The metaphor we use for electricity is so embedded in the language that we hardly notice: the metaphor is based on the notion of a flow of water and uses the word current.
We talk of a direct current or of an alternating current. Wonderfully, the rules for a direct electric current are very similar to those for a direct flow of water.
Water can flow through a pipe. Within limits, for the same duration of time, when you double the difference in pressure between the beginning and end of the pipe, twice as much weight of water flows through.
Water you can weigh directly. You cannot do the same with electricity. However, you can weigh changes that act as indicators of electricity. In particular, you can insert two pieces of silver, two electrodes, into a solution of silver nitrate and weigh the pieces before and after arranging what we now call a `direct current' to flow, the weight of one electrode decreases and the other increases by the same amount. It appears that silver is transferred from one electrode to the other.
When you double the number of devices that Volta came up with that you have arranged in a series, twice as much silver is transferred.
This outcome is very similar to what happens when you double the pressure of water in a pipe. For the same duration, twice as much water goes through. Doubling the number of cells in Volta's `battery' doubles a quantity that corresponds to pressure. (Eventually, we will come to call one unit of this quantity a `volt'.)
Of course, various complications bedevil people who undertake the actions of weighing water that has flowed through a pipe, or weighing silver electrodes; but the basic principle is there.
For electricity, the metaphor requires that a substance of some sort `flow' through a solid metal. Moreover, the metaphor requires that we come to think of the weight of silver transferred as a measure of whatever flowed.
Water does not flow through metal. Indeed, we make pots to hold water. But electricity flows through metal. On the other hand, neither water nor electricity pass through glass.
What would be a different metaphor for electricity? I am not sure whether one exists. I think of electric `currents' as movements of electrons or of other charge carriers.
However, I remember being taught to think of voltage as a `tension', a pulling apart, as well as a `pressure' a pushing. But the word `tension' did not help me; I could not imagine a crowd of electrons `flowing' when being pulled in two directions. The crowd had to be `pushed' from one side and `pulled' or `sucked' from the other. Then its members would all move or `flow' in the same direction.
The speed of a flow of water consists of the speed of individual drops of water. But the speed of electrical propagation is not the speed of individual electrons, but of the electrons' (or other charge carriers) ability to `push' others.
`Alternating current' meant elections moving back and forth.
I think of people like myself who do not want to learn too much about electricity, but who do hope to gain a sufficient understanding of it. I know about a circuit for water in nature: it falls from a cloud as rain, flows through the earth to a river, down the river into the ocean. There it gains solar energy, evaporates and rises into the air, becoming water in a cloud.
I want to be able to think about electrical circuits the same way.
Is the flow metaphor the best? Is the notion of a circle or circuit the best?
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