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Homework Help: Charging capacitor - no resistor

  1. Jun 5, 2012 #1
    1. The problem statement, all variables and given/known data

    When charging a capacitor with NO resistor in the circuit, the p.d of the capacitor IMMIDIATELY reaches a maximum....


    2. Relevant equations

    3. The attempt at a solution

    ....is the following reasoning correct:

    As stated by Kirchoff's Law, the sum of the potential differences across all components in a series circuit is equivalent to the the emf of the source...as there are no other compoenents in the circuit the p.d across the capacitor = emf of source immediatley.

    (Note: when a resistor is present the p.d across the resistor decreases intil the p.d across capacitor = p.d source)

    Is this correct?
  2. jcsd
  3. Jun 5, 2012 #2


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    Homework Helper

    I like your reasoning - but would like to see some reference to the idea that you have been instructed to assume the resistance of the wires is zero, and thus regardless of current flow, the p.d across them will be zero, and then an explanation that if real wires were used, their tiny resistance would mean a very short charge time, rather than an instantaneous charging.

  4. Jun 5, 2012 #3
    this was a question I made up myself based on a diagram in my book
    thanks so much, through, for confirming my reasoning :)
  5. Jun 5, 2012 #4


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    Gold Member

    You are also leaving out the internal resistance of the power source. There is no such thing as a totally ideal power source.
  6. Jun 5, 2012 #5


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    Staff: Mentor

    Also, any wire, no matter the resistance, has some inductance. Between that and the results of attempting to drive an infinite current (requiring infinite speed for finite charge carriers) you've left the realm of normal circuit theory and entered Maxwell's and Einstein's domain. Happy adventuring :smile:
  7. Jun 5, 2012 #6
    The circuit as posed is not a physics problem but a circuit theory problem. The battery has zero internal resistance as do the wires. Assuming you switch the discharged capacitor onto the battery you will have an infinite current for an infinitesimal time as the capacitor charges. The math can be handled by the theory of distributions, aka delta functions.
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