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Electric Potential and Potential Energy in a Circuit

  1. Aug 29, 2009 #1
    Hi

    I have a question about something I read. In a circuit, I was told that electrons move from a low potential to a higher potential but an electron's potential energy decreases. In other words an electron's potential increases while its potential energy decreases as it moves. How does this work? I thought that potential and potential energy were proportional? Thanks.
     
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  3. Aug 29, 2009 #2

    vk6kro

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    I would have thought so too.

    In a circuit, electrons travel from the negative to positive terminals of the power source and they can do work heating up resistors etc as they go. When they get to the positive terminal, they can do no more heating unless the power source gives them more energy and they can go around the circuit again.

    So, they have potential energy when they leave the negative terminal and use it up as they pass through resistors etc.

    Maybe there is some difference in the wording that isn't obvious.
     
  4. Aug 30, 2009 #3
    I couldn't make sense of it either but it's definitely true because an electron goes from the negative terminal to a positive terminal and since the negative terminal is at a lower potential than the positive its potential must increase but I don't understand why.
     
  5. Aug 30, 2009 #4

    vk6kro

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    since the negative terminal is at a lower potential than the positive

    We are talking about a circuit here, aren't we? In a circuit, we talk about relative potentials.
    Something might be more positive than something else but it isn't inherently at a higher potential because of it.

    I know Physics people have a concept of absolute electrostatic potential relative to the charge at infinity.
    But if you have a circuit isolated from the outside world, it is only the relative potentials that matter. The positive potential isn't at a higher or lower potential than the negative one. It is always positive relative to the negative terminal.
    This is current electricity, not electrostatics.

    I suspect you might get a different answer in the Physics section or even here if a physicist happens to read this heresy.
     
  6. Aug 30, 2009 #5
    But I thought that the positive and negative designations were made BECAUSE the positive termina's potential is higher than the negative terminal's potential.
     
  7. Aug 30, 2009 #6

    vk6kro

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    Where did you see that?

    Sounds like a definition from somewhere. Could you quote a reference for it?
     
  8. Aug 30, 2009 #7
    Here is a classic example of electrons going to the positive terminal in vacuum and doing work. In the (good?) old days, we played with vacuum tubes, At the negative terminal (or near it) was a filament. we ran current through it. and it got red (actually orange) hot, and it started to evaporate electrons. Near the filament was a grid, that modulated the electron flow. Further away was the plate, which was at several hundred volts more positive than the filament. Electrons that passed by the grid were accelerated by the several hundred volts, and hit the plate with lots of energy (100 milliamps at 300 volts is 30 watts). Often the plates got very hot and glowed because of the electron's energy loss when it hit the plate. All of this technology was replaced with little npn transistors about 50 years ago.
     
  9. Aug 30, 2009 #8

    negitron

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    Nope. Benjamin Franklin incorrectly believed that electric charge flowed from the positive terminal of a battery cell to the negative and today we still follow that convention; it appears in the symbols for diodes and transistors in which the arrows point in the direction of conventional current flow. We now know that charge flows from negative to positive, but the convention of positive-to-negative flow is so firmly established that we're sort of stuck with it. That's why electrons gain potential while losing potential energy. It's all Ben's fault.
     
  10. Aug 30, 2009 #9
    Oh okay, so the theory is designed to make sense if we're thinking of positive charge flow only. Thanks!

    EDIT: Can you check whether my understanding in the post below is correct? Thanks
     
    Last edited: Aug 30, 2009
  11. Aug 30, 2009 #10
    Well my mom's an electrical engineer and that's how she explained positive and negative terminals to me. She said that since voltage is potential difference, then the two terminals must have differing potentials whose difference is equal, in a normal battery, 1.5V. If we have a potential difference, then there must be a side with a higher potential than the other so she said the positive is the side with a higher potential than the negative and that the two terminals are simply relative. Her explanation made sense to me but it's quite possible that it's wrong because I haven't found an reliable source to verify what she said.
     
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