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Ohmic Devices and Conceptual Circuit Question

  1. Feb 15, 2012 #1
    1) An Ohmic device is one that obeys Ohm’s Law. According to your data, are carbon resistors Ohmic devices? Support your answer with references to your graphs and/or equations.

    2) When a typical circuit is switched on, the net displacement of electrons along a wire is about 1 mm per second. If electrons have such a low net speed, why do lights and devices appear to work instantaneously when switched on?

    R = V/I

    1) The lab portion is a bit ahead of lecture so I'm not totally sure about this one. Using Ohm's law i was able to calculate an experimental value with a percent error of only 2%, but is that enough evidence to prove it is an Ohmic device?

    2) This one I'm very lost with due to the lack of knowledge about circuits (since lab is ahead of lectures). Not really sure where to begin.

    Last edited: Feb 15, 2012
  2. jcsd
  3. Feb 15, 2012 #2


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    first of all, is the 2% under your equipment error or statistical error?
  4. Feb 15, 2012 #3
    Maybe this could help a bit with the second question.

    Even though the net displacement is 1mm/sec, that doesn't really mean that you have to wait for one second until the first electron pops out from the wire. Reason for that is simple, the number of electrons in your wire pro 1mm is much bigger than one, so you have to wait only a small portion of the second to get the first electron popping out from the wire. Hope this helps somehow
  5. Feb 15, 2012 #4
    wukunlin: The carbon resistor used in our experiment was manufactured to have a resistance of 147Ω and by measuring V and I of our circuit, then plugging that into Ohm's Law we calculated 150Ω, so thats where the 2% error is from. (sorry if this doesn't actually answer your question).

    Uncle_John: I'm beginning to understand this now, but I don't exactly understand what you mean by the "electron pops out of the wire." My thought process was that electrons have to travel down the wire from a power source to whatever object the wire leads to.
  6. Feb 15, 2012 #5


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    well, typically you should measure different points of V, and I, and then observe the gradient of the slope. If your graph is linear then your resistor is ohmic. To accurately find the gradient and the error margin of the gradient you will need to refer to some basic statistics texts.

    for your second question, basically what john's saying is, imagine you have a pipe connected to a faucet, the pipe is full of water initially, why do you see water spilling out of the pipe almost immediately when you turn on the faucet, despite the faucet only let out a very small trickle of water?
  7. Feb 15, 2012 #6
    I have a few graphs for this experiment and they're all linear, thanks a ton for the help with that question.

    So essentially wires are filled to max capacity and once their power source is activated the electrons have to travel a very small distance (so small that 1 mm/s is fast enough to appear instant) to activate the object the wire is connected to?

    Thanks again
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