Electrostatics
Senior 1 · Physics
Electrostatics
Lesson 2 of 5
24 min
Rub a balloon on dry hair and it sticks to the classroom wall. Rub two balloons on hair, hang them side by side on threads, and they swing APART. So charged objects sometimes attract and sometimes repel. Which one happens, and why? A simple experiment answers both questions.
You need: two polythene rods, two cellulose acetate rods, a woollen cloth, a dry duster, a paper stirrup, nylon thread and a stand.
Figure 2.1: A charged rod hangs freely in a paper stirrup, so even a small force turns it.
| Rod in the stirrup | Rod brought near | What happens |
|---|---|---|
| polythene (rubbed with wool) | polythene (rubbed with wool) | the hanging rod turns away: repulsion |
| acetate (rubbed with a duster) | acetate (rubbed with a duster) | turns away: repulsion |
| polythene | acetate | turns towards it: attraction |
| polythene, or acetate | an unrubbed (neutral) rod | turns towards it: weak attraction |
Two rods of the same material, rubbed the same way, must carry the same kind of charge, and they repel. Polythene and acetate carry different kinds, and they attract. Benjamin Franklin called the kind on the rubbed acetate or glass positive and the kind on polythene negative.
Important
Getting it to work. Hold each rod by the end you did not rub. Work on a dry day, away from sinks and steamy kettles: a film of moisture lets the charge leak away within seconds.
Definition
The laws of electrostatic charges
Example: two balloons both rubbed on hair are both negative, so they repel.
Every material holds on to its outer electrons with a different grip. When two different materials are rubbed together, their surfaces touch at millions of points, and electrons move from the one with the weaker grip to the one with the stronger grip. Wool holds its electrons weakly and polythene holds them strongly, so electrons move from the wool to the polythene.
Figure 2.2: Rubbing moves electrons from the wool to the polythene. The rod and the cloth end up with equal and opposite charges.
| Material rubbed | Rubbed with | The material becomes | The cloth becomes |
|---|---|---|---|
| polythene | wool | negative (gains electrons) | positive |
| rubber balloon | dry hair | negative | positive |
| ebonite (hard rubber) | fur | negative | positive |
| cellulose acetate | a cotton duster | positive (loses electrons) | negative |
| glass | silk | positive | negative |
Definition
Charging by friction is charging two different insulating materials by rubbing them together. Electrons move from one to the other, so the two objects gain charges that are equal in size and opposite in sign.
Worked example 1: a balloon and hair
After rubbing, a balloon has a charge of . How many electrons moved, and what is the charge on the hair?
These electrons came from the hair, so the hair is left with : equal and opposite.
Figure 2.3: The negative comb pushes electrons in the paper to the far side. The positive near side is closer to the comb, so the pull on it is bigger than the push on the far side.
The paper stays neutral overall; its charges have only been separated. The electric force gets weaker with distance (lesson 4), so the attraction of the closer positive side wins and the paper jumps up. A positive rod works the same way, pulling electrons to the near side.
Important
Repulsion is the only sure test of charge. If an object attracts a charged rod, the object may be charged with the opposite sign OR it may be neutral. If it REPELS the rod, it must carry the same kind of charge.
Worked example 2: detective work with three rods
Rod C is polythene rubbed with wool. Rod B repels rod C. Rod A attracts rod B. What can you say about the charge on each rod?
To decide, bring A near a POSITIVE rod, such as rubbed acetate. If A repels it, A is positive. If A attracts it too, A is neutral.
Worked example 3: glass and silk
A glass rod is rubbed with silk, and the silk gains electrons. Find the charge on the rod.
The rod lost these electrons, so its charge is .
Warning
Common mistakes. (1) Saying "positive charges moved from the wool to the rod". Only electrons move. (2) Deciding that an object is charged because it ATTRACTS a charged rod: a neutral object is attracted too. (3) Thinking rubbing makes charge. It only moves charge, so the two materials always end up with equal and opposite charges.
Tip
Like repels like, unlike attracts: the same rule as for magnetic poles. To remember which rod is which: "polythene picks up electrons, so polythene goes negative".
Friction charges objects every day: the blades of a ceiling fan rub against the air and collect a line of dust; you may feel a shock when you slide off a car or minibus seat in the dry season; plastic food wrap clings to a bowl; clothes rubbing together crackle. Try PhET: Balloons and Static Electricity (version 1.5.21): rub the balloon on the sweater, watch the negative charges move across, then bring the balloon near the wall and see the wall's charges separate as in Figure 2.3.
Static electricity: build-up of charge on insulating materials (Cognito). Watch how electrons move when two insulators are rubbed together, and how like and unlike charges repel and attract.
Practice
Key points
Sources and credits
The experiment write-up, the diagrams, the worked examples and the practice exercises are written by Ganzaa.
