With a labelled diagram, explain the principle, construction and working of a Van de Graaff generator. State one use.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the principle.
Charge given to the inside of a hollow conductor moves to its outer surface.
Sharp points cause corona discharge, so they can spray or collect charge.
2 · Draw the diagram. [Sphere S on an insulating stand; belt over pulleys P₁ (with motor M) and P₂ (inside S); combs C₁ and C₂; power supply to C₁; earthed chamber T]
3 · Name the parts. Hollow metal sphere S, insulating belt, pulleys P₁ and P₂, motor M, combs C₁ and C₂, and the earthed chamber T of high-pressure air.
4 · Explain the working. Corona at C₁ (about 10⁴ V) puts positive charge on the belt; the belt carries it into S; at C₂ the charge passes to the sphere and moves to its outer surface.
5 · Say why the voltage rises. The inside of S stays free of charge, so the process repeats and S reaches a few million volts; leakage sets the limit.
6 · State a use. It accelerates ions to high energies for the study of nuclear reactions.
7 · Close in one line. So the generator stores charge on the outside of a hollow sphere to reach very high voltages.
Scoring tip: Label at least six parts. The marks for this question usually split between the principle, the diagram and the working.
