why is potential differnece/voltage not a fundamental quantity?
Let's use heat as an analogy. You may have a large cup of water at room temperature. If you add a teaspoon of boiling water to the cup, heat will be transferred from the 1 teaspoonful of boiling water to the water in the large cup at room temperature. Heat flows from a body at a higher level of temperature to one that is at a lower level of temperature. The quantity of water in the cup which is larger than the quantity in the teaspoon does not dictate the direction in which the temperature flows. Voltage is similar, current always flows from a body at a higher voltage to one that is at a lower voltage.
Another analogy. When you pump gasoline from a gas station, into your car tank, you control the rate of flow by pressing lightly or strongly on the lever at the end of the tube. Initially you press it hard to get the tank filled up, but towards the end you ease on the lever to reduce the rate of flow so that (if you are like me) you can reach a round amount like $40.00 or $45.00 and current is like that, the rate of flow.
Voltage and current are related. Current through a material depends on the diameter of the wire and its resistance.
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