The electrostatic force

The ancient Greeks discovered that amber rubbed with fur attracts little pieces of straw. This phenomenon we now call electrification, derived from e^EKipov, the Greek word for 'amber'. The Greeks could hardly have realised what they were seeing was a tiny clue to a hidden force of enormous dimensions. The source of this force we call electric charge. This force also acts at a distance. It is much stronger than gravitation — in fact, stronger by a factor of about 1036!*

A major difference between the electrical force and the force of gravitation is that while there is only one kind of matter, and the force of gravitation is always attractive, there are two kinds of electrical charge. We call them positive and negative. (These names are purely conventional; we could equally have called them 'black' and 'white', or any two names which denote oppo-sites.) Opposite charges attract while like charges repel. This is the force which holds atoms together as the electrons which carry negative charge are attracted to the positively charged protons in the nucleus. The same force is also responsible for the chemical binding of molecules, with the result that matter is a

* The strength of interaction can be represented by a number called the dimen-sionless coupling constant, which has the value 1/137 = 7.3 x 10-3 for the interaction between charges, compared to 5.3 x 10-39 for the gravitational interaction.

tightly bound mixture held together by electrical forces. Electric forces are in action 'behind the scenes' in all chemical and biochemical processes.

The balance of positive and negative charges is so perfect that we cannot see the huge electrical tensions within a piece of matter. The positive charge of the nucleus of an atom is electrically neutralised by negatively charged electrons. 'Electrified' materials have a tiny surplus of electrons which results in net negative or positive charge (less than one charge in 1010), and as a result the electric forces 'are brought out into the open'.

Quantitative electrical experiments began in the 18th century, and various theories on the nature of the phenomena were put forward. In America, Benjamin Franklin (1706-1790) carried out spectacular and dangerous experiments during thunderstorms, which led him to recognise lightning as an electrical effect. Joseph Priestley (1733-1804), an English clergyman and schoolteacher, had met Franklin when he was in England, and they became great friends. Priestley carried out more precise if less adventurous experiments using a small pith ball and an electrified metal cup. He found that when the pith ball was suspended close to the outside of the cup, it experienced a force of attraction, but it experienced no net force when it was suspended anywhere inside the cup. This reminded him of Newton's calculations in relation to the force of gravity, and led him to infer that there was a similarity between electrical and gravitational phenomena. He concluded that 'the attraction of electricity is subject to the same laws as gravitation, and therefore according to the inverse squares of the distance'. Henry Cavendish (1731-1810) confirmed Priestley's hypothesis with a similar but more precise experiment, but he was an extremely shy man and did not publish his results, which only became known many years after his death.

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