Section 1.2 What is current?
Current is simply the flow of electrical charge — generally because electrons are moving through a wire. We can quantify current by measuring how much electrical charge (measured in coulombs) flows past a certain point per unit time (measured in seconds). TODO figure 8 The measurement coulombs / second is renamed the ampere in honor of French physicist André-Marie Ampère 1 .
Going back to the marble track analogy: current is simply the number of marbles to roll past a point per second. TODO figure 9
It's important to keep a clear distinction between voltage and current. Voltage is how far the marbles drop between two points on the track; current is how many marbles Voltage represents potential energy; current represents flow.
One more analogy that may help, borrowed from Kenn Ahmdal's book There Are No Electrons: In Ahmdal's "theory", there is no such thing as electrons; electric effects are due to microscopic green men who love to party. Voltage is how badly the green men want to party; current is how many of them are on the road going there. As we'll see soon, an increased desire to party often results in increased traffic, but the two ideas are not the same. Don't get them mixed up.