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How to keep circuit wiring cool so not to distort results

  1. Aug 17, 2015 #1
    1. The problem statement, all variables and given/known data
    A student is calculating the resistance of a piece of fuse wire. He takes 5 readings on the voltmeter and ammeter for different settings of a variable resistor.
    He wants to avoid heating effects in the wire. How would he do this?
    2. Relevant equations

    3. The attempt at a solution

    1. Have a fan blowing at the circuit.
    2. Have an air conditioning machine near by.
    3. If the middle part of the wire is covered in plastic insulation then place that part of the wire in a tub of room temperature water. I thought about a tub of ice but that may be too cold?

    This is High School physics.

    Thank you.
  2. jcsd
  3. Aug 17, 2015 #2


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    Hello again...
    (moderator might merge this with 827996)

    Another relevant equation is P = I * V : the power dissipated (converted into heat) in the resistor is the product of voltage and current.
    Together with V = I * R you get P = I2 * R so you want to keep the current low.
    (Alternatively you get P = V2 / R meaning you want to keep | V | low ....

    your ideas are fine -- note that avoiding the heating effect is something else than mitigating the consequences of the heating effect (a rise in temperature)

    And of course he/she looks at the graph to see if the high-current points deviate !

    Keeping the current limited is also possible by measuring from -2.5 V to + 2.5 V instead of from 0 to 5 V !
  4. Aug 17, 2015 #3

    1. The voltage is kept low by having fewer cells on a battery (or just by making changes in the variable resistor)? Does that also reduce the current at the same time since V is proportional to I in V=IR

    2. So how low is low? Is that determined by the graph? If the line starts to curve we know that heating is having an effect so we should wait until the wire cools down and take more readings in the voltage that kept the graph straight lined?

    3. How do produce a negative voltage?

    4. Is current dependent on voltage? So if the voltage of the battery is high then the current it produces is high?
    Can't understand how voltage and current are different. I know current is the rate of flow of coulomb of charge. But what is voltage?
  5. Aug 17, 2015 #4


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    First of all my compliments that you are trying to do this so thoroughly ! That kind of stamina will get you very far, but you should avoid losing sleep over the subject, because then it becomes counter-productive.

    To answer your items:
    1. I was referring to the voltage over the piece of fuse wire. As I understand it, you vary a series resistor and measure the voltage over and the current through the fuse wire. So yes, the battery voltage is relevant and the resistance of the variable resistor is relevant as well. For the fuse wire it doesn't matter which one is varied.

    2. As you hopefully distilled from the other threads, the current can heat up the wire and thus increase its resistance somewhat (until the wire melts and thus fulfils its useful function of protecting against too high a current). Pity the experts started making things more difficult than necessary.

    3. You simply reverse the connections at the power supply (the battery). Depending on the equipment, switching the connections on the meters is also necessary (but some meters actually put a minus sign in front).

    4. The whole experiment demonstrates that current through a resistance is dependent on voltage: different voltages, different currents. The relationship appears to be a straight line through the origin (right ?), meaning that the relationship can be mathematically described as I = constant * V . I use that way to write it because of your post #3 in the other thread. The other, completely equivalent way to write is of course more the familiar V = R * I Ohm's law.

    Good thing you ask about the distinction between voltage and current. Current is indeed rate of flow of charge. Now what's the other one ?

    In the analogy I remember form long ago, we compared current to rate of flow of water through a hose or a pipe and voltage to the pressure that pushes the water through. Voltage is tension: how hard the charge is being pushed. Generally voltage is a cause and current a consequence. Depending on the possibilities to 'ease the tension'. If there is no path, there is no current. A wide path offers low resistance, a narrow path offers more. Long path: more resistance, etc. The analogy only gets you so and so far, but by then you're hopefully familiar with the phenomena.

    Good luck !
  6. Aug 17, 2015 #5


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    Do you have a circuit diagram for this?

    What is the wording of the complete problem?
  7. Aug 17, 2015 #6
    "A student decides to use Ohms Law to help find the resistance of a piece of fuse wire. He takes 5 readings on the voltmeter and ammeter for different settings of a variable resistor.
    To get an accurate value for the resistance of the wire, the student needed to avoid any heating effects I the wire. Describe how student could reduce heating effects when carrying out the experiment".

    The circuit shows a 12V power source. In series the -ve terminal leads to a variable resistor that leads to a fuse wire and this leads to an ammeter and this leads to the +ve terminal. Connected in parallel to the fuse wire is a voltmeter.

    Also there is another question for the same experiment: "The student did not measure the voltage of the supply or the particular settings of the variable resistor. Explain why these measurements were not required.

    My answer is:
    The settings of the variable resistor and the voltage of the supply is irrelevant. All that we need to know is that the variable resistor can be made to produce a change in current. Hence a change in potential difference across the fuse wire.
    Comments on this please.
    Last edited: Aug 17, 2015
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