[Physics] How does charge flowing between emf terminals reduce voltage difference


I'm currently learning what electromotive force is and while reading my book's description of an ideal source of emf, I had difficulty understanding what these sentences mean:

The nonelectrostatic force maintains the potential difference between the terminals. If it were not present, charge would flow between the terminals until the potential difference was zero.

I don't quite get what it means by this. How does the nonelectrostatic force "maintain" the potential difference between the terminals? If it wasn't present and only the electric force remained, why would the potential difference decrease to zero?

The only thing I could think of was that the electric force from the E field inside the source of emf would still be exerting forces on charges from high potential to low potential, turning its electric potential energy into kinetic energy by doing work on the charges. I still don't know how that would reduce the voltage difference across both of its terminals to zero though.

Regarding the ideal source of emf, the book assumes there is a positive and a negative terminal with the positive terminal at a higher potential and the nonelectrostatic force going from low to high potential while the electric force goes from high to low, just in case that helps clear any ambiguity.

Best Answer

Before thinking about circuits, let's think about two conducting spheres of charge that I connect by a wire. Before I connect the wire, sphere 1 is at voltage $V_1$ and sphere 2 at voltage $V_2$, let's say $V_1>V_2$. I find it useful, in terms of thinking about what's going on, to notice that if the spheres are the same size then saying the spheres have different potentials is equivalent to saying that the 2 spheres have different charges residing on their surfaces (you can justify this by noting that the capacitance of a sphere is determined by its radius).

Now let's connect the spheres. What will happen? Well, a current will flow in the wire. This will take positive charge off of sphere 1 and deposit it on sphere 2 [strictly speaking if you want electrons to be charge carriers, then negative charge is flowing from 2 to 1; but in terms of thinking about what's going on it's easier to imagine, and mathematically equivalent to say, that positive charges are going from 1 to 2]. This in turn changes the voltages on the two spheres; $V_1$ decreases and $V_2$ increases. The process stops when $V_1=V_2$. Again, if the spheres are the same size this condition is equivalent to the charges on both spheres being equal.

OK, now imagine a battery hooked to a resistor and a switch, the simplest circuit imaginable. Before we close the switch, terminal 1 is at $V_1$ and terminal 2 is at $V_2$. At this point, it makes perfect sense to think of each terminal of the battery as being a sphere of charge. Then we close the switch, this is like connecting our spheres with a wire. Based on our silly model of a battery, you would the voltage between the two terminals of the battery (ie, $V_1-V_2$) to decrease until eventually it reached equilibrium with $V_1=V_2$. Clearly, a battery does not behave like two spheres of charge after the circuit is closed.

The whole point of a battery is that it maintains the potential difference between its two terminals. After we close the switch, a little bit of positive charge flows from terminal 1 to terminal 2 by going through the circuit. Naively this means that terminal 1 has less positive charge and terminal 2 has more positive charge, so terminal 1's voltage decreases while terminal 2's voltage increases. Inside the battery, some process takes place to to take the excess positive charge on terminal 2 and put it back onto terminal 1. Whatever this process is, it cannot be electrostatic, because positive charges following the electric field can only ever move from terminal 1 to terminal 2 [positive charges move from high voltage to low voltage, if the only force is electrostatic].

The details of what the battery does to maintain the potential difference varies depending of the kind of battery. A conceptually simply example of a battery is a Van de Graaff generator. In a Van de Graaff generator, you have a conveyer belt that literally carries the excess positive charge on terminal 2 and deposits it back on terminal 1, undoing the naive 'equilization process.'

Most useful batteries rely on some chemical process to maintain the potential difference. For example, one can use oxidation reactions to do this. The details involve some chemistry (there's a wikipedia summary at http://en.wikipedia.org/wiki/Electrochemical_cell), but essentially you put each terminal in a bath of ions, and the chemical energy of the reactions at each terminal [balancing oxidation and reduction] forces ionized atoms to carry electrons from terminal 2 to terminal 1.