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Physics question about current?

  1. Oct 9, 2011 #1
    Physics question about current?!?

    if current passes through a resistor, current goes down (I=V/R), but doesn't that violate kirchoff's current law, which states that current that goes in equals the current coming out? I'm so confused. also does voltage drop when current goes through a resistor?
     
  2. jcsd
  3. Oct 9, 2011 #2
    Re: Physics question about current?!?

    Yes the current that goes into a resistor will be the same that comes out. As long as you have a straight wire there will be no change in current. But whenever there is a wire that branches off into other wires the current will split according to kirchoffs current laws.

    Ohms law (V=IR) should be written as ΔV=IR. So the current moving through the resister multiplied by the resistance is equal to the change in voltage( voltage drop ) of every electron that passes through. Just remember that current always loses voltage while moving through a resistor.
     
  4. Oct 9, 2011 #3
    Re: Physics question about current?!?

    How can the current going into a resistor be the same as current going out, when a resistor is suppose to RESIST the current??
     
  5. Oct 9, 2011 #4
    Re: Physics question about current?!?

    Picture a big rectangular building in the middle of an open field with two doors.( This will be our resistor ) Now picture that a huge crowd of people need to get in and out of the room as fast as they can. The rate at which people are moving through room is governed by the size of the door. People can always move through the room because there is a opening. there is also a steady flow of people moving through the building. If the resistance was infinity the door would be closed. If the resistance was zero there would be no walls on the building.

    Does that help at all that is how my physics teacher explained it.
     
  6. Oct 9, 2011 #5
    Re: Physics question about current?!?

    say 100 ppl are traveling along a wire, and a building that lets ppl go in one by one, does that mean there's a build up of ppl outside the entrance? that wouldnt make sense cause something will blow up.
     
  7. Oct 9, 2011 #6
    Re: Physics question about current?!?

    Ya that is the problem with the analogy and I'm glad your brought it up because I wasn't sure if I should address it.

    You have to remember that the electrons repel each other. So for a instantaneous moment the do build up and then rush back towards the negative terminal of the battery. In that manner the resistor sets the tone for the entire circuit. The current that is travelling through the resistor is the same current that is traveling through the wire.
     
  8. Oct 9, 2011 #7

    russ_watters

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    Staff: Mentor

    Re: Physics question about current?!?

    They must stand in a single-file line. The "resistance" just reduces the flow rate through the whole circuit.

    However, you did just prove why the flow rate must even be the same going in and coming out of the resistor, didn't you...?
     
  9. Oct 9, 2011 #8
    Re: Physics question about current?!?

    When you change the flow rate of water coming out of a faucet isn't the flow rate of water coming in the same as the flow rate of water coming out? Isn't the faucet resisting the flow of water?

    All analogies break down at some point. It's been a long time, but I think the way I first "got it" was simply through practice. Just draw up different circuits using a battery and series connected resistors and apply ohms law. By starting off simple, with all resistors in series, you know right away that the current through all the resistors will be the same. Don't ask why. Physics doesn't answer why questions very well. Just use ohms law to calculate the current flow (use the sum of all resistors), then calculate the voltage drop across each resistor (they should all add up to the battery voltage). Eventually you will "get it" and you can move on to parallel connected resistors. Use the following equation to calculate the equivalent resistance of resistors in parallel:

    [tex]R_{eq}=\frac{1}{\frac{1}{R1}+\frac{1}{R2}+{...}+ \frac{1}{Rn}}[/tex]
     
  10. Oct 10, 2011 #9
    Re: Physics question about current?!?

    Are you serious????????? What do you think physics exists. Because intelligent and curious people were brave enough to question the world around them. Isaac Newton would turn over in his grave if he heard you say that.

    Sorry I just really don't like this attitude toward learning.
     
  11. Oct 10, 2011 #10
    Re: Physics question about current?!?

    ^This. Because of electric repulsion, the electrons repel each other to equal distance. This means the electrons move (almost) as one, and any change in current due to a resistor means the whole circuit slows down. Picture a circuit as an old-style record player. The pin is the resistor. If the pin is added to the record, the whole record slows down, not just the bit of the record behind it, due to the electromagnetic repulsion between the atoms and molecules in the record. That very same repulsion give the effect in the electric circuit.
    P.S. although I agree with you that the job of physics is to ask why, and turtle's attitude is unhelpful, Newton probably wouldn't be the best example. While being arguably the smartest person ever, he didn't have a very good personality, and would have turned in his grave over a lot of trivial things besides this one.
     
  12. Oct 10, 2011 #11
    Re: Physics question about current?!?

    Sorry, I should have made my point more clear. But yes, I am serious. Newton's work did not tell us why, it told us how.

    1) Why do two bodies attract gravitationally?
    I don't know

    2) How do two bodies attract gravitationally?
    [tex]F=\frac{GMm}{r^2}[/tex]

    So I should have said, physics answers how questions better than why questions.
     
  13. Oct 10, 2011 #12
    Re: Physics question about current?!?

    Okay maybe I misunderstood you but in that case of the current question there was all the reason to ask why.

    As for the universal law of Gravitation that is a great example and I don't think anyone should ever stop wondering why the hell the equation looks the way it does.
     
  14. Oct 16, 2011 #13
    Re: Physics question about current?!?

    Fixed.
     
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