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Homework Help: Ohms Law Question

  1. Jun 24, 2009 #1
    1. The problem statement, all variables and given/known data


    Graph of current vs voltage. What is happening to the resistance after the linear portion of the graph and why.

    2. Relevant equations


    3. The attempt at a solution

    the line is the resistance and it follows ohms law for a while (it is constant and independent of V and I), but then suddenly jumps up exponentially at a high value of I... I cant figure out what could cause this. it doesnt say whether this resistor is ohmic or not, but graphs of nonohmic resistors ive seen are exponential all the time not just at high current
  2. jcsd
  3. Jun 24, 2009 #2
    you must know that resistance is temperature dependent. notice the linear relationship between current and electric potential until the the curve. At the point of the curve, resistance is no longer a constant. thus, you see the curve.
  4. Jun 24, 2009 #3
    The curve appears is too crude to obtain information. Is it supposed to look linear toward the bottom?
  5. Jun 25, 2009 #4
    yes, its linear, then suddenly grows exponentially
  6. Jun 25, 2009 #5


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    What happens to a resistor as the current through it increases without bound?
  7. Jun 25, 2009 #6
    What if it is almost linear, but not exactly so?

    What circuit elements have you learned of? Can you think of any simple component network that would produce a curve like that given the circuit elements you have learned?
  8. Jun 25, 2009 #7


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    It's probably accurate to consider it linear up until the time it becomes exponential (of course no real part is truly linear but this is coursework, the land of ideals!); it seems a safe assumption that that was specified in the problem. And yes, there really is a real-world fundamental circuit element which will behave like that, if pushed.
  9. Jun 25, 2009 #8


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    You are sitting in a dark room and you walk over to the switch on the wall and you turn on the ????? and then you can see OK. What is the ?????.....?
  10. Jun 25, 2009 #9
    I think it's too crude.

    vk6kro thinks it's a light bulb. Could be.

    Exponentials look very flat at the bottom.
    http://upload.wikimedia.org/wikipedia/commons/5/53/Exponential.png" [Broken]

    Ambiguous homework questions are common enough. Is that your own rendition of the curve, or are you reposting it?
    Last edited by a moderator: May 4, 2017
  11. Jun 26, 2009 #10


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    If we took the graph as being absolutely correct, there is another possibility.

    If you had a resistor (like the heating element in a fan heater) which had a powerful stream of air being forced past it, you would get it behaving exactly like a resistor (and hence the straight line graph) while the stream of air was able to carry away any heat generated.

    There would come a point, though, when the temperature did start to rise and the resistance of the resistor would start to increase. After that, you could get the non linear behaviour shown on the right of the graph.
    Last edited: Jun 26, 2009
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