Static Electricity: Invisible Forces
Shuffle across a carpet, touch a door handle, and something invisible snaps between your finger and the metal. Nothing was plugged in and nothing was switched on. The whole effect comes from something far too small to see moving from one surface to another.
What you need
- One balloon
- Your own hair, or a wool jumper, if your hair is very short or damp
- A sheet of paper torn into pieces about the size of a fingernail
- A tap you can turn down to a thin stream
Before you start
Try it: the jumping paper
- Scatter your paper pieces on a table so they are not touching each other.
- Rub the balloon back and forth on your hair for about ten seconds. Keep going a bit longer than feels necessary.
- Hold the balloon a few centimeters above the paper and lower it slowly.
- Watch the pieces jump up to meet it. Some will stick, then fall off again a few seconds later.
- Now turn the tap down until the water is a thin, smooth stream, and bring the rubbed balloon close to it without touching. The stream bends towards the balloon.
What is actually moving
Everything is built from atoms, and an atom has three kinds of part. Only one of them ever leaves.
Choose a part to find out what it does and whether it can move.
- Protons — packed in the center, carrying a positive charge. They are locked in place and never move between objects.
- Neutrons — also in the center, with no charge at all. They add weight and nothing else here.
- Electrons — tiny, negative, and traveling around the outside. These are the ones that can be rubbed off onto something else.
So "static electricity" is a count. Rubbing moves electrons from one surface to the other. The surface that gained them now has more negative charge than positive, and the surface that lost them has less. Nothing was created; it was just shared out unevenly.
Why your hair stands up
Two different things happen at once here, and it is worth telling them apart.
- Your hair is pulled towards the balloon, because the hair is now positive, the balloon is now negative, and opposite charges attract.
- Your hairs also spread apart from each other, and that is the opposite rule at work. Every hair lost electrons, so every hair is positive, and matching charges push each other away. Each strand is trying to get as far from its neighbors as it can.
Every hair on your head is now positively charged. Why do the strands stand apart instead of lying flat?
- Charges that match push each other away
- Charges that match pull each other together
- The balloon is holding each hair up from above
The paper trick works differently again. A scrap of paper has no extra charge at all. When the negative balloon comes near, it pushes the paper's own electrons towards the far side of the scrap, leaving the near side positive — and that near, positive side is pulled up to the balloon. The water bends for a related reason: water molecules already have a slightly positive end and a slightly negative end, so they swivel to face the balloon and the stream leans over.
If nothing happens
Almost always, the answer is that the air is damp.
Water in the air gives loose electrons an easy path away, so the charge drains off as fast as you build it. Static works far better on a cold dry day than in a steamy bathroom, and freshly washed hair is usually a poor choice.
The snap you feel at a door handle and the flash across a summer sky are the same piece of physics. Only the size is different.