The Van de Graaff generator accumulates charge on a hollow metallic sphere to create high potentials (millions of volts). Principle: charge placed inside a conducting shell transfers completely to the outer surface regardless of the shell's existing charge (because $V_{inside}$ >= $V_{surface}$, so charge flows outward). A continuous belt carries charge from a source to the inside of the sphere. The limiting voltage depends on the radius: $V_{max}$ ~ $E_{breakdown}$ * R (for air, $E_{breakdown}$ ~ 3 x 10^6 V/m, so a 1 m sphere can reach ~3 MV). Applications: particle accelerators, X-ray generation, nuclear physics experiments. JEE relevance: understand the principle (not detailed design).