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A few questions about Resistance and DC Circuits

  1. Mar 18, 2008 #1
    I'm just sort of looking for some clarification on some of this...

    1) Say we have an ideal emf (battery, I guess...) connected in a circuit to a resistor and nothing else, why is the current the same on either side of the resistor? My textbook doesn't really explain this that well, but I think it's because charge is conserved, and if we pick any two points on the circuit the "flow" of electrons has to be the same for charge to be conserved.

    2) So because of this, can we think of resistor as applying a resistance over the entire loop as opposed to one part of it? This seems weird to me because when we draw a circuit diagram representing this situation the resistor is included in only one part of the circuit as opposed over the entire loop, is there a reason for this?

    3) Because of this resistors essentially have the effect of decreasing the current at any point in the loop by decreasing the the charge flow over a time interval, correct?

    4) When talking about DC circuits we can consider the ideal emf as the work done per charge. When we talk about an emf in terms of work, we're talking about the ability of the emf to (literally?) pump electrons through the actual emf device, correct?

    I have a few more questions, but if these are wrong there would be no point in asking them lol, so I'll save the rest for later...
    Last edited: Mar 18, 2008
  2. jcsd
  3. Mar 18, 2008 #2


    Staff: Mentor


    I wouldn't look at it this way at all. It may work for a simple circuit like this, but it certainly would not work for typical circuits, and it is not realistic even for this simple circuit.

    Well, a resistor is defined as a device where the current is proportional to the applied voltage, so I would say the resistor makes the current proportional to the voltage rather than saying that it "decreases" it. It then affects the current in the rest of the loop by conservation of charge as you mentioned above.

    Your wording is a little confusing, can you try again?
  4. Mar 19, 2008 #3
    The ability of the electric field to force a bunch of charged particles to go in the direction that is contrary to their spontaneous direction, to make them go in the direction that, all by themselves, those charged particles would naturally be repelled from.

    I've been an EE for over thirty years and I still have to visualize circuit behavior with an analogy to gravity. Current is like someone telling you to carry a bucket filled with so many rocks. Voltage is like someone telling you that the place you have to carry those rock to is somewhere at the top or the bottom of a mountain.
    Last edited: Mar 19, 2008
  5. Mar 19, 2008 #4
    That resistor symbol tells you, in effect: although there is an infinitely large number of infinitely small resistors throughout the length of this path, this one resistor conveniently represents the sum of all of them.
  6. Mar 19, 2008 #5
    The same reason that, if you have a garden hose, and water is going into one end of it at a rate of three gallons per minute, then water must also be coming out of the other end at a rate of three gallons per minute. It doesn't have a hiding place for it to store the stuff.
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