Triky question on resistance

Discussion in 'Homework Help' started by bwd111, Feb 12, 2014.

  1. bwd111

    Thread Starter Member

    Jul 24, 2013
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    Device the resistance to current decreases when temp increases. Im saying resistor cause I know its right but why would thermistor not be right as well?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I have no idea what you just asked.

    As for thermistor part. It is designed to change the resistance with temperature.
    Standard resistors do not change with temperature but only slightly. Sometimes negligible.
     
  3. inwo

    Well-Known Member

    Nov 7, 2013
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    Some increase and some decrease. Is there a question?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You might take a look at PTC resistors.

    Bertus
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Nearly all common types of resistor exhibit increasing resistance as temperature increases.

    This is the principle behind the correct functioning of a tungsten electric light bulb as a prime example.

    So your component is not a resistor.

    Do you know any other types of resistive component besides a thermistor (which it may be) ?

    http://www.barthelectronics.com/pdf...1 Voltage Coefficient Products_Pulse Page.pdf
     
    Last edited: Feb 12, 2014
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    What is the source of that question?

    My initial thoughts were PTC .... but the question as presented lacks clarity.
     
  7. inwo

    Well-Known Member

    Nov 7, 2013
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    "Some increase and some decrease. Is there a question? "


    Carbon resistors
    have several different forms. One is a mixture of ceramic powder and fine carbon granules held together with a resin type of glue. This has pretty much been discontinued since the advent of modern printed circuit boards where space is at a premium. The type most used these days is a small ceramic base on which a resistive pattern has been deposited. In both cases, the ceramic base is non conducting whereas the carbon forms a current carrying path; the more carbon, the less the resistance.
    This type of carbon has a negative thermal coefficient, i.e. the higher the temperature, the lower the resistivity. Its value depends on the details of the form of carbon but often is about -0.0005/°C near room temperature . So if you heat a 1000 Ω resistor by 10 °C then it will have a resistance of 1000 Ω *(1-0.0005*10) = 995 Ω.




    https://van.physics.illinois.edu/qa/listing.php?id=18564
     
  8. bwd111

    Thread Starter Member

    Jul 24, 2013
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    Here is the re-word A device whose resistance to current decreases when its temp increases is called
     
  9. WBahn

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    Mar 31, 2012
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    What kind of "devices" are fair game as an answer? Remember, we have absolutely no clue as to the context associated with this question.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I have absolutely no idea what you are talking about...! :confused:
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    A thermistor whose resistance decreases with temperature is called a negative temperature coefficient (NTC) thermistor.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    That's all true but OP said resistance to current decrease ?
     
  13. MrChips

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    Oct 2, 2009
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    You gotta read what the op is trying to say.
    He means "resistance to the flow of current"
    which is the same as simply saying "resistance".
     
  14. WBahn

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    Mar 31, 2012
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    @bwd111: Look up "positive temperature coefficient" and "negative temperature coefficient".
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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  16. bwd111

    Thread Starter Member

    Jul 24, 2013
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    That is how the question was written on the board at school word for word and people were answering the question. Guess you had to be there? The correct answer was a resistor
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Too ambiguous!
    I guess I would have failed that one. :confused:
    Max.
     
  18. bwd111

    Thread Starter Member

    Jul 24, 2013
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    I did AND that wasn't one of the choices . choices were -rectifier,transistor, resistor ot thermistor
     
  19. WBahn

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    Mar 31, 2012
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    It would have been really nice to have mentioned this list, particularly after I specifically asked

    Sheesh.

    As several people have been trying to lead you towards, the most reasonable answer (particularly of those given) is thermistor. While all resistors have a temperature coefficient (some positive and some negative), this is not by design by rather a fact of life. A thermistor, on the other hand, is a resistor that is specifically intended to have a large temperature coefficient. Some have positive temperature coefficients and others have negative temperature coefficients.

    So neither answer is truly correct for the question as asked, but thermistor is, in my opinion, the better one.
     
  20. MrChips

    Moderator

    Oct 2, 2009
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    Arrrgh! Why didn't you say so in the first place.:mad:

    Resistor is the wrong answer! Tell your teacher I say so.
     
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