There are two kinds of electric charge: positive and negative. Like charges repel each other; opposite charges attract. Around every charge there is an invisible "electric field" that determines what force any other charge will feel. This guide turns those abstract ideas into things you can see, using the simplest static electricity experiments there are.

Safety first (read this before you start)

  • Everything here is safe static electricity — the amounts of charge are tiny and cannot hurt anyone. Work in a dry environment, though: humid weather makes charging difficult.
  • Do not build up static near flammable gases or alcohol. This experiment has nothing whatsoever to do with household wiring — never try anything with a wall outlet.

Materials and equipment

  • One or two balloons, or a plastic ruler or pen barrel;
  • Paper torn into tiny scraps (the lighter the better);
  • Dry hair, or a piece of wool or synthetic cloth;
  • Optional: a very thin stream of tap water.

Three effects to see first

Effect one: picking up paper scraps. Rub a plastic ruler briskly on your hair a dozen times, then bring it near the scraps — they jump up and stick to it.

Effect two: bending a stream of water. Bring the rubbed ruler or balloon close to (without touching) a very thin stream of tap water. The stream bends visibly toward it.

Effect three: like charges repel. Rub two balloons on your hair, hang them from strings, and bring them together — they push each other apart.

What happened: rubbing transfers electrons from one object to the other, leaving one negatively and the other positively charged — that is charging by friction. A charged object can attract small, light objects, and it can push or pull on another charged object.
Interactive simulation: this experiment has a drag-and-play simulator in our Interactive Lab. The lab interface is currently Chinese-only — the controls are simple sliders and switches, so it is still easy to explore.

Recording what you see

No stopwatch or ruler needed here — but do write the observations down one by one, because each points to a different conclusion:

  • How high the scraps jump depends on how much you rubbed. A dozen rubs versus three or four makes an obvious difference: more rubbing transfers more electrons, so the object carries more charge.
  • The scraps stick, then spring away. Students usually think they've failed — in fact this is the best moment of the whole experiment. On contact, the scraps pick up some of the same charge as the ruler, switching from "attracted" to "repelled." Record the full attract-then-bounce sequence.
  • The water stream only bends; it never snaps over to the ruler. The water is uncharged — it is pulled by being polarized by the charged object, so the deflection fades fast with distance. Move the ruler from 1 cm away to 5 cm and the bending nearly vanishes: the force between charges falls off quickly with distance.
  • The angle between the two repelling balloons slowly shrinks over a minute or two. Their charge is quietly leaking away through the air and the strings — the more humid the day, the faster the leak.
Not working? It's almost always the weather or the materials: ① Humid air (in a rainy season this experiment barely works at all) — pre-dry the ruler and your hair with a hair dryer. ② Sweaty palms let the charge leak away through your hand — hold the ruler with a dry cloth instead. ③ Scraps too big and heavy — 2–3 mm squares are the most responsive. ④ An oily film on the ruler — wipe it with a dry tissue before rubbing.

The science: charges and fields

Nature has exactly two kinds of charge. The charge on a glass rod rubbed with silk is defined as positive; the charge on a rubber rod rubbed with fur is negative. The rules between them:

  • Like charges repel; opposite charges attract;
  • A charged object attracts small, light, uncharged objects (paper scraps, a water stream);
  • The force between charges grows stronger as they get closer.

To describe how a charge exerts force on other charges across empty space, physics introduces the electric field: the space around a charge contains a field, and any other charge placed in it feels a force. The field has a direction — defined as the direction of the force on a positive charge at that point — which is why field lines always run out of positive charges and into negative ones. Every arrow in the simulation is the field direction at that point.

Does rubbing actually "create" electricity? No. Charging by friction is really electron transfer: when two objects are pressed into close contact, the one that holds its electrons more loosely gives some up and becomes positive, while the one that gains them becomes negative. The total charge of the whole system never changes — that is conservation of charge. So the two rubbed objects always end up with equal and opposite charges: when the ruler goes negative, your hair carries exactly that much positive charge.

How to tell which kind of charge an object carries

"It attracts paper scraps" can't tell you the sign — positive and negative charges both attract light objects. The laboratory method is to prepare a "reference" of known sign first, then use repulsion to decide:

  1. Rub a rubber rod with fur; by definition it is negative;
  2. Bring the unknown object near this rod (hang both from strings). If they repel, the unknown is also negative;
  3. If they attract, the unknown is positive — or simply uncharged. This case must be double-checked with a positively charged glass rod.

Remember the rule: repulsion always means like charges; attraction has two possible explanations. That is why the sign of a charge can only be established by repulsion, never by attraction. A school electroscope works on the very same principle: touch a charged object to the metal ball and the charge spreads to the two thin metal leaves below; carrying the same charge, the leaves repel and spread apart — the wider the angle, the more charge.

Static electricity in everyday life

  • The crackle and tiny sparks when you pull off a sweater in winter; the little zap from a car door — charge built up between your body and the seat, discharging;
  • Hair that floats and spreads apart after combing — every strand carries the same charge;
  • Photocopiers, electrostatic air cleaners and spray painting all put the attraction between charges to work;
  • And the reverse: gas stations ban rubbing synthetic clothing, and fuel tankers drag a grounding chain, all to bleed static away safely before a spark can ignite fuel vapor. Static carries very little charge but very high voltage — around flammable gases, that is dangerous.

Three things students get wrong most often

  • "Rubbing generates electricity." Charge is never created from nothing — it only moves from one object to the other. The two objects end up with equal and opposite charges; the total is still zero.
  • "If it's attracted, it must carry the opposite charge." Not necessarily. Uncharged light objects (paper scraps, a water stream, hair) are attracted to charged objects too. Only repulsion proves like charges.
  • "Static electricity is the same as the electricity in wall outlets." It is not. Static involves a tiny amount of charge and a momentary discharge; an outlet delivers a sustained, powerful current and must never be experimented with. Everything in this experiment happens across a few millimeters to a few centimeters and has nothing to do with household wiring.

Tips for teachers and parents

  • Timing and groups: 20 minutes; works best in dry weather; the whole class can do it simultaneously.
  • Say it before hands touch equipment: keep away from alcohol and anything flammable; make explicit that this has nothing to do with household electricity.

The step most people skip. In very humid weather the experiment barely works — pre-dry the equipment with a hair dryer or move to an air-conditioned room. And make absolutely sure students see and record the scraps' "stick, then spring away" moment: it is the direct evidence of charging by contact.

How to tell they really understand. If a student can state "repulsion always means like charges; attraction has two possible causes," and explain that friction transfers electrons rather than creating charge, they've met the standard.

Take it further

  • In the simulator, set both charges positive and watch the field arrows "push each other apart"; then compare with the pattern for one positive and one negative charge.
  • Compare how well charging works on a dry day versus a humid one, and think about why humidity matters. (Hint: water molecules in humid air let charge leak away more easily.)
  • Build a simple electroscope: hang two small strips of aluminum foil from the lower end of a paper clip, push the clip through a plastic bottle cap into an empty bottle, and bend the exposed end into a little hook. Touch the hook with a rubbed ruler and watch the foil strips spread apart.
  • Try a no-contact experiment: press a rubbed balloon against a wall and it clings there for quite a while. The wall isn't charged — so why does it "hold" the balloon?

When a student can use "positive and negative charge, forces between charges, field direction" to explain why a balloon picks up paper scraps, this experiment has done its job.