Electrostatic Charge Investigation
Rub a plastic ruler on your hair and hold it over torn-up paper. The paper jumps.
That is the whole of electrostatics in one move, and everything else in this topic is built on it. You will need a ruler, a roll of sticky tape, a sheet of paper and a dry day.
The dry day is not a joke. More on that below, because it is the reason this practical fails in Durban in February and works perfectly in Johannesburg in July.
What charge is, and how to test for it
Charge is a fundamental property of objects that makes them experience and exert electrical forces.
You cannot see it, so you need a test, and this one works throughout:
An object is charged if it attracts small bits of paper.
The control most people skip
Rub the ruler, hold it near the paper, and the paper flies up. Obvious conclusion: rubbing charges things.
Now rub a sheet of paper the same way and hold it near the bits. Nothing happens.
So the charge was on the ruler, not on the paper, and rubbing does not charge everything equally. Without that second step the class has proved much less than it thinks, and it takes ten seconds.
Two pieces of tape, and the argument they make
This is the best experiment in the topic and it costs a roll of tape.
Use the matte kind that looks cloudy on the roll and goes invisible when you stick it down. Clear cellophane tape does not charge properly, and a class using the wrong tape will get nothing.
Work on a wooden or melamine desk, never metal. Metal conducts the charge straight away.
Part one: two tapes charged the same way
- Cut an 8 cm piece and fold the top centimetre over to make a non-sticky handle
- Stick it to the desk, then pull it off quickly by the handle. Do not let the rest touch your hand
- Test it against the paper bits. It attracts them, so it is charged
- Hang it from the edge of the desk, make a second one the same way, and bring it close
They push apart. Both were charged by the same method, so both carry the same kind of charge.
Like charges repel.
Part two: two tapes stuck together
- Stick one tape to the desk and the second on top of it
- Pull both off together, as a pair. Test it. If it is charged, run the non-sticky side over your closed lips to discharge it
- Now pull the two apart. Test each one. Watch what they do to each other
- Stick them back together and test again
| Stage | Charged? |
|---|---|
| The pair, pulled off together | No |
| Pulled apart, each tape | Yes, both |
| Brought near each other | They attract |
| Stuck back together | Uncharged again |
Opposite charges attract.
And here is the argument, which is the part worth slowing down for
Two uncharged tapes were pulled apart and both came out charged. Put them back together and the pair is uncharged again.
One kind of charge cannot cancel itself. If there were only one sort, sticking two charged things together would give you something more charged, not something neutral.
So there must be two kinds, and they must be opposites. That is where the words positive and negative come from. They are not arbitrary labels handed down; they are the conclusion of an experiment you can run with stationery.
Why it fails on a humid day, and why that matters in South Africa
Electrostatics is very sensitive to humidity. On a damp day the charge leaks into the air as fast as you can make it, and nothing works.
If your class rubs the ruler and the paper does not move, you have probably not done anything wrong.
| Where and when | What to expect |
|---|---|
| Highveld in winter, June to August | Excellent. Very dry. Everything works first time |
| Highveld in summer | Variable. Better in the morning, before the afternoon storms |
| Durban in summer | It may simply not work. The air is too wet to hold a charge on anything |
| Cape Town in winter | Wet. Difficult |
Plan this practical for the driest week you can find. Inland, aim for July.
And if it does fail, that is the result. Record it, explain it, and run it again when the weather changes. A class that has watched charge leak away in the damp understands charge better than a class for whom it always worked.
It is also why you get a shock off a car door in winter and almost never in a wet summer.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 3 |
| Topic | Electricity and magnetism |
| Status | Class experiments. Not a formal assessment |
| Marks | 40 on our worksheet. None is prescribed |
Neutral does not mean empty
Every object contains both positive and negative charge. What changes is the balance.
| Object | What that means |
|---|---|
| Neutral | Equal numbers of positive and negative charges |
| Positively charged | Fewer negative charges than positive. Electron deficient |
| Negatively charged | More negative charges than positive. An excess of electrons |
"It has positive charges in it" is not what positively charged means. Everything has positive charges, including a neutral object and a negatively charged one. It is about the balance, and that distinction is worth marks.
And rubbing does not create charge. It moves electrons from one object to the other, so one ends up with a surplus and the other with a deficit. That is exactly why the two tapes in part two came out opposite.
Static electricity outside the classroom
- A shock off a car door. Sliding across the seat charges you; the metal door is the first conductor you touch
- Clothes clinging out of a tumble dryer. Tumbling charges them and opposite charges attract
- Hair standing up after a jersey comes off. Every strand has the same charge, so every strand repels every other
- Dust sticking to a television screen
- Lightning, which is charge separation in a cloud on an enormous scale
All five happen more in dry weather, which is the humidity point again, arriving from a direction learners recognise.
If it does not work
| What you see | What caused it |
|---|---|
| Nothing works at all | Humidity. The commonest cause by a long way, and it is not a mistake. Try a dry day |
| The tape will not charge | Wrong tape. It must be the matte kind that looks cloudy on the roll, not clear cellophane |
| The tape charges and then goes dead | The sticky side touched a hand or the desk. Handle it only by the folded tab |
| Nothing charges on a metal desk | Metal conducts the charge away. Use wood or melamine |
| The ruler picks up nothing | Not rubbed enough, or a humid day |
| The pair in part two is already charged | Normal. Discharge it on your closed lips and carry on |
| The tapes stick to everything nearby | They are charged and everything is attractive. Hang them off the desk edge |
| Paper flies up, touches the ruler, then jumps off again | Correct, and worth watching for. See below |
The best accident in the practical
A paper bit is attracted to the ruler, touches it, picks up some of its charge, and is instantly flung away.
Nobody plans that demonstration and it happens every single time. Attraction, charging by contact and repulsion, in one second. Point it out before it happens and the class will watch for it.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Testing for charge, and the control | 8 |
| Like charges repel | 8 |
| Opposite charges attract, and the conservation argument | 10 |
| Neutral, positive and negative in terms of electrons | 6 |
| Humidity | 4 |
| Conclusion | 4 |
No mark allocation is prescribed. The worksheet and this split are ours.
The mark most often dropped is explaining what positively charged means. It is not "it has positive charges". Everything does.
If you have time
Bend a stream of water. Run the thinnest steady stream you can from a tap and hold a rubbed ruler beside it. The water bends towards the ruler. It works even on days when the paper is sluggish, and it is the most impressive thing in the topic.
Stick a charged balloon to the wall. Minutes on a dry day, seconds on a wet one. It makes the humidity point better than any explanation.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the charge tests, both tape experiments, the conservation argument and a humidity question
- Marking memorandum, including how to mark it fairly if the class ran it on a humid day and nothing charged
Related practicals
- Magnetism, Grade 10. The previous topic, and unlike charge, you can never isolate a single magnetic pole
- Series and parallel circuits, Grade 10. What happens when charge starts moving
Buy this experiment
Be told plainly: this practical needs a plastic ruler, a roll of matte sticky tape, a sheet of paper and a wooden desk. Your school already owns all four, and we are not going to pretend otherwise.
The one thing worth buying for this topic is an electroscope. It detects charge without contact, it demonstrates charging by induction, and it turns a tape-and-ruler lesson into something that also works from the front of a classroom. They live in the electrostatics range.
Get the tape right. Matte, cloudy on the roll, invisible on paper. It is the one specification in this practical that decides whether it works.