IGCSE Physics / Electric charge
Forces between charges and electrostatic effects
Edexcel International GCSE Physics (4PH1), chapter 9: Electric charge, specification point 2.25P, 2.26P. Free lesson.
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Charges of the same type repel, while opposite charges attract. A nearby charged object can shift electrons in a neutral object, creating an induced charge that explains effects such as a balloon sticking to a wall.
How charges attract and repel
Two charged objects can push or pull on each other without touching. The direction depends on whether their charges are the same or opposite. Same type of charge repels, so the objects push apart. For example, two rubbed polythene rods are both negative. Opposite charges attract, so the objects pull together. A negative polythene rod and a positive acetate rod attract. In the practical, a hanging rod swings away from a like charge and towards an opposite charge.
How a charged object attracts something neutral
A neutral object has no overall charge, but its electrons can move when a charged object is brought close. This creates an induced charge, which is a charge produced by the movement of electrons. Rubbing gives a balloon extra electrons, so it becomes negatively charged. The wall starts neutral. The balloon repels electrons in the wall, so they move away from the surface. The wall's surface is left positive. The positive surface and negative balloon are opposite charges, so they attract and the balloon sticks. A charged comb can make small pieces of paper jump up in the same way.
Why a stream of water bends towards a rod
A charged rod can attract a neutral stream of water because its charge moves electrons within the water. Explain the electron movement before stating the force, so each part of the explanation follows from the last. A negative polythene rod gained electrons when it was rubbed. Electrons in the water nearest the rod are repelled and move away from it. This leaves the side of the stream nearest the rod positively charged. This is an induced charge. Opposite charges attract, so the stream is pulled towards the rod.
How earthing removes charge
Earthing is joining a charged object to the ground with a conductor, such as a metal wire, so that the object loses its charge. The electrons flow to or from the ground. For a negative object, electrons flow from the object to earth. For a positive object, electrons flow from earth to the object.
Why hair stands up after removing a woolly hat
Rubbing moves electrons between the hat and the hair, so the hairs become charged. Each hair gets the same type of charge, and the hairs repel one another. The hairs push apart because like charges repel, so they stand on end.