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Question About Voltage and Charge

  1. Jul 13, 2007 #1
    I'm taking an introductory college physics course as part of my pre-med coursework, but unlike most pre-meds I actually like physics and want to understand the material beyond simply crunching the numbers. The equations are easy but some of the concepts are hard - espeically when you really think about them.

    We're studying basic e/m right now, and there's one issue I just can't resolve in my mind. I'm hoping one of you might have a moment to enlighten me, and I thank you in advance for your time and expertise.

    The question is how you can have high voltage without a great deal of charge, and vice-versa. For example, when you shuffle your feet across the carpet, you may acquire a potential difference of several thousand volts, right? As I understand it, this happens because you acquire lots of electrons. Yet when you subsequently touch a door handle, the shock isn't dangerous because, as a textbook will say, there's very little charge.

    To my half-informed mind, you can't have one without the other. Why doesn't building up high voltage require amassing a lot of charge as well? And conversely, how can a great deal of charge be stored up at relatively low voltage, such as in a car battery?

    Thanks very much in advance for your wisdom. My father, who was also a physician, knew e/m forwards and backwards and considered it by far the most important science. He'd hang his head in shame if I didn't mentally master this topic.
  2. jcsd
  3. Jul 13, 2007 #2
    Nope. Very few electrons, but the ones that are there are sitting in a very large potential.

    Voltage and charge are completely different concepts. Suppose you have a drainpipe flowing into a bucket, or whatever, so that it forms a waterfall. You can have a dribble or a torrent coming down it, but the volume of water flowing doesn't affect the height at which it's dropping from, does it?

    Potential is the 'height' through which your charge is 'dropping'. Current is the rate of charge flow.

    Confusion tends to arise because in real systems, the power in use tends to be constrained. It takes a considerable amount of power to bung a large quantity of charge around at a high voltage; progressively less as the voltage is lowered.
  4. Jul 13, 2007 #3
    think about what voltage is, its the sum of the e field across a distance so it is directly proportional to [tex]\vec{E}[/tex] and [tex]\vec{x}[/tex]. e field itself is not also solely dependent on charge, it is heavily influenced by the charge distribution. so that explains static shock, you have some amount of charge ( i don't know if it is a lot or a little ) and you get a breakout from your finger because of the small radius of curvature, which produces a very high e field. a car battery does not store charge exactly, more like it creates charge through a galvanic reaction, and these reactions happen at a low potential.
  5. Jul 13, 2007 #4
    Thanks for the reply. I already understand much of what you're saying. I'm familiar with the "height" analogy, and I fully understand the difference between voltage and current.

    If shuffling your feet across the carpet only nets a few electrons, why is the potential so extremely high? And how does a car battery store so much energy? Thanks again.
  6. Jul 13, 2007 #5
    did you not read my post?
  7. Jul 13, 2007 #6
    No, your post appeared on my screen only after I had replied to Soujourner01.
  8. Jul 13, 2007 #7
    Your question involves many concepts: charge, voltage, current, capacitance, electrical networks and biological effects of electric current.

    So its difficult to answer.

    "For example, when you shuffle your feet across the carpet,
    you may acquire a potential difference of several thousand volts,
    right? As I understand it, this happens because you acquire lots of

    The last statement is not true "this happens because you acquire lots of
    electrons "

    V = Q / C

    where V = potential between you and the earth
    Q = number of electrons ( = charge )
    C = capacitance between you and the earth

    The capacitance is very small, so whit a small number of electrons, you
    will get a high voltage ( thousands of volts )

    When you touch a door handle it flows a lot of current ( I = V / R )
    but during very little time ( because there are few electrons ),
    so it wont harm you. The voltage drops in very little time.

    If you touch an Integrated Circuit you can damage it.

    "how can a great deal of charge be stored up at relatively low voltage,
    such as in a car battery? "

    This is not a good example, because in a battery the charge is not
    stored, is created.

    But, anyway, if you touch a battery the current through your body will be

    I = V / R

    V = 12/24 volts

    this current is much smaller than in the previous example, so you dont
    even notice it.

    You can get many much power from the battery if you connect a lower
    resistance, like the electrical motors that start your car.
  9. Jul 13, 2007 #8
    Thanks for that helpful reply. That leaves only thing still unclear to me: so what exactly makes a 12V battery supply current only at 12V and not 11V or 13V?
  10. Jul 13, 2007 #9
    the reduction potentials of each of the individual galvanic cells is about 2V, there 6 cells.
  11. Jul 13, 2007 #10


    User Avatar
    Gold Member

    Its based on chemical interactions and the battery's ability to have concentrations and deficits of charge on its terminals.

    This should answer your question further:
    http://www.pz.harvard.edu/ucp/curriculum/circuits/s8_background.htm [Broken]

    EDIT: Just one quick correction with regards to your quote. You seem to be implying that a battery supplies current. This is however incorrect. A battery only provides the [electric] potential difference i.e. driving force to set charge in motion which creates current. By definition, current is charge per unit time - I = q/t.
    Last edited by a moderator: May 3, 2017
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