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Basic questions about electric charge

  1. Apr 28, 2013 #1
    Hello, I just started to read up on some basic electromagnetism and have some questions on:

    1) Why do the electrons flow from the glass rod to the silk when you rub it? Why the preference for the silk? And if glass is an insulator, how do the electrons 'escape' the atoms their orbits (looking at the picture classically)?

    2) How do you tell which material becomes negative when you rub two materials together, so how do you determine which object loses some of its electrons?

    3) When electrons flow through the ground (say from your body), why does this not affect the electrical characteristics of the ground?

    4) How does a charged lose its charge if you place it on an insulator? Can it lose it to the air, if so, by what mechanism (how would you think of the electrons interacting with the air molecules?)

  2. jcsd
  3. Apr 28, 2013 #2
    1 and 2 are both questions of electrostatics, and unfortunately I don't have a good explanation for those. If I had to make an educated guess, it's likely due to the way the valence electrons are configured in whatever atom or molecule is of interest.

    3 is a simplification we make when we generalize the concept of ground. "Ground" is an ideal that doesn't truly exist, but in most cases, it's "close enough" to consider the planet to be a sink of both infinite positive and negative charge.

    4 is also because insulator and conductor are also simplifications. A perfect insulator would have infinite resistance, so no charge could possibly move within it; such a thing does not exist in day to day life. A perfect conductor has no resistance, and charge instantaneously moves in response to an electrostatic field; again, such a thing doesn't really exist, though superconductors are close. All insulators conduct some amount of electricity, it's just a question of how much.

    Also related to 4, you might want to look up "corona discharge", and "electric arc", as those are how high voltages dissipate into the air.

    Electromagnetism represents a huge amount of knowledge, so don't expect to understand it all at once; keep asking questions :)
  4. Apr 30, 2013 #3
    ...and because a ground has an infinite number of both kinds of charges then adding a relatively small amount of either wouldn't affect the overall charge?

    So it's the resistance of an insulator that prevents electrons from moving within it. If there's no perfect insulator, then all insulators would eventually conduct some charge, albeit at a very low rate, right?

    I don't know the details yet but would it be true that the 'stronger' source of charge you have, the higher the resistance of the insulator must be?

    Those definitely look cool! Yeah, there's always lots to learn. :D
  5. Apr 30, 2013 #4
    I don't know the details yet but would it be true that the 'stronger' source of charge you have, the higher the resistance of the insulator must be?

    Think of electricity as water through a pipe:

    Ohm's law states that I=V/R or Current=Volts/Resistance

    If you plug up the hole (adding resistance) the current gets stronger. The opposite is true. I suggest using this comparison always when dealing with anything related to Ohms law, since it has worked for me so far.
  6. Apr 30, 2013 #5
    Precisely. It does matter if you're dealing with a spacecraft or something; in a case like that, the assumption of ground isn't necessarily valid. It does work great on something as large as earth though.

    Indeed. You'll find even glass conducts significantly with a high enough voltage applied. You'd be amazed what conducts current when you zap it with a Tesla Coil! And even the very low rate part isn't necessarily accurate. Ohm's law is a linear relation, so if you apply 1000 times the voltage, 1000 times the current flows; Apply 1,000,000 times the voltage, 1,000,000 times the current flows. So even if something only conducts 1 mA at 1V, it's going to conduct several thousand amps when you hit it with lightning!

    In general yes, though as you get into very high voltages, you have to start considering other effects like dielectric breakdown, or ionization of gases. Basically, If you apply a strong enough electric field, your resistive material will suddenly start conducting current, such as when lightning arcs through the air to ground, or a capacitor fails when you overcharge it.

    Yup yup. Look into how we derive potential(voltage) from charges. Also look into how charge creates the electric field. It sounds like you're mostly interested in electrostatics, so we'll stay out of time-varying stuff for now, such as the magnetic field.
  7. May 3, 2013 #6
    Alright! Thanks guys. Couple of more question on this topic:

    5)I read that an atom is ionization is the loss or gain of electrons. So if a an object becomes charged, then 'some' of it's atoms have lost/gained electrons. So is it correct to say that some of its atoms are ionized?

    6) If you have a long metal rod in contact with some source of charge on on end and you touch the other end, could you think of the charges on the source end 'pushing' the electrons in the metal into your hand?

    (by the way, no one has detailed input for the first two? :p)
  8. May 3, 2013 #7
    the earth can be viewed as a huge spherical capacitor. so you can dump a huge amount of charge on it without appreciably changing its voltage. also im guessing charge dumped on the earth from various sources around the world will have a random averaging effect.
  9. May 4, 2013 #8
    I broke down and did some googling for you: Both 1 and 2 should be answered by this section and the accompanying links:

    5)Not in the sense you're thinking so far as I'm aware. We generally use the term ionization for gases and plasmas. I don't *think* you can say a solid has become ionized. This is mostly due to the fact that solids become charged through a different mechanism. Electrons in metals, for example, are not tied to a particular atom, but just sort of exist in a pool throughout the material. This is why metals are such good conductors; the electrons don't have to be stripped from an atom to start conducting.

    6) Very much so. A voltage potential is what "pushes" electrons around. If current flows, that is, charge moves, a potential is always involved. Electrostatics is odd though, in the sense that the idea of a complete circuit becomes much fuzzier. If the voltage is high enough, just about everything will act as a ground, absorbing or giving up some amount of charge.
  10. May 6, 2013 #9
    Excellent - thanks again!
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