History and the electric current convention(Expansion – tell me more)



How charged particles move, and the consequences of historical decisions

Read more about which way the charged particles actually move–and find out whether it is important at this level.

In the metal conducting materials that we use to make electric circuits, it is the negative charged particles (the electrons) that are physically able to move around:

By convention, however, the electric current is taken as flowing in the opposite direction towards the negative terminal.

Why should it be that the conventional electric current is taken as being in the opposite direction to the motion of the electrons? The answer is that the convention arose historically.

Early experiments by William Gilbert (1544–1603), physician to Queen Elizabeth I, investigated electrical charging by friction of many substances. By comparing, for example, glass rubbed with silk and ebony rubbed with wool, Gilbert concluded that there were only two types of charge and that charged particles of the same kind repelled, whereas opposites attracted. He called those produced by the action of friction on fur as positive and those by friction on rubber as negative.

It was noted in later experiments that when charged objects were brought into contact with the Earth through a conductor, a small charge flowed for a short time. When cells were invented, it was observed that a cell's carbon electrode behaved in a similar way to fur, and a silver electrode in a similar way to rubber. Thus, an excess of electric fluid (positive charge) appeared to be transferred from the positive carbon electrode (anode) to the negative silver electrode (cathode).

The idea of positively charged flow remained in favour until the work of Joseph John Thomson (1856–1940). In a study of the flow of electricity through gases, (technology used today in neon signs), Thomson isolated a beam of negatively charged particles of much smaller mass than atoms. These originated at the negative terminal (cathode) in his gas tube and Thomson realised his cathode rays were composed of negatively charged electrons. This fundamental particle, the electron, was soon associated with all atoms and shown to be responsible for both static charging and electrical currents in metals.

By this time, however, the convention had been established that electric current ran from the positive terminal (e.g. carbon electrode) to the negative terminal (e.g. silver electrode) of a cell. This flow of charge is a conventional current. In most wires, negative electrons flow in the opposite direction to this conventional current.



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