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Variance of resistance with significant current flow

  1. May 27, 2008 #1
    The value of a real-world resistor ( ) changes with significant time as the resistor conducts a current.

    Answer choices

    Never Rarely Sometimes Usually Always

    Explain why.
     
  2. jcsd
  3. May 27, 2008 #2
    I would say rarely.
    If a resistor heats up significantly, it's resistance would change.
    See: h ttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html
    This question seems overly-vague to me.
     
  4. May 27, 2008 #3
    I see. So the current causes the resistor to heat? Is this the case always, that current heats up all resistors with time as it conducts a current? I agree it is extremely vague. It's an odd question, however, he never hinted that heat was part of the solution or reasoning. Thanks for your help. Much appreciated.
     
  5. May 27, 2008 #4
    Current always produces some heat, but it is negligible in real-world resistors by design, unless there is a power surge.
     
  6. May 27, 2008 #5

    Gokul43201

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    Reminder to both members involved to please read the Forum Guidelines before proceeding.

    For homework/coursework questions the OP is required to show their own effort first and helpers are required to not provide anything more than tutorial help, and that only after the OP has show some original effort.

    See the Rules tab on top, or the link in my signature.
     
  7. May 28, 2008 #6
    I understand.
     
  8. May 28, 2008 #7
    I would give up in this folder and take your question to the Electrical Engineering forum.

    I think maybe you mean to ask about the long term change in resistance: asking "how does the value of a real-world resistor change after a significant time conducting current?"

    It's not absolutely clear whether you mean to ask about the change in resistance after it becomes hot, or instead, after it's been functioning in a circuit for a while.
     
  9. May 28, 2008 #8
    Thank you. I did manage to find some information about real world resistors. The reason that resistors in the real world cannot function in perfect accordance with Ohm's Law is that in any given situation, a range of variables, including outside heat, external distortion, and ambient noise, all minutely affect the flow of current through a resistor.

    Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this.
     
  10. May 28, 2008 #9
    Well done. Usually you don't want to subject the resistor to more than half of it's power rating. But what you really want to talk about is this. In the real world, materials change resistance with temperature. Metals become more resistive. Carbon becomes less resistive. Semiconductors can be either. How much they change with temperature is called the temperature coefficent. A resistor that has increased resistance with temperature has a positive temperature coefficient. The whole idea, in most cases, is to keep the magnitude of the coefficent as low as possible. A part rated at 50 PPM, for instance, should change less than 50 micro ohms per ohm of initial resistance per degree centigrade over its range of operation.
     
    Last edited: May 28, 2008
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