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Charging a capacitor

  1. Jul 7, 2009 #1
    when we say batteries create the potential difference between the two ends of the capacitor..what does it mean?how are does a plate become negatively charged and positively charged?please explain
     
  2. jcsd
  3. Jul 8, 2009 #2
    I had a lecture yesterday, and I imagined it all. Batteries have unequal potential, which basically means there are more electrons on the negative side, which mkes it unstable. Conductors have conduction electrons which can be easily displaced and replaced. A capacitor causes the circuit to break. So the extra electrons deposit on the plate and the other plate is said to be positive with respect to it.
     
  4. Jul 8, 2009 #3

    berkeman

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    http://en.wikipedia.org/wiki/Battery_(electricity [Broken])

    .
     
    Last edited by a moderator: May 4, 2017
  5. Jul 8, 2009 #4

    berkeman

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    Batteries are not unstable. Please see the link that I posted.
     
  6. Jul 8, 2009 #5
    sir,why are you advertising here..u r a mentor!
     
  7. Jul 8, 2009 #6
    Potential difference is the instability resulting from the work done to create it, and this instability drives the elecrons round the circuit to the other end until there is no potential difference(a condition of stability)
     
  8. Jul 8, 2009 #7
    It may help to think of the battery as an electron pump.Electrons are pumped from the negative side of the battery to one of the capacitor plates and away from the second capacitor plate to the positive side of the battery the current being the same on both sides.The result is that the plates charge up,one side being negative and the other side being positive.As the charge builds up the voltage across the capacitor plates increases and since this opposes the battery voltage the current falls.The capacitor is fully charged when the voltage across its plates becomes equal and opposite to the battery voltage.
     
  9. Jul 8, 2009 #8
    does that mean at the time when the capacitor is fully charged,and supposing the potential difference is 2v,the battery will have a negative potential of same magnitude..according to your explanation.?
     
  10. Jul 8, 2009 #9
    Yes, the potential difference across the capacitor will increase and eventually become equal in magnitude to the potential difference(e.m.f.) of the battery.It's like having two equal voltage batteries in a circuit which are pushing against eachother.
     
  11. Jul 8, 2009 #10
    That is drastically wrong.If according to Dadface's interpretation, the battery is an electron pump, then it must have energy to pump the electrons which comes from the work done in keeping the electrons separated from positively charged atoms in a battery.The atoms and electons are separated by a partition inside the battery, so the electrons can only take the long way out by substituting conduction electrons in the wire and repelling them to the positive end of the battery.
    In the case of a DC source applied to a capacitor the circuit is broken and we say electons are deposited on the plate of the capacitor but there is no current in the circuit.You cannot think something like when the battery is removed the electrons are withdrawn by the battery because even at this point, the battery is more unstable than the wire.
     
  12. Jul 8, 2009 #11
    There is a current whilst the capacitor is charging(and when it discharges).The current falls exponentially with time and at a rate that depends on the time constant(RC) of the circuit.The charging current falls to zero when the p.d. across the capacitor plates becomes equal to the battery voltage and it is this latter point that I think needed clarification.:smile:
     
  13. Jul 8, 2009 #12
    That's right and obvious.That's why it is called charging and discharging.
     
  14. Jul 8, 2009 #13
    I think perhaps you misunderstood the point that pras quantum was making and I misunderstood the point you were making.You did refer to a d.c. circuit and "no current" but I wanted to clarify this.
     
  15. Jul 8, 2009 #14
    Your initial explanation is perfectly right.I should have read it before.Also, I made a mistake in saying "but there is no current in the circuit" without a proper head or tail of it.There is null current between the times of charging and discharging.
     
    Last edited: Jul 8, 2009
  16. Jul 8, 2009 #15

    berkeman

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    If you are referring to my footer, it is not commercial advertising. It concerns Amatuer HAM radio, which in the context of my footer, is used for emergency communications in disasters and other emergencies. We volunteer our time and equipment, there is no charge.
     
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