current

Discussion in 'Homework Help' started by bigjohn, Mar 12, 2009.

  1. bigjohn

    Thread Starter New Member

    Mar 12, 2009
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    I am investigating an experiment between the current in a wire and its heating effect. I have devised a circuit where I use a variable resistor to change the current which will be measured on an ammeter. The wire will run through water in a styrofoam cup. I will leave the circuit running for a fixed period of time and measure the increase in temperature in the water. This will give me a graph with axises of temperature and current and should be a straight line.
    There is one problem. As the wire heats up the resistance and so the current will change, thwarting my results. Any ideas?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The water may, to some extent, provide a heat sink and thereby offset the degree to which the temperature rise changes the resistance. Stirring the water during the test might improve the cooling efficiency - but that will make it easier for some of the heat to escape from the insulated cup.

    If you know the wire temperature coefficient of resistance value you may be able to include an adjustment for the temperature change (but I'm not sure about the maths?) - if it is really going to be a significant factor.

    Another approach would be to verify that the input energy (V x I x time) is equivalent to the heat required to raise the water temp from its start to end value.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Perhaps I'm way off message if it's the external controlling resistance that actually is your concern. I was focussing on the heating water in the cup. :rolleyes:

    Yep, could be a problem - unless you have access to a power supply which you can run in current limiting mode. Some DC power supplies have a control which limits the current being drawn to a user set maximum value.

    As long as it doesn't damage the supply itself to run it this configuration for an extended period.

    You could build yourself a current regulator if you have or know someone with the electronics skills.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The temperature change has also to do with the amount of water.
    The specific heat capacity of water is : 4.184 J/(g·K) (liquid at 20 °C)
    The heating by the current must be pretty much to be noticed.

    This data comes from the wiki page on water.
    http://en.wikipedia.org/wiki/Water

    Greetings,
    Bertus
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Why should the graph be a straight line? Energy input is proportional to the square of the current.

    Why should that be a problem?
    You say you are measuring temperature and current you will have corresponding temperatures and currents to plot on your graph, they will plot the same curve, just not be evenly spaced.
     
  6. bigjohn

    Thread Starter New Member

    Mar 12, 2009
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    Thank you everybody for the quick replies. The advice and the links will help in the procedure of the experiment.
     
  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Unless the wire is running INCANDESCENT, the change in the resistance will be unmeasurable. Your experimental setup should give good valid results.

    eric
     
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