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Why do batteries in parallel add in current but not voltage?

  1. Mar 29, 2015 #1
    Consider two fixed (-) source charges in free space. if you look at the field at any point created by the two charges, the resulting potential is due to the superposition of the two fields.

    for two batteries in parallel then, what happens to the two fields at the junction such that superposition does not apply?
  2. jcsd
  3. Mar 29, 2015 #2


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    They don't. Putting similar batteries in parallel does increase the possible current that the pair can provide (over what one of them can provide) but what they ACTUALLY provide is dictated by the load (see Ohm's Law) not by whether you have one alone or two in parallel.
  4. Mar 29, 2015 #3

    Doug Huffman

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    Batteries in parallel add the area of the plates, while in series the plate potentials are added.
  5. Mar 29, 2015 #4
    okay, but my question still remains; what happens at the junction such that the potentials don't superimpose?
  6. Mar 29, 2015 #5


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    I don't get why you think they SHOULD superimpose since they are potentials. It would be like saying that if you have a rock sitting on a 3 foot high post and then next to it you put another rock on it's own 3 foot high post, now you have to say that the two of them are sitting together on a 6 foot high post.
  7. Mar 29, 2015 #6

    Doug Huffman

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    Do potentials 'superimpose' as waves do?
  8. Mar 29, 2015 #7
    please tell me which one of these you disagree with and why

    * the potential is a property of the electric field.

    * the electric field in a circuit is guided by its conductors

    *i have one power source in a circuit which produces an electric field

    *i have another power source in the same circuit which independently produces its own electric field

    * since the two sources of electric fields are in the same circuit, their electric fields must interact somehow

    the fields interact in series why shouldn't they interact in parallel? the interaction is magic to me right now. i don't know how it happens, please do enlighten.
  9. Mar 29, 2015 #8
    as a matter of fact yes they would i had two oscillating electric fields occupying a common region of space.. but luckily i'm just talking about a static case
  10. Mar 29, 2015 #9


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    Given that the two field generators could be miles apart, I don't see why their fields should interact. Two voltage sources do, taken together, create the ability to drive more current than either on separately and that's true even if the sources are miles apart
  11. Mar 29, 2015 #10


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    That's a wrong analogy. In fact it corresponds to putting two e.g. electrons in a potential field. The right analogy is putting two watermills next to each other. If one watermill can increase the height of water from h1 to h2 and the other exactly the same, then there is no superimposing. Don't try to visualize it because I don't mean first increasing the height from h1 to h2 using the first watermill and then increasing it from h2 to h2+(h2-h1). I mean the watermill are exactly the same in the same heights. But I think we shouldn't push this analogy too far!
  12. Mar 29, 2015 #11


    Staff: Mentor

    If you put two non-ideal batteries in parallel, a circulating current will be produced that tends to make the two sources equalize in voltage. Consider a 12v battery in parallel with a 6v battery. The internal resistances of the sources and the resistance of the connections determine the magnitude of the current. If you consider the resistance of the connection to be zero, then the voltage of the two sources will be forced to the same intermediate value.

    It sounds like you forgot that a voltage is a difference in potential between two points; not one. By analogy, consider 1 foot high milking stools. Three stools on top of each other (series connection) allow you to step up three feet. Three stools on the floor (parallel) allow three times as much weight to be lifted one foot.
  13. Mar 29, 2015 #12
    actually i hadn't considered this scenario. but, glad it was brought up.


    the green represents the electric field emitted by the left battery. the blue, by the right. and so when their fields equalize the potential = 0.
    This agrees with my intuition. so i really get it.

    But here's where i'm having a hard time. If i attach wires to the top and bottom, this is what i have pictured in my mind as far as what goes on. i know i have something wrong, i just don't know what.


    so now you can understand why i see the two fields adding at the junction point
  14. Mar 29, 2015 #13


    Staff: Mentor

    Your arrows are depicting the currents, not the voltages. Yes, opposing currents superimpose and can cancel or add.
  15. Mar 29, 2015 #14
    no not current nor voltage. i get how the current ends up going that route, but i was depicting the electric field.

    first of all, is my first picture correct in depicting the E-field?
    what about my second picture? if i have the wrong idea about the junction, why's the incorrect?
  16. Mar 29, 2015 #15


    Staff: Mentor

    You seem to have current and voltage mixed up. Forget the word field, that doesn't help in cases like this.

    I hate water analogies, but in this case it mighthelp. Imagine the wires to be like canals that connect water basins at unequal heights. The heights are analogous to voltage potential and the flow in the canals are analogous to electric current. Of course the heights and flows are related (water flows downhill) but they are not the same.

    Electric potential does not "flow" through wires.
  17. Mar 29, 2015 #16
    so do i hahah

    i do not have voltage and current mixed up.

    (a ended up writing a lot but i can save you some time; basically i want to understand what's going on at the junction from a fundamental standpoint. and from such a standpoint, you can't explain current without even mentioning the electric field)
    for cases like this the only time the field does not need mentioning is when "voltage stays the same in parallel" applies. i don't see how this principle applies here which is why i'm trying to fall back on a more fundamental quantity

    which is why i keep referring to the thing that does "flow" through wires.
    i get that the electric field is rather unimportant from an engineering standpoint (for this problem), but i'm trying to understand this from a fundamental perspective. i think it's only fair to talk about all contributing fundamental quantities that are in the problem, and explain the phenomenon via those fundamental quantities, ie the electric field.

    voltage stays the same in parallel yes. but this isn't "just because." do correct me if i'm wrong ---> but i can reason that (in a circuit with a voltage source and say.. two resistors in parallel) potential stays the same in parallel because the electric field essentially splits at the parallel junction. and this splitting doesn't have any effect on the field itself (which is where the potential comes from!) and that's why potential stays the same in parallel.

    my original question deals with a case where there are two independent fields entering a junction.

    say i didn't short the two top and bottom canals (as shown in the picture), you have to talk about potential first before you can talk about any current flowing. and to know exactly what's going on for this case, i don't see how one can avoid talking about the electric field and determine the potential (height) at the two canals.

    basically, if i'm understanding you correctly you're saying: potential stays the same in parallel. but as i've explained earlier, i can see how that's the case for a circuit with one voltage source powering two loads in parallel, but i don't see why "voltage stays the same" should apply here.
    Last edited: Mar 29, 2015
  18. Mar 29, 2015 #17


    Staff: Mentor

    The gravitational field is the source of the potential energy difference of water with height. But do you need to use superposition of gravitation to figure out which way the water flows in a three-way junction? Do you have to imagine the gravity field "splitting" at a junction?

    Even though you deny it, everything you have said makes it sound like you imagine fields as acting like currents. That's wrong.
  19. Mar 29, 2015 #18
    ah, i can understand how i would make myself sound like that. but no, the field forms in a particular path / way, and the charge carriers "ride" the field. so although no, field doesnt = charges.. but the charges do go where ever the field takes it. so.. my goal is to figure out WHY the field forms the way it does at the junction, and i'll have no more gaps in my understanding.

    yes precisely. the 'field' itself isn't really important in this case because i already have an intuition what's a function of what. ie, the energy being a function of height.
    But your analogy is incomplete. Allow me to make it more accurate. Imagine the universe was T shaped, and in the top portion you have a pipeline as wide as that part of the universe, the pipe is filled with water. At each end there is a massive, negatively dense object (positive gravity). Midway along the pipeline, there's a junction extending down the vertical part of the T part of the universe, where the pipeline here is also as wide as this vertical part of the universe. at the junction, something warps the space in that region in a way that will make sense if you continue reading this ridiculous example (haha...)

    ...Now we have reconstructed the problem in the form of a silly gravity circuit.. picture the positive gravitational field lines extending from each end towards the center, where it then becomes "redirected" (warped/gravitationally lensed) down the vertical T part. the water will want to be pushed towards the center junction, but then what? you have a superposition of fields so they potentials still add!!!....
    Last edited: Mar 29, 2015
  20. Mar 29, 2015 #19


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  21. Mar 29, 2015 #20


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    Consider the situation if you had 2 capacitors instead of 2 batteries. The positive plates are both charged to +10v and the negative plates at -10v, both with respect to ground, for a total voltage of 20v across the plates.

    Now, imagine taking a wire and connecting the two positive plates together. What's the voltage of the wire with respect to ground? Assuming the wire has negligible capacitance compared to the capacitors, it's 10 volts! A wire attaching the two negative plates together is charged to -10v. If it helps, you can imagine attaching the wires to the plates while they are still neutral and then charging them to +10v and -10v by using the wire connecting each pair.

    What you're asking is equivalent to asking why does applying +10 volts to the wire connecting the positive plates charge each one to +10v instead of +5v. The answer to that question is that the two plates must be charged to the same potential as the applied voltage. If I apply +10v to the wire prior to connecting it to the plates, current will flow until the voltage of the wire is equal to the applied voltage. (Obviously, since if the voltage of the wire is different from the source then a difference in potential between the source and the wire exists and current will continue to flow) If I then connect the two plates to this wire, there will be a difference in potential between the plates and the wire, and by extension between the plates and the voltage source, and current will flow until both plates are at +10v. The same process applies to the negative plates too.

    Now, let's connect a resistor between the positive wire and ground. The resistor sees the ground as 0v and the wire as +10v. So it has 10v of potential across it and current will flow accordingly. Now attach a wire between the neutral side of the resistor and the negative side of the circuit. The resistor now has +10v applied on one side, and -10v on the other side, so the voltage across it is 20v. In the end you wind up with two capacitors at 20v in parallel with each other, and a resistor with 20v applied across it, not 40v.

    Does all that make sense?

    Also, consider that if the voltage added together you would have a violation of conservation of energy. Let's say a battery can provide Y number of charges with X amount of potential energy each. The battery can then provide a total energy equal to YX. Putting two of these batteries in parallel means that the total energy capable of being provided by the batteries is 2YX. A resistor connected to these batteries sees 2Y charges pass through it, each with a potential energy of X, giving you 2YX energy used by the resistor. In other words, in this simple circuit the resistor consumes all of the energy. Let's say that 2YX = Et, or total energy. Each battery provides YX energy, or 1/2 Et, so adding them together gives you 2YX or Et and the resistor consumes all of this Et energy.

    Now, if voltages added, the voltage across the resistor would be twice as high as in the above example. This means that each charge that passes through the resistor has energy equal to 2X. That means that the total energy consumed is 2Y2X = 4YX = 2Et. But how can this be? This requires that each battery provide Y charges with 2X energy. (Or 2Y charges with X energy) This obviously cannot happen since we stated that each battery can only provide X energy to Y charges. You'd have a case where a battery could provide twice as much energy as normal just by connecting it in parallel with another battery without any change in the chemistry or physics of the battery. This is a violation of the conservation of energy and cannot occur.

    Note that when batteries are connected in series you have an opposite situation. Since each battery can only supply Y charges, and they are connected in series, only Y charges can be given up to the circuit. (only Y charges can pass through the battery because of the way the underlying chemistry works) So if one battery provides Y charges at X energy each, two batteries in series provides Y charges with 2X energy each to get 2YX energy. So you can put several 1.5 v batteries together in series to get the higher voltage you need to run many electronic devices.
    Last edited: Mar 29, 2015
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