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Direction of Electron Flow with Real Batteries

  1. May 6, 2013 #1
    Hello all - a quick question that I can't seem to find an answer to online that I trust...

    While the conventional current model describes flow of positive charges around a circuit, we know that current is actually the flow of electrons (at least in common metallic conductors). Which leads to the question: which of these two resolutions regarding direction of current flow is correct (assume a simple circuit with one light bulb and a common AA alkaline battery).

    1. The terminal of the battery labelled "+" is actually at a higher potential than the terminal labelled "-" (i.e. is accurately labelled). When the circuit is completed, electrons flow out of the negative terminal of the battery, lose potential energy as they transit the circuit, and return to the cell at the higher potential positive terminal. This means that the direction of current determined on circuit diagrams (from (+) terminal to (-) terminal) would actually be opposite the real direction of flow of electrons.

    2. The terminal of the battery labelled "+" is actually at a lower potential (i.e. is really the negative terminal of the battery) than the terminal labelled "-" (which is really positively charged). When the circuit is completed, electrons flow out of the incorrectly-labelled (+) terminal, lose potential energy as they transit the circuit, and return to the cell at the (-) terminal. This means that while the labels on the battery wrapper are incorrect, the direction of current flow established in circuit diagrams is correct.

    Both possibilities recognize and resolve the difference in direction between conventional and real currents, but they do so differently: one preserves the direction of current flow from diagram to real circuit (by mislabelling the battery), while one requires remembering that current flow is opposite.

    I ask because someone once told me that in automotive circuit designs, charges actually flow from the positive to the negative terminal of the battery (from which, if that is accurate, I would infer that option #2 is correct). Wouldn't this difference matter in applications involving diodes? Or is that directionality issue also taken care of in the "labelling" of the diodes? Is this distinction completely inconsequential?

    Forgive me if the question is an ignorant one - and thanks for the help,
     
    Last edited: May 6, 2013
  2. jcsd
  3. May 6, 2013 #2

    phyzguy

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    (1) is correct.
     
  4. May 6, 2013 #3
    Thank you!

    What about in house wiring? The "hot" wire is general considered positive and the neutral/return is considered (-). How can that be? The "hot" wire must contain electrons at higher potential energy, since it is the one that'll blast you, no?

    So does that mean that in home wiring, the positive is really negative, but in automotive wiring, the positive is really positive? Then again, how can that be? If I'm not mistaken, it's the positive in auto wiriting that'll give you a little jolt too, not the negative.

    I think I must be missing something fundamental here when it comes to the significance of which charge is which...please set me straight?
     
  5. May 6, 2013 #4

    nsaspook

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    This might help.
    http://amasci.com/miscon/eleca.html#cflow
     
  6. May 6, 2013 #5

    phyzguy

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    House wiring is AC. The neutral wire is connected to ground, so it is at the same potential as the Earth around you. The "hot" wire alternates 60 times per second from being 110 volts above ground to being 110 volts below ground. Since you are about at ground potential, touching the neutral wire will send no significant current through you since there is no potential difference. If you touch the hot wire, an alternating current will flow through your body.

    It's not the absolute potential that matters, it is the potential difference that drives current flow. In a car, the negative terminal of the battery is connected to the car chassis. So if you touch the metal of the car with one hand, and the negative terminal with the other hand, there is no potential difference between your two hands, so no current will flow. If you touch the metal of the car with one hand, and the positive terminal of the battery with the other hand, there is a 12 volt potential difference between your two hands, so current will flow.
     
  7. May 6, 2013 #6
    And it (electrons) will actually flow from the negative terminal of the battery through your body back to the positive terminal? I know it generally doesn't matter - I'm just trying to picture what actually happens.

    What if I were grounded and touched, sequentially, the (+) and (-) automotive battery terminals. Would one of those be at roughly the same potential as the ground, and I therefore not experience the current?

    Thanks again for the assistance - the last response was hugely helpful.
     
  8. May 6, 2013 #7

    jbriggs444

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    If you are standing on the pavement in a slightly salty puddle and if the car's tires are insulating well then the effect is as if the car is a capacitor with a [very] small capacitance.

    On a positive-ground car, if you touch the positive terminal, which is wired to the car's frame then the car will reach a neutral potential with respect to the ground. If you then touch the negative terminal, the car will reach a +12 volt potential with respect to the ground. Multiply 12 volts by the capacitance of the car with respect to the ground and that's the amount of charge that flowed through you.

    Electrons from the car will be flowing into your hand. Ions will be flowing through your body, taking on electrons at your hand and giving up electrons at your feet. Whether these are positive ions flowing toward your hand, negative ions flowing toward your feet or some combination of each, I don't know.

    If you switch back to the positive terminal, electrons will flow back into the car and ions will flow through your body in the reverse direction.

    If the car's tires are not insulating well and you touch the negative terminal while standing in that salty puddle then you've just completed a 12 volt circuit. Not a very smooth move.
     
  9. May 6, 2013 #8

    phyzguy

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    Suppose you connect the negative terminal of the battery to ground, and you touch your right hand to ground and your left hand to the negative battery terminal. Both hands will be at the same potential, so no current will flow.

    Now move your left hand to the positive battery terminal. There is a 12V potential difference between your two hands, so current will flow. The current is made up of electrons flowing out of the negative battery terminal, through your body, and into the positive battery terminal.
     
  10. May 6, 2013 #9
    Got it - because the circuit is not complete, there is no sustained charge flow -- the car is not an infinte source of charges. Once the potential difference is established, current ceases. Right?

    I'm still not quite confident on the positive/negative battery issue, but I think it comes down to a problem with my truly beliving that potential does not matter - only potential difference. I need to think about this for a while longer, and I think it will clear up.

    Thanks for the help and patience. I'm a biochemist by trade - one who is fascinated by physics, but with admittedly many gaps in understanding. Working on them :)
     
  11. May 6, 2013 #10
    Think about it as jumping between mountains. If one of the mountains is 3000 m high, a jump to another high mountain is still a small jump if the other is 2999 m high. Only the difference of altitude matters, as long as you dont miss the other mountain.
    Same goes for electricity.
    If you build a box powered by a wall socket with two potentials of 3000V and 2999V with an adequate resistor in between a tiny current would rise according to I = 1/R. Its a small jump.
    However, suddenly the box explodes and the interesting wellknown aroma of burnt electronics fills the room. The voltage, that was extremely high with respect to the ground, ionised the air which became conducting and grounded a large spark of electricity to the case of the apparatus.

    Thats the same as missing the other mountain, and the reason why 2999V and 3000V can make a circuit with both low and extremely high diffential voltage and I remember that confused me alot back then.

    Also remember that potential designated by +/- is just a definition, like we call the direction towards the sky up and the opposite direction down.

    In this case your body could be a form a part of an equivalent circuit consisting of a capacitor in series with the battery and a resistor.

    The capacitor behaves like a bucket that can be filled with electrical charges instead of a material.
    Current will the flow in your body until the the capacitor ("the bucket") is filled, that is when it has reached the same potential as the voltage applied over it.
    The size of the "bucket" is called capacitance.

    http://en.wikipedia.org/wiki/RC_circuit
     
  12. May 6, 2013 #11

    nsaspook

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    A small quibble.
    A capacitor is not 'filled' with charge, charge is driven from one plate to the other, the number of charges within the device as a whole does not change. A capacitor stores 'energy' in the form of separated charge.
     
  13. May 6, 2013 #12

    AlephZero

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    I understand why you are asking the question, but for analysing circuits you will be better off forgetting about this level of detail and just working with "conventional" current flow. The choice of "positive" and "negative" was made long before anybody knew about electrons, and the fact that humans call the charge on electrons "negative" doesn't have any deep physical significance.

    Also, except for metal wires, there are many situations where "electrical current" is NOT the same as "the flow of electrons". It may be a flow of positively or negatively charged particles (and the negative ones are not necessarily electrons), or even a mixture of positive and negative charges moving in opposite directions at the same time.

    (And to understand how many common electronics cmponents work at the physical level, you need to forget about "charged particles" altogether, and think in terms of quantum mechanics instead.)
     
  14. May 7, 2013 #13
    Worth mentioning it isnt a bucket neither ;)
     
  15. May 7, 2013 #14
    Note well: there are no electrons flowing through your hand.

    Flesh is an electrolytic conductor, not a metal. When you touch the (+) and then the (-) sequentially, there are brief flows of real, actual, genuine positive and negative ions inside your hand. No mobile electrons. They sum up to create the expected (conventional) current. That's why we use conventional current: it's because electron current doesn't exist inside people, and it doesn't exist in dirt, oceans, etc. If you don't want to deal with conventional current, and instead want to know what actually happens, then you're forced to deal with ion flows. If instead you want something simple, then use Conventional Current where all charges have the same polarity (that's what it's there for.)

    Note that if you're grounded, then you're touching an electrolytic conductor (the Earth) where no electron flows occur! :) You could assume that Earth is a metal. But if you're going to break loose from reality and hide all the complicated charge flows, then you might as well behave like physicists and engineers, and replace them all with positive charges.
     
  16. May 7, 2013 #15
    Remember that potential is one way of describing electric fields, and for this reason the Potential behaves something like distance.

    What is the potential of an object? Meaningless. It's like asking what an object's altitude is. Altitude from what, the center of the Earth? Sea level? The local floor? The problem is that a single object has many different altitudes at the same time, since altitude varies depending on the choice of zero-reference. And, one point in a circuit never has a single potential, instead it has many potentials at the same time.

    D-cell battery: the flat end might be -1.5V, but only in relation to the positive end (which we've declared to be zero reference.)
     
  17. May 7, 2013 #16

    CWatters

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    I agree with AlephZero. You only really need to start thinking about which way electrons go when you start looking at how semiconductors are made/designed. You don't even really need to worry about electron flow when designing semiconductors into a circuit.
     
  18. May 7, 2013 #17

    nsaspook

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    Not really, I'm beginning to really hate bad/incomplete/wrong analogies about electricity. At this level of discussion let's stick to facts and leave analogies behind if possible.
     
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