Physics

Van de Graaff generator

Electrostatics · High voltage

How can a machine on a school bench build up hundreds of thousands of volts — and why doesn’t the spark kill you?

A Van de Graaff generator uses a moving rubber belt to carry electric charge up into a hollow metal ball, the dome. Charge keeps piling onto the outside of the dome until it reaches hundreds of thousands of volts and a spark jumps through the air. The total amount of charge is tiny, so the spark stings but is normally harmless.

A Van de Graaff generator uses an insulating belt to carry charge continuously into a hollow metal dome, where it spreads over the outer surface and raises the dome to a potential V = Q/(4πε₀R). The voltage climbs until charging is balanced by leakage or until the surrounding air breaks down in a spark.

Key facts

Dome capacitance (R = 12 cm): C = 4πε₀R ≈ 13 pF
Dielectric strength of dry air: ≈ 3 MV/m at sea level
Classroom maximum voltage: ≈ 200–400 kV
Charging current: ~1–10 µA (classroom); up to ~mA in accelerators
Charge on dome at 300 kV: ≈ 4 µC
Invented: Robert J. Van de Graaff, 1929 (Princeton)

Parts you can explore (13)

Metal dome
Insulating column
Rubber belt
Upper roller
Lower roller
Lower comb
Upper comb
Motor
Belt charges
Dome surface charge
Grounded sphere
Spark discharge
Base

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