Van de Graaff generator generates :

Correct — D, high voltage low direct current. A Van de Graaff generator, built by Robert J. Van de Graaff in 1929, is a machine for producing an extremely high steady potential. A motor-driven insulating belt runs between two pulleys; charge is sprayed onto it at the bottom and carried up inside a large hollow metal sphere, where a comb collects it. The trick that makes the machine work is a property of hollow conductors: charge deposited on the inside of a conducting shell moves at once to its outer surface, and the field inside the shell stays zero, so fresh charge can be brought in however high the sphere's potential has already climbed. The potential goes on rising until leakage through the surrounding air balances the rate at which charge arrives, which is why the largest machines are sealed in tanks of insulating gas under pressure. Big ones reach some five million volts, a classroom one around a hundred thousand. What they cannot supply is much current — the belt carries only a trickle of charge per second — and that trickle flows steadily in one direction, so the output is direct, not alternating. That combination, an enormous potential difference at a tiny direct current, is what makes the machine useful: it is a particle accelerator, driving protons and electrons up to high energies for nuclear physics, for sterilising food and for generating energetic X-ray beams.

• (a)electrostatic charge — The sharpest wrong option, because it is not a false statement — the machine is an electrostatic generator and accumulating charge is exactly what the belt does. It fails as an answer because it describes the internal mechanism rather than the output, and because it would equally describe a glass rod rubbed with silk. The pairing of options (c) and (d), identical but for the words alternating and direct, shows what the question is actually asking.
• (b)magnetic field — Nothing in the machine is built to produce a magnetic field. Magnetic fields come from currents and from magnets; this is an electrostatic device whose whole business is potential difference.
• (c)high voltage low alternating current — Right about the voltage, wrong about the current. The belt carries charge in one direction only, so the output is direct. The device that supplies high-voltage alternating current is a step-up transformer, and a transformer cannot work on direct current at all.

The Van de Graaff generator is the standard demonstration of two ideas at once. The first is that the electric field inside a hollow conductor is zero and any charge given to it resides on its outer surface, which is what lets the machine keep adding charge to an already highly charged sphere. The second is that potential and current are independent: a source can sit at millions of volts and still deliver only microamperes, which is why touching a classroom generator makes the hair stand up rather than doing harm, while a mains socket at 230 volts is dangerous.

The item is decided by the second half of the phrase, not the first. Three of the four options are about the right area and only one of them names the right kind of current, so read the options as a pair and ask whether charge on the belt reverses direction. It does not: it is sprayed on at the bottom, carried up and taken off at the top, always the same way round, so the current is direct. It is worth being clear about why 'electrostatic charge' loses even though it is true of the machine. A question asking what a device GENERATES is asking about its output — the thing it exists to supply to something else — and what a Van de Graaff supplies is a very large potential difference. Charge accumulation is how it gets there. Hold the contrast with the transformer alongside: a transformer changes one alternating voltage into another and is helpless with direct current, while the Van de Graaff produces direct current and cannot be stepped in the way a transformer is.

• A Van de Graaff generator produces very high voltage direct current at very low current levels.
• It was built by Robert J. Van de Graaff in 1929.
• Large machines reach around 5 million volts; tabletop demonstration models around 100 kilovolts.
• A moving insulating belt carries charge to a hollow metal sphere, where the charge moves to the outer surface and leaves the interior field at zero — which is why charging can continue indefinitely.
• Its main scientific use is as a particle accelerator, driving protons and electrons to high energies; it is also used to sterilise materials and to produce energetic X-ray beams.

• Choosing 'electrostatic charge' because it is true of the machine; the question asks what it generates as output, and the option set is testing direct against alternating current.
• Assuming that a high voltage must mean a dangerous current; the two are independent, and this machine supplies almost no current.
• Confusing it with a transformer, which needs alternating current and cannot raise a direct voltage.

As an output question like this one, as a mechanism question about the belt and the hollow sphere, or as part of a set on electrostatic shielding and the field inside a conductor.

• practice — not a real PYQ

  In a Van de Graaff generator, charge given to the inner surface of the hollow metal sphere:

  • (a)stays where it is placed
  • (b)moves at once to the outer surface
  • (c)spreads evenly through the metal
  • (d)leaks back along the belt

  Answer(b) moves at once to the outer surface — charge on a conductor resides on its outer surface and the field inside remains zero, which is what allows the sphere to be charged to very high potentials.

• practice — not a real PYQ

  A Van de Graaff generator is used mainly for which one of the following purposes in physics?

  • (a)Generating electricity for domestic supply
  • (b)Accelerating charged particles to high energies
  • (c)Producing strong magnetic fields
  • (d)Measuring very small currents

  Answer(b) Accelerating charged particles to high energies — its very high potential is used to drive protons and electrons down an evacuated tube for nuclear physics experiments.