Total capacitance and polarity

Discussion in 'Homework Help' started by HerculePoirot, Mar 6, 2011.

  1. HerculePoirot

    Thread Starter New Member

    Mar 6, 2011
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  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    How could it matter? Capacitance, or at any rate ideal capacitance is not a polarity-sensitive property. The capacitance of the individual capacitors dopes not change with polarity, nor does that of the equivalent capacitance that they make up.

    The situation could be different if the capacitors had some property of varying according to the applied voltage, as happens with, say, varicap diodes, but this is a rather special case.
     
    Last edited: Mar 6, 2011
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You will also find electrolytic capacitors which can only be operated in one polarity, but this is a feature of a particular technology, not of capacitance in general.
     
  4. HerculePoirot

    Thread Starter New Member

    Mar 6, 2011
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    Yes, I got that, but why is this so? I know the facts, but I want an explanation of the facts.

    Let me quote Wikipedia and explain my confusion:
    "In electromagnetism and electronics, capacitance is the ability of a body to hold an electrical charge."

    So, then there is positive charge on one side of the capacitor, and a negative charge on the other side, right? If I have another capacitor with a reversed polarity, and have them connected in series, wouldn't the charges "mess with each other"?
     
  5. mjhilger

    Member

    Feb 28, 2011
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    When a circuit is built, the caps are almost always uncharged (= 0 volts). A caps voltage is the integration of the current through the device. So if we start at 0 and the caps are connected in series ---||---||---, then the current travels the same direction through both caps (say left to right ->). So the polarity of the charge on each is +- +-. if the current is the other direction (right to left <----) again starting at 0; then the polarity on the caps is -+ -+.
    Polarity of a cap is a property of the type of that particular manufacturing process. An ideal cap is 2 conducting surfaces seperated a distance with some material in between. The material may be air or something else; that material is the dielectric (this material prevents the actual conduction, it is an insulator). Certain types of dielectrics are sensitive to the direction of greatest charge (polarity)and can break down; so as the charge accumulates and the voltage builds one way is fine, if the cap is turned the other way and the voltage builds, the dielectric breaks down. If the dielectric breaks down, in the simple statement the cap becomes a straight conductor; however the breakdown of certain dielectric materials can cause a fire or small explosition. All dielectrics are subject to an upper limit of voltages across the distance as a high enough voltage will cause any material to break down and conduct. As is the case of lightning, when a high enough charge is accumulated on a cloud the air between it and the ground say, the air begins conducting. The lightning bolt is a plasma stream of the streaming current of the discharge. In that case the air - normally a good insulator, begins conducting. Air as a dielectric is not polarity sensitive.
     
    Last edited: Mar 6, 2011
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