Experiment 17: Static Electricity — Balloon and Paper

Rub a balloon on your jumper, hold it near scraps of paper, and the scraps leap up as if bewitched. The trick is static electricity: separated charge exerting force at a distance, governed by an inverse-square law, F ∝ q/r² in pattern. In this experiment you will rub, hover and count lifted bits — then use the simulation to show that doubling the distance quarters the force.

The physics behind the practical

Rubbing transfers electrons between surfaces: the balloon gains electrons and becomes negatively charged (order of a few microcoulombs, μC, in the sim's terms), the jumper is left positive. Charge is conserved — rubbing *separates* charge, never creates it. The balloon's field then reaches out and polarises nearby neutral paper: electrons in the paper are repelled slightly away from the balloon, leaving the near side of each scrap slightly positive — and opposite charges attract, so the neutral scrap is pulled towards the balloon. Attraction of neutral objects is always this polarisation trick; it is also why the same balloon sticks to a (neutral) wall or ceiling.

Coulomb's law gives the force between two charges as F = kq₁q₂/r², with k = 9 × 10⁹ N·m²/C²: the force grows with either charge and falls with the square of the distance — halve the distance, quadruple the force. That steep fall is the whole experimental story: at 25 cm the balloon lifts nothing, at 4 cm it lifts everything. Humidity sabotages static by coating surfaces with a microscopically damp, slightly conducting film that lets the charge leak away — static experiments famously work better on dry days.

Variables and controls

In the virtual lab

Set Balloon charge to 6 μC and lower the balloon with the Height above paper slider: at 25 cm the readouts show a small Electric force and Paper bits lifted: 0 of 12; at 16 cm, a couple of bits twitch up; at 8 cm most of the tray is airborne; at 4 cm all 12 stick. Doubling the charge at a fixed height roughly doubles the lifted count; the force readout climbs as 1/r². Follow the force-vs-distance curve in the Data Visualization — steep cliff near the paper, flattening with height: the inverse-square signature again. Log charge, height, force and bits lifted for at least four heights.

Hands-on: the balloon lift-off

Tear 12 similar tissue bits onto a dry tray. Rub the balloon on a wool jumper exactly 10 times, then lower it slowly from about 3 cm above the bits and count what sticks. Repeat at 2 cm and 1 cm. Compare with a balloon rubbed 20 times, and — if your climate allows — on a humid day vs a dry day. Discharge between conditions by touching the balloon to a metal tap. Charge is small and the shock risk trivial here, but keep static away from anyone with a pacemaker concern about deliberate shocks, and never take balloons near flames — and never charge anything from a wall socket, ever.

A worked investigation

Sim data at q = 6 μC, bits lifted from the tray of 12:

Step 1 — Verify the inverse square on the force. Taking the readout F = 0.90 mN at 12 cm: at 6 cm, expect 0.90 × (12/6)² = 0.90 × 4 = 3.6 mN; at 24 cm, 0.90 × (12/24)² = 0.90 ÷ 4 = 0.225 mN. Force lives and dies by distance².

Step 2 — Why bits lift in jumps. Gravity on each bit is fixed; the electric force rises steeply as distance falls. A bit lifts only when F exceeds its weight, so the count rises in steps — 0, 2, 5, 9, 12 — rather than smoothly: a threshold phenomenon, the same shape as a magnet picking up pins one by one.

Step 3 — The bench echo. With 10 rubs and a dry tray, bits at 1 cm: 10 lifted; 2 cm: 4; 3 cm: 1. Fewer than the sim's count at similar settings — real balloons leak charge to the air and the bits are heavier — but the trend with distance is identical, which is the physics being tested.

Step 4 — Uncertainty. "Bits lifted" is a coarse, countable variable: ±1 bit between repeats is normal, and the height judgement is ±0.5 cm — at 4 cm that is ±12% in distance, ±27% in force. Coarse variables suit discovering the trend; precise force laws come from the sim readout and (in research labs) electrometers, not from counting tissue.

Conclusion and evaluation

Force and lifting matched Coulomb's pattern: ∝ charge, ∝ 1/distance², acting on *neutral* paper through polarisation. The method's enemies are charge leakage (humidity, handling, time between rubbing and testing) and inconsistent rubbing — both controlled imperfectly here by counting rubs and working briskly. The deeper lesson stands: electrostatic forces are strong only at short range because the universe divides them by r²; that is why the balloon must nearly touch the paper to work its magic.

Safety

Dry hands only; balloons away from flames and from faces if anyone has a latex allergy; static shocks are trivial but deliberate shock pranks are off the table; nothing in this experiment ever involves mains electricity.

Common mistakes

• Rubbing "hard" instead of consistently — charge depends on rubs counted and materials paired, not enthusiasm.
• Testing slowly after charging — the balloon leaks charge to the air while you chat; rub, then test at once.
• A humid day written off as "static doesn't work" — the damp film is conducting the charge away; that is a result, not a failure.
• Counting bits at different hover heights — height is your variable; fix it per count.
• Claiming the paper became charged — before contact the scraps are neutral and polarised; attraction proves polarisation, not net charge.

Exam Experience

Question 1 — calculation [4 marks]. A balloon's charge is unchanged while its distance from a paper bit doubles from 4 cm to 8 cm, and then halves to 2 cm. Using an initial force of 1.6 mN at 4 cm, calculate the force at 8 cm and at 2 cm.

Scoring guide:

• F ∝ 1/r², so doubling r divides F by 4 [1]
• F(8 cm) = 1.6 ÷ 4 = 0.40 mN [1]
• Halving r (4 → 2 cm) multiplies F by 4 [1]
• F(2 cm) = 1.6 × 4 = 6.4 mN [1]

Question 2 — evaluation and design [4 marks]. A class wants to test the claim "humidity kills static". Design a fair comparison of bits lifted on a dry day vs a humid condition.

Scoring guide:

• Independent variable: humidity condition (dry room vs humidified — e.g. near a steaming kettle, safely, or a humid day) [1]
• Dependent: bits lifted at a fixed balloon height, out of the same 12 bits [1]
• Controls: same balloon, same 10 counted rubs, same bits (dried between conditions), same height and timing after rubbing [1]
• Repeat 3× per condition and compare means against spread before accepting the claim [1]

Multiple choice 1. A charged balloon attracts a *neutral* scrap of paper because:

• A. The paper was secretly charged all along
• B. The balloon's field polarises the paper — the near side gains opposite induced charge, which is closer, so attraction wins
• C. Gravity reverses near charged objects
• D. The paper becomes a magnet

Answer: B. Induced charge separation puts opposite charge nearer the balloon than the like charge, so the net force is attractive — no net charge needed.

Multiple choice 2. Two balloons rubbed the same way are brought near each other. They:

• A. Attract, because both hold charge
• B. Repel, because both carry the same sign of charge
• C. Do nothing — balloons cannot affect each other
• D. Discharge instantly and fall

Answer: B. Rubbing loads both with the same sign; like charges repel, which you can feel as the balloons push apart on their strings.

Going deeper: how much charge is a microcoulomb?

Charge sounds abstract until you count it. One coulomb is about 6.24 × 10¹⁸ electrons' worth of charge, so the balloon's 6 μC is roughly 6 × 6.24 × 10¹² = 3.7 × 10¹³ electrons — thirty-seven trillion extra electrons, spread over the balloon's skin, and still so few, proportionally, that the balloon's total electron count changed by less than one part in a billion. Static effects are surface whispers with outsized consequences, purely because electric forces are intrinsically strong.

Scale the other way for awe: a lightning bolt transfers of order 15 C — about two and a half million times the balloon's charge — in a fraction of a second, which is why it splits trees while the balloon only lifts tissue. Between those extremes sit the Van de Graaff generator (microamps of current at hundreds of kilovolts, enough for hair-raising demonstrations) and the photocopier and laser printer, which paint with charged toner using exactly this lesson's polarisation-and-attraction trick, pixel by pixel. Electrostatics is not a party trick with delusions of grandeur; it is an industry that also does parties.

One more angle: why balloon and jumper?

Rubbing separates charge because the two materials *rank differently* in their grip on electrons — the idea behind the triboelectric series, a league table of electron-greed. Balloon rubber sits far from wool in that table, so their pairing transfers charge eagerly; two identical balloons rubbed together would transfer almost nothing, having equal claims. Choosing materials from distant rows of the series is the practical secret of every static demo, from the balloon lift to the Van de Graaff's belt. When a static experiment underwhelms, suspect the pairing before the physics.