Mr. Scandrett's ClassroomOS

Physics · Lesson 14

Van de Graaff Balloon

Electrostatics Coulomb's Law Triboelectric Effect Induced Charge

A small balloon rubbed against hair or wool acquires a static charge, then leaps toward — or flies away from — a Van de Graaff generator's dome. No wires, no magnets, just the invisible electric force described by Coulomb's law, visible at human scale.

Who Was Robert J. Van de Graaff?

Robert J. Van de Graaff (1901–1967) was an American physicist who built the first Van de Graaff generator in 1929 at Princeton. Using a silk ribbon and a tin can, he could accumulate millions of volts of static electricity. The machine became essential for nuclear physics experiments — accelerating particles into atomic nuclei — and is still used as a teaching tool in science classrooms worldwide.

The Core Concept

When a balloon is rubbed against hair or wool, electrons transfer between the two materials by the triboelectric effect. The balloon gains extra electrons (becomes negative) or loses them (becomes positive). The Van de Graaff dome is always positively charged.

Three Scenarios

+ Balloon Dome is +, balloon is +. Like charges repel — balloon flies away.
Neutral Dome polarizes the balloon — near side goes negative. Weak attraction.
− Balloon Dome is +, balloon is −. Opposite charges attract — balloon swings toward dome.
Induced charge (induction): A charged object can attract a completely neutral object by pushing charges within it to one side, creating an induced dipole. The near side becomes oppositely charged, giving a net attractive force even with no net charge on the neutral object.

Interactive Simulator

The dome is always positively charged. Set the balloon charge using the panel controls. Watch the string deflect and the force arrow change direction.

Dome Charge+5 μC
Balloon Charge−2 μC
ForceAttract

Real-World Applications

🔬
Classroom Demos Van de Graaff generators make electrostatics visible — hair stands on end, sparks jump, balloons fly. Abstract concepts become tangible.
🎨
Paint Spraying Charged paint droplets repel each other → uniform, fine coating. Used in car manufacturing for a perfect finish with minimal waste.
💨
Air Filtration Electrostatic precipitators use charged plates to attract and trap dust, smoke, and pollen — cleaning industrial exhaust and indoor air.
🖨️
Xerography Photocopiers use a charged drum to attract toner particles to the right spots, then transfer and fuse them onto paper. Pure Coulomb force at work.

Practice Problems

Use F = kQq/r². k = 9 × 10⁹ N·m²/C².

Easy1. A positively charged balloon will be attracted to a negatively charged dome. True or False?

Hint: Opposite charges attract — positive and negative pull toward each other.

Easy2. Two identically charged objects will do what when placed near each other?

Hint: Like charges (both + or both −) always repel each other.

Medium3. A neutral balloon near a positive dome becomes polarized — its near side goes negative. This process is called:

Hint: Induction = charge redistribution caused by a nearby charged object, without direct contact.

Medium4. The dome has Q = +5 μC and the balloon has q = −2 μC at r = 0.3 m. Calculate F = kQq/r² (give the magnitude in N, 2 sig figs).

Hint: F = 9×10⁹ × 5×10⁻⁶ × 2×10⁻⁶ / 0.09 = 90×10⁻³ / 0.09 = 1.0 N

Challenge5. The electric field E at distance r from a sphere of charge Q is E = kQ/r². If a 1 μC charge sits 0.5 m from the dome (Q = +10 μC), what force does it feel? [0.36 N]

Hint: E = 9×10⁹ × 10⁻⁵ / 0.25 = 360,000 V/m; F = qE = 10⁻⁶ × 360,000 = 0.36 N