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Electric discharge question.

  1. Jul 30, 2010 #1
    I posted someones answer here, with my questions regarding it at the end. The answer you see under this sentence was made in response to my question. Please read this answer and answer my questions at the end of it.

    Let us try an analyze this statement. You mentioned three "buzz words": "(electric) discharge" and "potential difference". Let me try and answer the second one first.

    You know that two opposite charges attract, right? This means, we need to act with some external force to just keep them at a certain distance. If we move them apart, keeping the necessary external force just enough for compensating the attractive Coulomb force between the charges, then this external force will do some positive mechanical work (because work is defined as force times the displacement in the direction of the force). This is similar to the work your hand does to raise a stone against the gravitational pull of the Earth.

    This mechanical work increases the potential energy of the system of two oppositely charged charges. You might place these charges on the plates of a capacitor (which is nothing more but two conductive plates, usually metallic, separated by some insulator, let's say dry air). One plate will be positively charged and the other plate will be negatively charged. It takes extra mechanical work to move some more positive charges from the negative plate to the positive plate, or, conversely, negative charges from the positive plate to the negative plate. Electrons are negatively charged. The mechanical work necessary to move a unit of charge is defined as potential difference. As you pile more and more opposite charge on the plates, it requires more and more mechanical work to move the same unit charge between the plates. Of course, moving the same amount of charge in the opposite direction will liberate the same energy as the mechanical work necessary to move it in the forward direction. This energy is usually given off as kinetic energy of the free charged particle.

    Now, let's talk about discharge. Discharge is the effect we see from avalanche ionization of gases. You know that atoms are composed of nuclei and electrons circling around them. It takes a certain amount of energy (or work) to remove one electron from the atom, i.e. to ionize it (it's called ionization energy). This energy can be supplied in any number of ways. For example from electromagnetic radiation (UV, X, gamma rays), bombardment by charged particles and so on. Here, we are specifically concerned with the following possibility of ionization. Let's say some atom gets ionized by some random event (always present background ionizing radiation). The electron will, in general, collide with other atoms, and, ultimately may find an ion with one electron short and recombine giving off the extra energy in form of electromagnetic radiation (photon) with the right frequency.

    But, suppose this ionized atom and electron are in the vicinity of a charged capacitor as described above. The free electron gets accelerated in the electric field of the capacitor and gains kinetic energy due to the potential difference. The bigger the potential difference, more energy the electron will gain on the same distance. Suppose the kinetic energy that it gains between two collisions is big enough to ionize the atom it collides with. This is called secondary ionization and it is the criterion for the onset of an avalanche of electrons and ions. Finally, an electric current starts flowing from the free electrons (and ions) that move and the free electrons finally reach the positive plate and decrease its positive charge, while the positive ions reach the negative plate and recombine with some of the free electrons on the negative plate and decrease its negative charge. This decreases the potential difference and the process will end once the potential difference drops under a certain critical value.

    During the discharge, the gas gets heated violently and expands, creating the characteristic sound. There is also light from the recombination processes. This is manifested as an electric spark.[/QUOTE]

    Paragraph1. I understand that two opposite forces attract, so the external force needed to keep these to attractors apart decreases as they are pulled further apart? I do not understand what mechanical force is, or how it comes to play.

    How does mechanical work increase potential energy, is it because the energy needed to keep the Electron and Proton away form each other decreases, using the excess energy as mechanical force? And dry air is an insulator because it lacks moisture, which is a conductor (why does moisture conduct)? How does the mechanical work move charges from opposite plates, and why does the force needed to move charges from one plate to anther increase as more charges are moved?
    Here is something I do not understand

    "Of course, moving the same amount of charge in the opposite direction will liberate the same energy as the mechanical work necessary to move it in the forward direction. This energy is usually given off as kinetic energy of the free charged particle."

    I will ask the questions for the nest paragraph after someone answers these.
  2. jcsd
  3. Jul 31, 2010 #2
    I will try to answer your question , Electric potential (volt) is the work per unit charge to move a charge , independent of the path , And the electrons in a circuit move because of the electric field . And to have a potential difference you must have different electric field strengths between the two points to get a voltage , If the E fields strengths are the same there will be no potential .
    things conduct because they have free electrons, not bound to the atom .
    And if you have time i would watch walter lewins lectures on E&M , they are good .
    this one talks about Electric potential , and might help
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