Static electricity · Process
Charging by rubbing
Rubbing does not make charge. It moves electrons from one surface to the other, so the two objects end up equally and oppositely charged — and the total is exactly what it was before.
Start here
The rod comes out negative. What about the duster?
A polythene rod rubbed with a woollen duster picks up small pieces of paper, and a careful test shows the rod is negatively charged. The duster was neutral before you started, and so was the rod.
What is the duster now?
Positive, by exactly as much. Every electron the rod gained came off the duster, so the duster is left short of electrons and its protons are no longer balanced. Nothing was created and nothing was lost — charge was separated. Hang the duster up on a thread and it will attract the rod, which is the next lesson.
Every atom holds positive protons in a nucleus and negative electrons around it, and in a neutral object the two exactly balance. The protons are locked into the nuclei and cannot go anywhere. Only the electrons can move — which means every piece of static electricity you have ever met is a story about electrons and nothing else.
Rubbing two surfaces together presses them into contact at millions of tiny points. Some materials hold their outer electrons more tightly than others, so at those points of contact electrons cross over from the looser material to the tighter one. Pull the surfaces apart and the electrons stay where they went.
So the object that lost electrons is left with more protons than electrons: it is positively charged. The object that gained them has more electrons than protons: it is negatively charged. Both are charged, by exactly the same amount, in opposite directions — and nothing was created. This only works with insulators, because a conductor would let the charge run straight back or away to earth through your hand.
At the bench · two dry insulators, rubbed together
Pick the pair. Count the rubs.
Change a control to begin
Two objects, both neutral to start with, both dry. Choose what is in each hand and how many strokes you give them.
Commit first. You rub a polythene rod with a second, identical polythene rod. What happens?
In your left hand
In your right hand
8 strokes
Electrons that crossed
—
—
Left-hand object
—
—
Right-hand object
—
—
The two added together
0.0 nC
nothing was created
The figure
Which way the electrons go
Materials can be put in an order by how tightly they hold their outer electrons. Rub any two together and the one higher up this list loses electrons to the one lower down, so the higher one ends up positive. The further apart they are on the list, the more electrons cross.
- Human hairmost likely to end up positive
- Glass rod
- Acetate strip
- Wool duster
- Cotton clothmiddling — poor at either job
- Polythene rod
- PVC pipemost likely to end up negative
Nothing on this list is a conductor or an insulator by accident: every one is an insulator, which is what lets the charge stay where the rubbing put it. Try the same experiment with a metal rod held in your hand and no charge builds up at all — it runs away through you.
Key fact
Rubbing two insulators together transfers electrons from one surface to the other. Protons never move. The object that loses electrons is left positive, the one that gains them is negative, both by the same amount — so no charge is created and the two together are still neutral.
Think again
“Rubbing makes the charge — the friction creates it.”
Nothing is made. Every electron that ends up on the duster was on the rod a moment earlier, and the two objects together carry exactly the charge they carried before you touched them: none. The rubbing supplies contact, not charge. It matters because it presses the surfaces together at millions of points and then peels them apart, and each of those contacts is a chance for an electron to change sides. Press two surfaces together firmly and lift them straight off and you get some charge without rubbing at all.
“A positively charged rod has had positive charge added to it.”
Nothing positive was added. It has had electrons taken away, and the protons that were always there are now unbalanced. This is worth being fussy about, because it is the reason the two objects always end up opposite: one loses exactly what the other gains. Protons sit in nuclei, bound by a force enormously stronger than anything a duster can supply, and in ordinary matter they never go anywhere.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Predict the signs
A glass rod is rubbed with a wool duster. Glass sits above wool on the list, so which statement is right?
Rung 2 · The one that catches people
A student rubs a balloon on a jumper, finds the balloon negative, and says the friction created the negative charge. What is right?
Rung 3 · Explain
A polythene rod is rubbed with a woollen duster and ends up negatively charged. Explain what has happened, naming what moves and what does not, and say what the duster is now.
Rung 4 · Take it somewhere new
A road tanker delivering fuel is connected to a metal spike in the ground before any fuel is pumped, and the pumping is stopped if the connection comes off. Explain what is being prevented, and why the same precaution is pointless for a plastic bottle of water.
Key note
In a neutral object the protons and electrons balance. Rubbing two insulators together presses their surfaces into contact at many points, and electrons cross from the material that holds them less tightly to the one that holds them more tightly. Protons never move. The object that loses electrons is left positively charged and the one that gains them negatively charged, by exactly the same amount, so no charge has been created — it has been separated. It only works with insulators, because a conductor lets the charge escape.
Going further
The order of materials is called the triboelectric series, and the honest truth is that published versions disagree with each other. Where a material lands depends on how rough its surface is, how clean it is, how humid the room is and even which way it was rubbed, and some pairs reverse if you swap a polished sample for a scratched one. The list is a reliable guide for glass, polythene and PVC, and a rough one in the middle — which is why cotton, sitting near the middle, is a poor choice for a demonstration.
Humidity is the reason this experiment fails on a wet day. Water is a far better conductor than any of these materials, and a film of it a few molecules thick on the surface is enough to let charge creep away as fast as the rubbing separates it. Winter is the season of static shocks for the same reason: cold air holds very little water, so indoor air that has been heated is extremely dry, and charge separated by your shoes on a carpet has nowhere to go until you touch a door handle.
Before this lesson
Connects to
At GCSE this becomes
- Charge measured in coulombs, sparking and the breakdown of air, and the uses and dangers of static charge in industry.
Where to next
- Next: Forces between charges
- Previous: Building and measuring a circuit
Electric circuits
Ask Mr Badmus AI
Got two materials and want to know which one ends up negative?
The bench is a teaching model. The order of the seven materials is the commonly published one, but real triboelectric series disagree with each other and a material's position shifts with surface roughness, cleanliness and humidity — so the direction of transfer for two materials close together on the list should be treated as a likely outcome, not a certainty. The number of electrons is generated by a simple rule — it rises with the number of steps apart on the list and with the number of strokes, but the stroke term levels off towards a ceiling rather than climbing without limit, because a real charge leaks away and because the air eventually breaks down. The figures are chosen to give charges of the order of a few nanocoulombs, which is typical of a rubbed rod, and are not measurements. Charges are rounded to one decimal place.
Lesson content © MrBadmusAI.