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Electric forces: static electricity

الأسبوع 7 · الوحدة الثانية «مجالات القوى» (افتتاحية) والدرس الأول «القوى الكهربية» (١): الكهربية الساكنة ونوعا الشحنة والمجال الكهربي ومانعة الصواعق · 3 دروس
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Static electricity: charging by rubbing

Science Prep 1 Term 1, Unit 2: pp. 32–38
GoalExplain how rubbing charges objects by moving electrons, and why charge stays on some objects and escapes from others.
New words
static electricity · electric charge that stays on an object instead of flowingelectric charge · a property of matter that gives rise to electric forcesinsulator · a material that does not let charge move through it easily
Explanation

Some forces act without touching, because objects have fields around them: electric, magnetic and gravitational fields. Static electricity is a first example of an electric force. Wool that crackles when you pull it off, or a small shock from a door handle, are static electricity. When two suitable materials are rubbed together, they become charged and can pick up light scraps of paper. Charging means that electrons move: one surface loses electrons and the other gains them. Charge is not made from nothing, it is moved. On an insulator, such as plastic, the charge stays on the rubbed part. A metal rod held in your hand will not stay charged, because the charge flows through your body to the ground, so it needs an insulating handle. A fuel truck drags a chain to the ground so that any charge on it can flow away safely.

Worked examples
charged balloonpaper bits
A balloon rubbed on wool picks up small paper pieces and can stick to a wall for a short time. The rubbing moved electrons and the balloon is now charged.
fuel truckchain to ground
A fuel truck has a chain hanging to the ground. As fuel moves and the truck moves, charge can build up. The chain carries it to the ground before a spark can form.
Nour rubs a metal spoon on wool while holding it in her hand and finds that it does not pick up paper. A metal is a conductor, so the charge flows through her hand to the ground. A plastic spoon works better because plastic is an insulator.
Class activity

Try it on a dry day, with an adult. Rub a plastic comb or pen on dry hair or wool and bring it near small bits of paper. Rub a balloon the same way and see if it sticks to a wall. Bend a very thin stream of tap water with a charged comb. Then try a metal spoon held in your hand. Damp air can spoil the results, so try again another day if nothing happens. Never do these tests in a thunderstorm.

Practice
1
Which particles move when an object is charged by rubbing?
2
Why does a fuel truck drag a chain on the ground?
3
Why does a metal rod held in the hand not stay charged?
4
Is charge created when we rub? Explain.