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Constant Current in Batteries? How?

  1. Apr 21, 2012 #1
    I do not know the exact electrochemical reactions which make a battery work, but I understand that the essential anatomy of a battery is a cathode and an anode separated by an electrolyte, and the anode absorbs electrons. When the battery is connected to a wire, the electrons move from the anode tot he cathode, producing a current.

    I have two questions.

    Due to the electrical potential difference, it would follow reason that the electrons would accelerate toward the cathode when a wire is connected. If this is the case, then how is there a constant current in the battery if current is defined by dq/dt (change in charge over time)?

    Secondly, what is the difference between two batteries of different voltage? How is the electrical potential difference lowered or increased? (more of an engineering question)

    Thanks in advance for responses. I am new to to these forums and hope to learn as well as contribute.
     
  2. jcsd
  3. Apr 21, 2012 #2
    Constant current batteries are not produced in primary cells, such as lead-acid, NiCd, Alkaline, NiMH, etc., because constant voltage works better. Cells are constructed to produce constant voltage. As load current is demanded, the voltage drops due to internal resistance. By minimizing resistance, better voltage stability is obtained.

    Nuclear cells are still being developed. These cells work better in the constant current mode. The voltage changes as the load varies but the current reamains approximately constant. Any battery bought for commercial use is the constant voltage type.
     
    Last edited: Apr 21, 2012
  4. Apr 21, 2012 #3
    So the current in a wire connected to a standard 12 V battery is not constant?
     
  5. Apr 21, 2012 #4

    phinds

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    The current in a wire connected to a standard battery is a function of the resistive load to which the battery is applied.

    If you mean JUST a wire then you have a short circuit and all bets are off (except for the fact that you are likely to get a VERY hot wire and/or battery)
     
  6. Apr 21, 2012 #5
    I guess I'm not being very clear, as the response does not answer my question. I am not asking about the engineering of the battery itself which allows it to function (except for my second question on how batteries increase voltage), but rather how the basic principles which allow batteries to work are leading me to believe something which does not occur.
     
  7. Apr 21, 2012 #6

    jtbell

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    No. Crudely speaking, the atoms in the wire get in the way, so there is a terminal velocity. It's not like a cathode-ray tube in which the electrons travel through vacuum.
     
  8. Apr 21, 2012 #7
    An electric wire has millions-trillions of electrons. Therefore, the process never ends up. Electrons keep flowing and flowing till there is a potential difference.
     
  9. Apr 22, 2012 #8

    phinds

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    Seems to me you have cause and effect backwards here. Electrons don't flow "until there is a potential difference", they flow BECAUSE there is a potential difference.
     
  10. Apr 22, 2012 #9
    Actually it's not a cause and effect thing, but rather, a chicken and egg thing. In order to create a PD (potential difference) charges must move and separate. Then when that happens, other charges can be influenced by the associated electric field due to charge separation and move as result.

    Is PD "caused" by charge motion, or is charge motion "caused" by PD? Actually it's both. The problem with thinking that charges move because of PD lies with the flow inside the battery. The charges inside are literally moving AGAINST the electric field, i.e. "uphill". Hence these charges are not moving BEACAUSE of PD, but rather their motion gives rise to the PD. The charges outside the battery are going "downhill", and they move due to E field influence or PD.

    Claude
     
  11. Apr 22, 2012 #10

    phinds

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    That's a good explanation and certainly seems right for a battery. thanks.
     
  12. Apr 23, 2012 #11

    And the current in a given wire does not just change overall depending on the resistive nature of the rest of the things in the circuit, it also changes from moment to moment as the other parts of the circuit operate. For instance, a bulb slowly heats up after being turned on, and becomes more resistive in the process, thereby drawing less current from the battery through the wire.
     
  13. Apr 26, 2012 #12
    I know that current is directly proportional to voltage but how is voltage directly proportional to current ?
     
  14. Apr 26, 2012 #13

    phinds

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    HUH ? Do you know Ohms Law?
     
  15. Apr 27, 2012 #14
    Our teacher said so when he was describing Ohm's Law. I agree that current is directly proportional to voltage but how is the reverse possible.
     
  16. Apr 27, 2012 #15

    phinds

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    Well let me ask you the question in math. If A is proportional to B, it B proportional to A?
     
  17. Apr 29, 2012 #16
    Yes, it is...
     
  18. Apr 29, 2012 #17

    phinds

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    Then what's your problem?
     
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