foolios said:
I have a small background with AC power and am trying to understand DC.
I recently considered paralleling/serializing some batteries and realized how different a DC circuit is.
Since a complete circuit of a battery is + to -, thus creating a short when connecting the two.
Why then is it possible to connect the + and - of two separate batteries?
Why doesn't this cause a short circuit?
What little I know is that the battery wants to send electrons across to the side that has less, right?
Why doesn't this happen then when the two batteries are connected in this way?
First, please note that the physics governing both AC and DC are fundamentally the same. The physical laws (represented by us as equations and natural language expressions) that govern electromagnetism are the same when concerning immediate voltages and currents. Each of AC and DC is a special case in these equations. For instance, the defining characteristic of DC is that the immediate voltages and currents are constants and therefore inductances and capacitances can be substituted for a closed and open circuit, respectively. By AC normally it's meant an electrical system with currents and voltages having a common period. Fourier analysis can be readily applied so that the quantities expressed as a function of time can now be expressed as a function of frequency. The
phasors used in analysis of AC circuits are the Fourier component of the quantities in question. For each of voltage and current, if that quantity varies sinusoidally with time, it has a single frequency and so a single phasor is enough to describe it. If a quantity varies periodically and non sinusoidally with time there will be more than one frequency component (those are the harmonics in power systems) and one phasor is needed to describe each of them.
AC systems can be connected in series just like DC systems. You probably have practical AC systems in mind where a conductor is grounded and so connecting conductors at a different (phasor) potential would make a short circuit because ground is
already a common connection point and by making a second one, you make a complete circuit. The same thing wold happen with a grounded DC system. Grounding is NOT a property of DC or AC but of specific systems. What is called a “ground” is also a matter of convention. You can also, for instance, have 2 isolation transformers (akin to 2 batteries) powered by a single power source, and connect one terminal from one transformer to the other (whether of same or opposite polarity) and no short circuit will arise.
Supposing that a battery has no net charge (in the sense of used in
self capacitance,
not in the usual sense of energy stored electrochemically) and has plane symmetry then both terminals will have an electric potential equal in magnitude and opposite in sign. There will be a electric field between them. When you connect the positive terminal of one battery with the negative of another, the electric field will create a current which redistributes the charges so that the terminals connected together will be at the same potential, and the other 2 unconnected terminals will have a difference of potential equal to the sum of the voltage of both batteries. This current is very short and of negligible magnitude; thus it's a transient in the broadest sense of the word but it's negligible for practical purposes.
Note that the electric field is different with the batteries alone than with them connected. When alone, every batteri will have one terminal at negative potential and the other at positive potential, but when connected, the connected terminals will be a the same potential.
The electrons contain no information of where they originated from. It's the electric fields generated by the power source what makes them return to the power source they came from (in a steady state operation).
If you connect 2 power sources in series and ground the mid point, then you have a
split phase power system.
It's hard to analyze circuits from a field viewpoint. Circuits are a very useful simplification. I don't have the knowledge necessary to do so, so I can only give you a rough qualitative explanation. I recommend you to check the book
The Fields of Electronics by Ralph Morrison. It's about circuit behavior explained in terms of electromagnetic fields. Beware, it contains some errors. For instance, I have seen a problem asking the reader to find a force in kilograms (even if kilograms-force are meant, the corresponding clarification and conversion factor is missing). Test charges are described as making or being made a work in units of volts (if it's work, it can't be in volts, electromotive force is probably what is meant). A capacitor in vacuum with square parallel plates of 1 m
2 at a distance of 1 m is once implied to have 1 F of capacitance. I wouldn't be surprised if there was many other errors like this as well, but it's an useful book and I don't know of another similar to it.
Your question becomes trivial to answer with circuit theory: There's no short circuit because there's no complete circuit when connecting opposite terminals of different batteries.
Regards.
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