# Where do the charge in a series/circuits (?) come from?

so I just entered the very (bemusing) area of electric charges in physics in my grade 12 syllabus and I am struggling. I've been able to do well with other physics concept, but when it came to electricity, I just became so stumped. So here's what I don't understand, how does Charge exactly move through a circuit wire? I have two theories:

1.) there are free-floating electrons in wires, right? So if you connect a battery to a wire, potential difference (don't even know how this works tbh) in the battery causes the free moving electrons in the wire to move to the other end...does this mean:: 1.a) that the wires would eventually run out of electrons that can freely move and therefore lose current?

2.) electric potential difference in the battery allows the battery to generate electrons that can move through the wire, bump with the free-floating electrons in the wires and move through to the end of the wire 2.a) To me this seems to be the most reasonable hypothesis.

Also, lastly can someone explain what electric potential difference looks like and how it occurs in batteries. I heard it has to do something with chemical reactions or something?

You are right - electrons "circulate". That is, they move in a circuit (that name looks a bit like "circle" ... can you see?). The power source (battery) can indeed push electrons into the wire at one end, collect them at the other end, then give them some more energy and let them go again.

At your level, a good analogy might be a roller coaster. There is a certain number of cars on the circuit. There is a machine that carries the cars (electrons) to the top (the battery), then lets them go on the rails (wire). Under the influence of gravity (electric field, "potential difference") they then move around the circuit until they get to the bottom of the track. If the track is steeper (stronger electric field), they will gain more energy as they move a certain distance.

In this analogy, you could have a section where there is friction on the rails (a resistor, a light bulb) and the cars would lose energy there - heating up the resistor or lighting the lamp. How much heat you can generate depends on the number of cars, and how high the roller coaster is.

All this is a very imperfect analogy - as you learn more about electricity I hope you will get a better intuition. But this is not a bad place to start.

As for how batteries generate a potential difference - that does indeed have to do with chemistry in the battery, but might be a better question for a separate post (probably on the chemistry.stackexchange site as that's more about chemistry than physics). There are many descriptions online - for example, this page on wikipedia or this slighly more extensive one

Floris answered quite well for the difference of potential. Regarding what is actually happening in the wire, I would add the wire looks more like a road filled with electrons. You add electrons at one end and this will push the electrons at the other end, and the number of electrons (of charges) passing at a given milestone through a given time will give you the current. The electrons actually move quite slowly in a wire (it depends on the wire and the current but the typical order of magnitude is centimeters or millimeters per hour). However, since there are a lot of electrons, it can make a big current. There is a detailed Wikipedia page on this subject.

• Very good point about the density of electrons and drift velocity. Indeed my answer skipped over that - for elementary understanding of electronics i worried it might be more confusing rather than illuminating. Indeed, in metallic conductors there are so many electrons that the can represent significant current (density) at even very low drift velocity - and in the case of AC currents they barely leave their starting position... Dec 11, 2016 at 2:59

Electrical circuits lend themselves readily to analogies that are 95% good, but cause grief when the remaining 5% is considered, as you are doing now! The analogy of an electric current being rather like a water current, just moving electrons under electric "pressure" instead of water molecules under pump pressure, is like that. In reality, while electrons do drift slowly down a wire, that's not at all what what an electric current is. The right model (for classical physics) is that an electrical current is an electromagnetic wave traveling in the wire (and if it's an AC current it'll also be radiating out from the wire), as described by Maxwell's equations. Fortunately, the much-simplified circuit descriptions work well most of the time. But it can be frustrating when one asks the sorts of questions for which the simplified models don't give good answers.