A circuit could be as simple as a conductive wire connected end-to-end, but useful circuits usually contain a mix of wire and other components which control the flow of electricity.
Depending on the situation, positive charges, negative charges, or both may move.
When a battery electrolyte is tested the result is the specific gravity of the electrolyte.
Charge will not flow in a circuit unless there is an energy source capable of creating an electric potential difference and unless there is a closed conducting loop through which the charge can move.
Copper and aluminum wire are the most common conductors used in the electrical trade.
Each atom has a center nucleus, where the protons and neutrons are densely packed together.
If 5 coulombs of charge flow past point A diagram at right in 10 seconds, then the current is 0.
The standard metric unit for current is the ampere.