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Conservation of current?

  1. Apr 10, 2015 #1
    In an explanation of a textbook's diagram it says : "Conservation of charge requires that whatever charge flows into the resistor at point A, an equal amount of charge emerges at point B. Charge or current does not get “used up” by a resistor. So the current is the same at A and B. "
    I can understand the conservation of charge but isn't current = charge divided by time?
    Wouldn't the resistor SLOW the charge move, isn't the reason we use them in the first place?
    So, how come the current is the same at A and B? I'd expect at B the current would be less as resistor would limit the charge passing through itself over time t ?
     

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  3. Apr 10, 2015 #2

    A.T.

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    What do you mean by "SLOW"? Are you confusing distance by time with charge by time?
     
  4. Apr 10, 2015 #3
    I mean charge by time. Wouldn't resistor change the amount of charge transferred in seconds?
    "So the current is the same at A and B" means charge per time is equal at each point, doesn't it? How could that be? What's a resistor for then?
     
  5. Apr 10, 2015 #4

    A.T.

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    Change compared to what? To without the resistor, yes. But the current must be the same at both ends of the resistor, because charge cannot accumulate in the resistor.

    It resists the flow, so you need more voltage for the same current.
     
  6. Apr 10, 2015 #5
    But the voltage is not the same. If it was why would electrons flow from A to B?
    At B voltage is lower, R is the same how come I-current is not changed??
    Am I missing something here?
     
  7. Apr 10, 2015 #6

    A.T.

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  8. Apr 10, 2015 #7
    I know Voltage and current are different things but I just don't get why current at B is same with A, sorry maybe I'm too thick.
    I need to read similar topics and do a further search and then go on.
    Thanks anyway.
     
  9. Apr 10, 2015 #8

    jtbell

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    To make a crude analogy, consider a stream of cars on a highway, moving at 100 km/h. As they enter a town, they slow down to 50 km/h. As they leave, they speed up again to 100 km/h. None of the cars "vanish" inside the town.
     
  10. Apr 10, 2015 #9

    phinds

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    Another analogy, that I like better than jtbell's, and I'll explain why, is that current is exactly like a bicycle chain. It has to move at exactly the same rate at all points in a simple circuit loop. Think of it as the power source pulling on the bottom end of the chain, forcing it to circulate evenly all around the circuit.

    The reason I like this better than jtbell's analogy (not at all that his is wrong in any way) is that his might be mistakenly interpreted as implying that the cars outside the town are still going 100mph while the ones in the town are going 50mph. This is not possible.
     
  11. Apr 10, 2015 #10

    A.T.

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    Of course it's possible. The cars just need to reduce their distances by half to keep the same car / time rate constant.
     
  12. Apr 10, 2015 #11

    OmCheeto

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    I was going to agree with you, about this not being possible, but then I looked at who made the statement, and scratched my head for a couple of minutes.
    It is in fact possible.
    Current is a "count" of electrons per unit time.
    Although the speed of the individual cars traveling through town may be lower than on the highway, there are more paths, and hence, the number of cars entering, passing through, and exiting the town would be the same.

    Perhaps we need fourth analogy! "Ticket counters at a football stadium"
    Now, the ticket counters would be the opposite of resistance, as each counter can admit one fan per second.
    So the flow of fans into the stadium would be the reciprocal of the number of counters.
    But the flow of fans before and after the counters, would be the same.
     
  13. Apr 10, 2015 #12

    phinds

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    But what happens as the first car entering town slows down and the one leaving town at that time doesn't slow down. Does that not cause a problem? I'm not getting how that's possible.
     
  14. Apr 10, 2015 #13

    phinds

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    Yes, if you change the paths, I agree. That's like having some of the current go through each of 2 parallel paths in an otherwise serial loop, which causes no problem but is getting away from my bike chain analogy.
     
  15. Apr 10, 2015 #14
    Because there is potential difference between points A and B, the work of electric field is not zero between these two points and it would accelerate the electrons but the resistor keeps the velocity about the same by converting the work of electric field to heat (via collisions of electrons with the conductor atoms).
     
  16. Apr 10, 2015 #15

    russ_watters

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    I prefer a water analogy because the above analogies each only discuss one flow scenario, whereas the OP appears confused about the relationship between two different ones.

    Say you have a large tank with a small pipe coning out the bottom, with a valve in it that is all the way open. This is like a wire with no resistors. The flow rate (x GPM) is the same in the entire pipe.

    Now close the valve most of the way. This is like installing a resistor in the wire. Now the flow is lower (perhaps 0.5x GPM), but as before it is the same everywhere in the pipe.
     
  17. Apr 10, 2015 #16

    A.T.

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    When the first car enters the town, how can one already be leaving it? If they drive in a continuously filled looping convoy, which one is the first?

    I see no problem.
     
  18. Apr 10, 2015 #17

    phinds

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    Again, I don't follow you. Let's say there are 100 cars around the loop. 10 of them are in the town. The 10 in the town all slow down. How do the other 90 not have to slow down as well.

    Alternatively, the car next in line to leave town slows down and then the one behind it slows down, and so forth, and as each car leaves town, it speed back up. Doesn't this scenario also cause a problem?

    EDIT: sounds to me like maybe I'm taking my bicycle analogy too literally. Is that the case?
     
    Last edited: Apr 10, 2015
  19. Apr 10, 2015 #18

    jtbell

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    Suppose you have a line of cars all going down the highway at the same speed. None of them has yet reached the edge of town, where they all have to slow down.

    The first car enters town and slows down. The second car is still going at its original speed, so it gets closer and closer to the first car until it also reaches the edge of town and slows down. Now the first and second cars are closer than they were originally, and the third car has started to come closer and closer to the second car. Etc.

    A similar process happens at the other edge of town, but now the cars get further apart as they successively increase their speeds.
     
  20. Apr 10, 2015 #19

    phinds

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    And how is this not a change in the cars per unit time at a given point in the road just inside town. If I'm following this analogy right, the cars per unit time correspond to charge flow per unit time, which is current. So now, in a serial circuit you have differing currents in different parts of the circuit. Am I misreading the analogy?
     
  21. Apr 10, 2015 #20

    jtbell

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    The rate at which cars pass a given point equals their linear density along the road times their speed. In units, (cars/hr) = (cars/km) · (km/hr). Inside town, if the speed is half what it is outside town, the density is double what it is outside town, that is, the cars are half as far apart inside town as they are outside of it.
     
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