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Direction of current

  1. Apr 19, 2012 #1
    I have a hard time in understanding the direction of current. Let there be two points A and B such that the two points are connected by wire. A has higher potential than B. So the electrons flow from A to B. How is the direction of current measured. Is it positive or negative ? Also what is the exact definition of current? Is it defined as the rate of flow of positive charge per unit time or negative charge per unit time?

    Now let us consider a cell. It has two terminals positive P and negative N. How do the electrons flow? From P to N or from N to P? My textbook says that negative charges flow from P to N. P has higher potential than N and so electrons naturally flow. If that is the case how do the electrons on reaching N again go to P inside the cell because N has lower potential than P? Is there any mechanism going in the cell?
     
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  3. Apr 19, 2012 #2

    HallsofIvy

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    Long before people knew anything about electrons, people were aware of electric current and arbitrarily assigned a direction to that current. Unfortunately, the direction assigned to the current, which became standardized, turned out to be opposite to the actual flow of the electrons.
     
  4. Apr 19, 2012 #3
    Now do you mean that the current flows from N to P?
     
  5. Apr 19, 2012 #4

    NascentOxygen

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    These two statements are contradictory.
     
  6. Apr 19, 2012 #5
    I forgot to mention about the field. The field is set up due to negative charge near A and naturally due to repusion, A must have higher potential with respect to electrons present in A. B is farther away and has lower potential than A. So electrons travel from A to B. Hope my description is right.
     
  7. Apr 19, 2012 #6

    NascentOxygen

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    The term "higher potential" has a precise definition in electronics. You cannot bend it to your whim.
    Electrons travel from a point of lower potential to a point of higher potential. Only if your description matches that of everyone else's can you be correct.
     
  8. Apr 19, 2012 #7

    sophiecentaur

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    It is a big help if you forget, entirely, about electrons before you launch on elementary (or even advanced) circuit theory. Conventional current ALWAYS flows from Positive to Negative so just do all your thinking on that basis. Once in a blue moon, you may need to be concerned with where some electrons may happen to be going but, since we stopped using Cathode Ray tubes, those occasions are increasingly rare. (In solid state devices you can never be sure what is actually carrying the current)

    When you need to consider electron flow then, of course, electrons flow TOWARDS the positive. There is no contradiction in any of this. No one 'got it wrong'. If you can handle the fact that, in arithmetic, a minus times a minus is a plus, then you can handle the (inconvenient?) fact that electrons don't happen to flow in the direction of conventional current.

    In fact, if you base your 'understanding' of electricity on the idea of 'things' flowing round a circuit at all, then you are bound to find yourself struggling eventually and using dodgy analogies like water, which involve Kinetic Energy, Pressure etc. Treat Current, Potential and Charge as brand new quantities for most purposes, follow the 'rules' of how to deal with them and Circuits will just roll over on their backs and allow you to analyse them.
    Once you allow yourself to get to that stage, you will find the 'electron conundrum' is no longer a problem.
     
  9. Apr 21, 2012 #8
    You said that electrons travel from lower to higher potential but how and why?
     
  10. Apr 21, 2012 #9

    phinds

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    sophiecentaur has already explained it completely. I don't think it can be said any better. It's an arbitrary definition. Get over it.
     
  11. Apr 21, 2012 #10
    Well i will try to study about current arbitratily without considering the direction of flow off electrons because i dont have time.
     
  12. Apr 21, 2012 #11

    NascentOxygen

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    Electrons are attracted to points of more positive potential. That's how God designed them.

    In the study of electronics, we usually find it easier to think of current flowing from points of more positive potential to points of lesser potential. But this uses the notion of current as something that carries positive charge away from the higher potential. We now know that in copper wire and all metals there are no positive charge carriers, there are only negative electrons, and they flow the other way--towards points of higher potential. So if you are not careful, it is easy to get oneself confused! As others have warned you. Best to not think of electrons, think of current.
     
  13. Apr 21, 2012 #12
    Now i understand. The higher potential for positive charge is lower potential for negative ones and vice versa. Whenever we talk about potential as you mentioned we should talk of positive charge and description should be based with respect to positive charge. Thank you nascentoxygen
     
  14. Apr 21, 2012 #13

    NascentOxygen

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    I didn't exactly say "we should talk of positive charge" did I? Because then we'd be talking about something that doesn't exist--assuming we are talking about our electricity supply in wires. By all means, if you wish, you can think of positive charges, just don't say it out aloud for others to hear or they may question whether you even know what you're talking about. :wink:

    Best to think and speak in terms of current, that's the convention. :smile:

    Lesson #1: In science, you must learn to conform to convention. 5sq6g.gif
     
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