IGCSE Physics / Electric charge
Charging by friction revision notes
Edexcel International GCSE Physics (4PH1), chapter 9: Electric charge, specification point 2.23P, 2.24P. Free lesson.
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Rubbing two insulators transfers electrons between them, so each becomes charged. Knowing which material gains or loses electrons helps you explain the charge and predict how charged rods behave.
How to charge and test insulating rods
An insulator can be charged by rubbing it with a dry cloth. A hanging rod can turn freely, so its movement shows whether another charged rod attracts or repels it. A rubbed polythene rod swings away from a second rubbed polythene rod, but towards a rubbed acetate rod. A rubbed rod can also be touched on the cap of a gold-leaf electroscope. If it is charged, the thin leaf rises. Keep the rods and cloth dry. Dampness lets charge leak away.
Why rubbing transfers charge
Rubbing does not create charge. It transfers charge that is already present. Before rubbing, the rod and cloth are neutral because each has equal amounts of positive and negative charge. During rubbing, electrons move from the cloth to the polythene rod. The rod gains electrons and becomes negative, while the cloth loses electrons and becomes positive. Only electrons move in a solid. The positive charges stay where they are. A material that gains electrons has more negative than positive charge. A material that loses electrons has more positive than negative charge.
Which charge each rod gets
Polythene and acetate transfer electrons in opposite directions when rubbed with a cloth. The cloth ends up with the opposite charge to the rod because it loses or gains those same electrons. Polythene gains electrons from the cloth, so the polythene becomes negative and the cloth becomes positive. Acetate loses electrons to the cloth, so the acetate becomes positive and the cloth becomes negative.
Explain the charge on the cloth
If an acetate rod becomes positively charged, it has lost electrons to the cloth. The cloth has gained those electrons, so it becomes negatively charged.