1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Resistance of thermistor in hot water as it cools

  1. Aug 23, 2015 #1
    1. The problem statement, all variables and given/known data
    A student investigates how the resistance of a thermistor varies with temperature.
    Student immerses a thermistor into a 50 degrees Celsius beaker of water and measures the resistance on an ohmmeter as the water cools. Student adds ice to achieve lower temperatures.

    Q1. The water was allowed to cool down slowly during the experiment. How did this improve accuracy of the results?

    Q2. The graph of the results produces a downward slope.

    2. Relevant equations
    V = I R

    3. The attempt at a solution
    A1. If the water is cooled to fast then the electrons that were released into the electron cloud will not be able to return to their bound state fast enough (so they will be in limbo) so the resistance will be recorded artificially low because electrons are still free.

    A2. Is this a negative correlation? I heard that from someone but not sure what it means.

    Thank you
  2. jcsd
  3. Aug 23, 2015 #2


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Hello agagin ! More familiar territory ?

    You must have seen some time that a thermometer brought from a cold place to a hot place needs some time to settle: the temperature has to even out with the temperature of the environment. Same for the thermistor, only because it's pretty small this goes fairly quickly. But throwing in ice is very drastic in my opinion. Student would have to make sure the thermistor doesn't get shocked, for instance by keeping a good distance between ice and device ....

    Electron clouds are not the thing to worry about in this experiment. (I tried it and I didn't see them in limbo :wink:).

    Good thing you explicitly state that you aren't sure. Compliments for your 'courage'.

    Gives me a chance for a sermon not to use terms you aren't sure of. It's one of the frequent sources of miscommunication and worse.
    Here you are correct, however -- thus I undermine my sermon, happens all the time.
    Point is that communication has two ends and there will be an enormous amount of receivers that interpret "negative correlation" as "no correlation".
    So much better to circumscribe with e.g. "correlated with a negative correlation coefficient". Up to you.

    I still miss an explanation for this negative slope in your attempted solution ...

    [edit] upon second reading of the problem statement: perhaps the ice was added at the end to find a last low-temperature point and student waited long enough for the temperature of the whole lot to settle.
  4. Aug 23, 2015 #3


    User Avatar

    Staff: Mentor

    What material does a manufacturer use to make a thermistor?
  5. Aug 23, 2015 #4
    These are the readings:

    Temp Celsius = resistance ohms
    10 = 12.62
    15 = 8.47
    20 = 6.61
    25 = 5.45
    30 = 4.25
    35 = 3.54
    40 = 2.79
    45 = 2.11
    50 = 1.12

    Hello BvU. Thanks for the reply

    The negative slope occurs because the electrons have less energy at lower temperatures so are less able to carry current and this appears as increased resistance.

    Still don't see why (in a physics point of view with physics vocabulary) that water must be cooled down slowly for more accurate results. You didn't agree with my electron cloud theory which is okay

    I don't understand what you're saying. Is a downwards slope a negative correlation because as the temperature increases the resistance increases? Is it more accurate to say the "resistance is negatively correlated to the temperature"?

    The ice was added I think at regular intervals to cool the water.
  6. Aug 23, 2015 #5
    The book does not say. Its High School Physics
  7. Aug 23, 2015 #6
    question 1 - you might want to think about the response and resolution of your meter.

    If something was changing quickly, and you were taking a measurement every minute, or a measurement every second; and you plotted both your results; what do you think the main difference would be?
  8. Aug 23, 2015 #7


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Come to think of it, there are two postitions where the temperature needs to be the same for this experiment: where the thermistor is located and where the thermometer is located. That can be achieved only if you allow enough time for the temperature to even out.

    Google thermistor and look under NTC for a device that has lower resistance at higher temperature.
    according to the link I gave you, the denomination "negative correlation" is used for something that decreases when the independent variable increases. So it's quite justified here!

    You are right in this ! Compare with this : more charge carriers means lower resistance. But it's above high school physics level, I'm afraid.
  9. Aug 24, 2015 #8
    The student (it's a she in the book again BuV) did mix the water regularly

    I missed the link to the correlation stuff previously. Just seen it . Was useful. Thanks
  10. Aug 24, 2015 #9
    If I record readings every second I will see only small changes in values. If I record readings every minute I will see a larger jump.

    So for the answer to the question : The water was allowed to cool down slowly during the experiment. How did this improve accuracy of the results?

    I would say: The water was allowed to cool down slowly so that the thermistor had time to respond to the changes in temperature. Otherwise the reading of resistance may still be high at that temperature and changing every second. Only after several seconds would the correct 'stable' reading be shown ... this would be the correct value
  11. Aug 24, 2015 #10


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    And the same for the temperature measurement.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted