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Thread: What is capacitance

  1. #1 What is capacitance 
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    Hi. I was wondering how would you define capacitance in chemistry terms. For example current flow is basically energy makes bond break and electron forms a bond with the next atom and so on. So what is capacitance if you want to look at it in terms of molecules?

    P.S. I'm a layman in chemistry and know nothing about it so maybe my explanation for current flow is wrong for which I'm sorry.


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  3. #2  
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    Quote Originally Posted by fine View Post
    Hi. I was wondering how would you define capacitance in chemistry terms. For example current flow is basically energy makes bond break and electron forms a bond with the next atom and so on. So what is capacitance if you want to look at it in terms of molecules?

    P.S. I'm a layman in chemistry and know nothing about it so maybe my explanation for current flow is wrong for which I'm sorry.
    I can have a go.

    First of all, flow of electric current involves those electrons that are NOT bound to any particular atom. So no "bond break" is involved. There is, in metals, a continuum called the conduction band, made up of the overlap of the outermost orbitals of the atoms. Electrons in this "collective orbital", if I can call it that, no longer belong to any atom in particular. It is sometimes referred to as a "sea" of electrons. It is these electrons that flow when an electric current passes through a conductor.

    Capacitance, at the atomic level, is a buildup of electrons on one plate of the capacitor, which thereby becomes negatively charged, and a deficit of them on the other plate, which becomes positively charged. The greater the area of close contact between the plates (while maintaining the insulation between them), the greater the amount of +ve and -ve charge that can be stored, because the electrostatic attraction stabilises the imbalance of charge, allowing more to accumulate for a given voltage.


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  4. #3  
    KJW
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    Quote Originally Posted by fine View Post
    I was wondering how would you define capacitance in chemistry terms.
    Capacitance is a quantity in the field of electronics or at a more fundamental level, physics. The only aspect relevant to chemistry is the dielectric constant of the material between the plates. The dielectric constant depends on the permanent dipole moment and the polarisability of the compound. It also depends on temperature because higher temperatures reduce the net effect of the permanent dipole moment due to thermal randomisation of the orientations, but polarisability is unaffected.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  5. #4  
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    Thanks for the replies but I was wondering about it more fundamentally like what happens with atoms or molecules that causes capacitance. I get that it's a physics phenomenon but it's real and if it's real the atoms and molecules must be up to something for it to occur.
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    Quote Originally Posted by fine View Post
    Thanks for the replies but I was wondering about it more fundamentally like what happens with atoms or molecules that causes capacitance. I get that it's a physics phenomenon but it's real and if it's real the atoms and molecules must be up to something for it to occur.
    Capacitance has no specific relationship to atoms and molecules. It is nothing more than a relationship between charge displacement and the electric field associated with that displacement. Atoms and molecules can have net charge due to ionisation, so that's the way they can form part of a capacitance. Neutral atoms and molecules can affect capacitance if they form dipoles.

    The relevant constitutive relationship that defines capacitance is C = Q/V, where Q is the amount of charge displaced, and V is the voltage associated with that displaced charge. The canonical undergraduate scenario has a pair of parallel plates, where an applied voltage causes some electrons to move from one plate to another, but any population of displaced charge will do. No atoms or molecules are essential. Protons over here, electrons over there = capacitance, too.
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  7. #6  
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    Quote Originally Posted by fine View Post
    Thanks for the replies but I was wondering about it more fundamentally like what happens with atoms or molecules that causes capacitance. I get that it's a physics phenomenon but it's real and if it's real the atoms and molecules must be up to something for it to occur.
    I had a go at this, admittedly in a rather hand-waving way, in post 2. And tk421 has referred to the way in which the properties of the molecules in the dielectric affect the capacitance, which I did not deal with.

    If you can say a bit more about why you find those descriptions unsatisfactory, perhaps we can take it further.
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