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Question on current flow through polarized capacitors

  1. Apr 21, 2009 #1
    If a current enters the negative terminal of a polarized capacitors, does the negative plate have to be fully charged before it passes through to the positive plate.
     
  2. jcsd
  3. Apr 21, 2009 #2

    vk6kro

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    If a current enters the negative terminal of a polarized capacitors, does the negative plate have to be fully charged before it passes through to the positive plate.

    No, the plates do not get charged at all. It is the dielectric that holds the charge and it is charged by the difference in voltage between the plates.

    Polarity is only important because electrolytic capacitors need it to be the right way around to form the dielectric.
     
  4. Apr 21, 2009 #3
    so if I understand this properly, a polarized capacitor can fully charge without discharging through the positive terminal.
     
  5. Apr 21, 2009 #4

    vk6kro

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    Absolutely not.

    It is just that a polarized capacitor must not be connected with the + terminal negative with respect to the - terminal.
    They are often used where there is a DC voltage present and this must be connected the right way around.
    They can explode in a very spectacular and messy way if you do it the wrong way.
     
  6. Apr 22, 2009 #5

    vk6kro

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    Any kind of capacitor charges through both plates and also discharges through both plates.

    The electrolytic capacitors have a very thin insulating layer on Aluminium which can be destroyed by electrolytic action if you put a reversed voltage on the capacitor.

    They are quite rugged and you can put an alternating voltage on them, but it is essential that they do not have a steady DC voltage of the wrong polarity on them.

    This coating is not part of the charge applied to the capacitor. The charge is actually in the coating which acts as a dielectric.
     
  7. Apr 22, 2009 #6
    Thank you for the response, it has been very helpful. But if a voltage is applied to the capacitor, will the current flow directly through the capacitor, or will the voltage have to charge the dielectric before the current can pass through.
     
  8. Apr 23, 2009 #7

    vk6kro

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    The DC current only goes into the capacitor to charge it. When you discharge the capacitor, it comes back out again.
    It does not really "pass through" the capacitor.

    This applies to any capacitor.

    The charge on the capacitor eventually gives the capacitor a voltage on it equal to the applied voltage and at that time no more charging current flows into the capacitor. You can take the capacitor and put it away and the charge will still be there.
    When you connect it to a conductor between the plates of the capacitor, current will flow out of the capacitor into the conductor. If you leave it there long enough, there will be no more current flowing and the capacitor voltage will drop to zero.

    Electrolytic capacitors behave exactly like other capacitors except that if there is a dc voltage present it should be the right way around.

    What might not be obvious is that AC voltages and currents can pass through capacitors while DC voltages and currents cannot pass through them.
     
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