor. …”