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Equivalent emf

  1. Aug 19, 2015 #1
    I heard that, emf cannot be parallely connected provided that the value of the emf are not same.

    Is this correct? Like, what would happen when batteries with 3V, 4V, 5V are parallely connected? And the equivalent emf?
    Last edited: Aug 19, 2015
  2. jcsd
  3. Aug 19, 2015 #2
    Well , yes . Imagine two cells connected to each other . If conditions are perfectly ideal , then current in that loop would be infinite ( From Kirchoff's loop law ) .

    However , if they both have large enough internal resistance , then you could connect them ( Why ? ) .
    And the third case - If internal resistance tends to negligible , similar to the first case , current would ...
  4. Aug 19, 2015 #3


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    The 5V battery would discharge through the lower voltage batteries. The resultant emf is going to depend on the relative resistances of the batteries. Also, the current that flows will depend on the resistances. Some batteries will be seriously harmed by such an arrangement.

    If there was some good reason for hooking these in parallel (I can't think of a good one off hand, but there might be) you might be able to get somewhere by putting in a diode at the correct locations.
  5. Aug 19, 2015 #4

    The two cells connected in which way? Just connected? Or, with an external resistance also?
  6. Aug 19, 2015 #5
    Either way , doesn't matter . Consider the loop containing both the cells only . It doesn't matter what the external resistance is .
    Last edited: Aug 19, 2015
  7. Aug 19, 2015 #6
    Well, it will harm the other batteries, but will be of some work for itself, right?
  8. Aug 19, 2015 #7
    Then the similar case would happen with a cell connected with a single wire(short circuit). The current flow will be infinity.

    But does it actually happen?

    (Sorry, I was troubling in posting messages, so I rewrote it)
  9. Aug 19, 2015 #8
    I think you should first state what conditions you are assuming .
  10. Aug 19, 2015 #9
    Well, I have lackings in this. Does the internal resistance include the resistance of the wire?

    I don't know what you meant by conditions. I'm just talking about a normal cell, where two ends of the emf are short-circuit. So, then also, the current flow will be infinity, right? (From the thought you said for the two connected cells).
  11. Aug 19, 2015 #10
    What I am trying to say, is that, if the cells connected in parallel faces infinity current, so does the short-circuit (I knew it doesn't??). So, what makes parallel cells more impossible?

    I understand the fact that the cells will be damaged, and it is impractical to do so. But what happens, if connected? What about the equivalent emf?
  12. Aug 19, 2015 #11
    The first part - Cells produce current by converting chemical energy into electrical energy . The reaction in cells involves movement of ions , and th e resistance offered inside the cell , to their movement is the internal resistance .
    When electrons move through the circuit , there is an opposition to their flow . This constitutes the resistance of the wire ( It is considered negligible in many cases ) .

    You seem to be mixing the concepts of ideal circuits to real ones . A circuit with enough resistance can have , say , batteries of 5V and 4V connected in parallel .
  13. Aug 19, 2015 #12
    But what if they don't have enough resistance?
  14. Aug 19, 2015 #13
    See post #2 , or #3 .
  15. Aug 19, 2015 #14
    That means, only the 5V battery is going to work?

    What equivalent emf? Suppose, if the internal resistance be 1 ohm, for two cells 5V and 4V parallely connected? The question may seem silly, but is popping in my head.
  16. Aug 20, 2015 #15


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    It's a straightforward circuit to sketch. You need to represent each of these non-ideal components by something equivalent but comprising all ideal elements, then apply circuit theory such as Ohms Law.

    First step: sketch the equivalent circuit, and post your sketch here.
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