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Electrification by Friction

If a polythene rod is rubbed with cloth, the rod can lift small pieces of paper. This can be explained by stating that the rod becomes charged. A charged object will attract a neutral object until it touches and then it is repelled. It is concluded by the experiment that there are two types of charge. It has been decided that the charge that occurs on the polythene rod when it is rubbed is negative and the charge on a perpex rod when it is rubbed shall be called positive. As with magnetism, like charges repel and unlike charges attract.

Insulators and Conductors

Charge does not build up on metalic rods when they are rubbed. The charge can escape through the body to earth, hence metals are known as conductors. Materials such as ebonite and silica do not allow charge to low in this way and are called insulators. For insulators to be effective they must be kept dry and the air around them must also have a low humidity.

Polythene rod charged by rubbing a woolen cap, rod shows negative charge, the woolen cap shows a positive charge. This implies that

positive and negative charges are created in equal quantities by friction. If electrons move away from the wool to the polythene the rod will become negatively charged. The nuclei that remain in the wool have lost electrons and so they become positively charged. The process of rubbing effectively increases the area of contact between the wool and the polythene rod, since transfer of charge occurs when the two materials are in contact.

Charging by Induction

A charged polythene rod can be used to transfer charge onto an insulated conductor without the two object coming into contact. The negatively charged polythene rod is brought close to the conductor. The electrons within the conductor are repelled by the charged polythene rod and the positive ions are attracted to the polythene rod. The conductor has a separation of charge on it. If the negatively charged side is momentarily earthed, the negative charge is able to flow to earth but the positive charge remains attracted to the negatively charged polythene rod. When the rod is taken away from the conductor, the positive charge on the conductor distributes itself evenly around the surface of the conductor leaving it charged.

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