Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding Capacitors - Form & Function

  1. Jul 22, 2006 #1
    Hello peeps, im currently studying capacitors and simple electronic circuit design, but im getting the feeling im not quite fully understanding things because i keep forgetting key concepts and how they fit together in an abstract sense.

    (please forgive the long post, its an attempt to get everything out and shake out any misunderstandings and uncertainties, I'll Add my succinct questions to the end)

    Some Background:
    I am trying to learn what capacitors are used for, and how to go about selecting the right capacitor for a given problem or circuit design.

    I understand that capacitors are used for storing charge and as filters, in the form of band-pass/stop high/low-pass filters. I beleive that this filtering is due to the frequecy response of the capacitor as an oscillating system, in which it takes time to fully charge a capacitor plate and that this is a property of the capacitor material and the current and voltage. I think of this analogus to a pendulum oscillating system attached to some damping device.

    I have learned how a capacitor stores charge by seperating charges with a dielectric (non-conducting) material, and that there exists an electric field between any two seperated charges. The energy held by a capacitor is a measure of the potential work that the capacitor can do, when the seperated charges are allowed to flow back to equillibrium.
    I beleive that i am correct in saying it is and applied voltage accross the capacitor that maintains the charge seperation accross the dielectric material?

    In terms of circuit design i have seen how a capacitor may be used in conjunction with a resitor to create a filter, but i cant say i fully understand how the two components work to acheive this.

    Im terribly confused. I dont quite understand what an electric field is, other than a seemingly abstract concept which sais that "If you roll this ball of charge away from that ball of charge, then the work you put in you will get out eventually when they roll back together again" (conservation of charge & energy laws).

    So my main Questions:
    • [1]What properties make a good dielectric material? (on a molecular and/or subatomic level of detail).

    • [2]In a resistor, electrical charge is converted to thermal energy, by what mechanism is this acheived? (again, this is mostly a question of molecular mechanics i guess).

    • [3]If i was designing a circuit that required a certain frequency filter, what would help me decide on the Capacitor type and rating and the resitor type and rating?.

    (you'll notice my complete lack of mathematical skill here, sorry in advance)
  2. jcsd
  3. Jul 22, 2006 #2
    In that case you should probably suspend study about capacitors for now and read about electric fields and forces. Mathematically speaking, an electric field is just a vector function of three dimensional space. Physically speaking, it is the force experienced by a unit positive charge when it is placed in a region where the field exists.

    • To quote from wikipedia (http://en.wikipedia.org/wiki/Dielectric_materials#Some_practical_dielectrics)

      You can also google this up (sorry this is not an adquate response to quetson 1).

      Electrical charge is not converted to thermal energy! Charge flows in the circuit and is conserved. Flow of charge gives rise to a current which causes Joule heating (dissipation of energy) in a resistor. As a current I flows through a resistor of resistance R, a potential difference V = IR (assuming Ohm's law) develops across the two terminals of the resistor. The power dissipated in the resistor is equal to [itex]V\timesI = I^{2}R = V^{2}/R[/itex].
  4. Jul 22, 2006 #3


    User Avatar

    Staff: Mentor

    No, it's not. Electric charge is never created or destroyed in a circuit, it's simply moved around from one place to another. The thermal energy that appears in a resistor comes from electrical potential energy associated with the charge. As the charge (electrons) flow through a resistor, they lose some of their electrical potential energy, which becomes kinetic energy of the random motions of the molecules of the resistor, i.e. thermal energy.

    If you're studying electronics from a source (textbook or Web site or whatever) that doesn't go into the underlying physics, you should pick up a decent undergraduate introductory physics textbook. New ones are expensive, but a fifteen-year-old used one will work fine. These parts of physics haven't changed in over a hundred years, after all. :smile: You'll find that they cover energy, electric forces and fields, and electric potential energy, and then cover capacitors, electric circuits, etc. as applications of those concepts. It sounds like you're trying to do it backwards. :yuck:
  5. Jul 22, 2006 #4
    Appologies, the dissipation of heat by a resistor is what i was thinking of. By what process is this heat disipated from the flow of charge according to the power law?

    I am familiar with Coulombs law for point charges and the electric field, to some extent.

    Thanks for the reply. Your right i dont fully understand electrical fields, i find the concepts self referencial. I'll just have to bang my head some more and hope it goes in eventually.
  6. Jul 22, 2006 #5


    User Avatar
    Gold Member

    The US military put out a book on hands-on electronics (around WWII, I think) called The Radioman's Guide. It's a decent basic no-frills book on basic electronics and I used it to help me understand basic circuits before tinkering on my own guitar amps and eventually those of others, too. You can probably find copies in used book stores and libraries.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook