Static Electricity, Grade 8
Rubbing does not make charge. It moves electrons from one thing to the other.
That one sentence carries the whole topic. Nothing is created when you rub a balloon on a jersey: electrons move off the wool and onto the balloon, so the balloon ends up negative and the jersey ends up positive by exactly the same amount. A class that has this does not ask where the charge came from.
Everything below needs a balloon, a woollen jersey and a tap.
What is actually happening
| Question | Answer |
|---|---|
| What moves? | Electrons, and only electrons |
| Why not protons? | They are held in the nucleus of the atom |
| An object is negative because | It has gained electrons |
| An object is positive because | It has lost electrons |
| The cloth you rubbed with | Always ends up oppositely charged |
Like charges repel. Unlike charges attract. And a charged object attracts a neutral one, which is the part most learners cannot explain. For that, see the paper pieces below.
Which way do the electrons go?
You will often see a single rule: rub the rod and it becomes negative. That is only true for some materials.
| Rubbed with wool | Ends up |
|---|---|
| A plastic or perspex ruler, an ebonite rod, a balloon | Negative. It takes electrons from the wool |
| A glass rod | Positive. It gives electrons away |
Which way the electrons move depends on both materials, not just the rod. With three rods and three cloths a class can work out the order for itself, which is a far better lesson than being handed a rule that is half right.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 8 |
| Term | 3 |
| Strand | Energy and change |
| Topic | Static electricity |
| Status | Informal activities. The assessed Term 3 task is the electromagnet project |
| Marks | 45 on our worksheet. None prescribed |
Practical 1: a charged balloon and small pieces of paper
- Tear tissue paper into pieces about 5 mm across
- Rub the balloon on a woollen jersey, twenty firm strokes in one direction
- Hold it just above the pieces
They jump up and stick, and after a few seconds some drop off again.
The paper has no charge of its own, and it is still attracted. The charged balloon pushes the like charges in each piece of paper to the far side, leaving the near side oppositely charged, and the near side is closer, so the pull wins. The pieces that fly off have picked up charge from the balloon and are now being repelled.
Practical 2: two balloons, the cleanest result in the topic
- Rub two balloons the same way on the same cloth
- Hold them side by side by their threads
They swing apart. Both took electrons from the same cloth, so both are negative, so they repel.
Now bring the cloth near one of them. It swings towards it. The cloth gave its electrons away, so it is positive, and unlike charges attract.
Do these two together. One shows attraction of a neutral object, the other shows repulsion of like charges, and between them they are the whole rule.
Practical 3: separate salt and pepper
Mix a teaspoon of salt and pepper on a plate and lower a charged balloon slowly towards it. The pepper jumps up first.
Both are attracted, but pepper flakes are lighter, so the same force lifts them while the salt stays put. Ask the class whether this separates by charge or by mass. It is mass, and the question is better than the trick.
Practical 4: bend a stream of water
Run the thinnest steady stream the tap will give, and bring a charged balloon close without touching it. The stream bends towards the balloon.
Water molecules have a positive end and a negative end. Near a charge they turn to face it, and the whole stream is pulled over. This is the most reliable demonstration in the topic and it needs nothing but a tap.
Practical 5: why it fails on a wet day, which is the one nobody prints
Every demonstration on this page works beautifully in dry winter air and fails in a damp coastal summer. Teachers conclude they did something wrong. They did not.
- Get the balloon lifting paper properly
- Breathe out heavily onto it three or four times, or hold it briefly over a steaming kettle
- Try again straight away
It barely works. Leave it to dry and it comes back.
Damp air conducts, so the charge leaks away instead of sitting on the surface. Run the lesson early in the day, in a dry room, with dry hands. And if the weather beats you, teach this instead: it is a better result than the one you planned.
How long does the charge last?
Charge the balloon, then test it against the paper every 30 seconds until nothing happens.
| Conditions | Typical |
|---|---|
| Dry day | Two to five minutes |
| Humid day | Under 30 seconds, sometimes nothing at all |
Lightning
Inside a thundercloud, ice crystals and water droplets collide in the rising air and transfer electrons. The lighter positive pieces are carried to the top, the heavier negative charge collects at the bottom, and when the difference grows large enough the charge jumps: to another cloud, or to the ground.
The flash and the bang leave together. Sound is slower, so the gap tells you the distance: about 3 seconds per kilometre. Nine seconds is about three kilometres away.
What to do in a thunderstorm
- Get inside a building, or into a car with a metal roof. The shell carries the charge around you
- Do not be the tallest thing around, and stay away from lone trees
- Stay off landline telephones and away from taps and plumbing. Both are metal paths to the outside
- Wait 30 minutes after the last thunder before going back out
One piece of advice that is printed wrong
You will see it written that you should not run in a thunderstorm, because running creates friction and static electricity that can attract the lightning.
That is not true. Running does not attract lightning. The charge a person can build up is nothing next to a storm carrying millions of volts.
Running to shelter is the right thing to do. It is worth correcting out loud, because the rest of that list is good advice, and a learner who spots one false reason tends to throw out the whole list. A cellphone does not attract lightning either. The landline warning is the sound one, because it is wired to the outside world.
Static electricity in everyday life
| What you notice | What is happening |
|---|---|
| A shock from a car door | Sliding off the seat charged you. Touching metal lets it jump |
| Clothes clinging out of a tumble dryer | They rubbed together and swapped electrons, so some are positive and some negative |
| The warning not to get back into your car while refuelling | The same shock, next to petrol vapour. The spark itself is harmless. What it can ignite is not |
| Hair standing up after a jersey comes off | Every hair has the same charge, so they all repel each other |
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| Nothing happens at all | Humid air, or damp hands | Dry room, dry hands, early in the day |
| It worked yesterday, not today | The weather changed | Use it. Run Practical 5 and make that the lesson |
| The paper sticks, then flies off | It has taken charge from the balloon | Normal, and worth explaining |
| The balloon will not charge | Rubbing too gently, or a synthetic jersey | Twenty firm strokes on wool, or on clean dry hair |
| The water stream will not bend | The stream is too thick | Thinner. A steady thread, not a gush |
Safety
This is the safest practical in the Grade 8 year, and it is worth saying so rather than inventing hazards. Nothing here plugs in.
- Ask about latex allergies. One balloon per learner, never shared
- Uninflated balloons and burst pieces are a choking risk where small children are about
- A Van de Graaff generator is a different matter. Nobody with a heart condition or an implanted medical device touches it, and it is discharged before anyone approaches
- The lightning section is about staying safe outdoors, and is best taught that way rather than as a scare
How the 45 marks are made up
| Section | Marks |
|---|---|
| What is actually happening | 12 |
| Your experiments | 18 |
| Lightning | 9 |
| Every day | 6 |
Nothing is prescribed for this topic. The worksheet and this split are ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 45 marks, including the humidity test, the thunder gap calculation and the printed lightning advice to check
- Marking memorandum, with what to accept for the neutral paper question and the five answers that look right and score nothing
Related practicals
- Electrostatics with sticky tape, Grade 10. The same rules, investigated a different way
- Make and use an electromagnet, Grade 8. The assessed Term 3 project, where the charges move instead of staying put
- Atoms, elements, compounds and mixtures, Grade 8. Where the electrons and protons came from in the first place
What you need to run it
Balloons, a woollen jersey and a tap. That is the honest answer, and this page is not going to pretend otherwise.
The one piece of equipment worth owning is the electrostatic friction rod kit: three rods and three cloths, at R550. It is the only way to run the experiment that matters here, where a class tries every combination and works out for itself which way the electrons go. A rule you discover is worth more than a rule you are given.
Individual friction rods in glass, ebonite, perspex and nylon are stocked separately from R30 if you already have cloths.
The rest of the range is in electrostatics apparatus.