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: Draw a circuit diagram to measure resistance of wire

  1. Oct 19, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    This is how I drew it (in pencil) , but there should be a rheostat connected in series with the power supply, why?i can already calculate the resistance straight away by V/I
  2. jcsd
  3. Oct 19, 2015 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    There may be additional reasons, but a very basic one is that if you connected that circuit without additional resistance, such as from a rheostat, bad things would happen because you'd have a short circuit.
  4. Oct 19, 2015 #3


    User Avatar
    Homework Helper

    A piece of wire has a very low resistance. That means that you are in effect short circuiting the power supply with the wire. The reostat adds additional resistance in the circuit to keep the current drawn from the power supply within the limits of the power supply, so it is more for practical than theoretical reasons that it is needed.
  5. Oct 19, 2015 #4


    User Avatar

    Staff: Mentor

    You need control over the voltage applied to the unknown resistance, else it is likely to be destroyed before you can take your measurements. Your circuit shows a fixed voltage source connected to the unknown R, and with R being unknown you can't know ahead of time whether this voltage will be safe for the unknown resistor. A rheostat set to maximum can offer protection, and its inclusion shows you have considered this angle.

    Had you shown the voltage source to be variable, then the rheostat would probably not be considered a necessary inclusion.

    The function highlighted by andrewkirk is also appropriate, because every voltage source has a limit to the current it can supply.
  6. Oct 19, 2015 #5


    User Avatar
    Homework Helper

    You can vary the current in the wire by changing the reostat setting and thereby obtain a range of I, V values from which you could draw a graph and relate the gradient of the graph to the wire's resistance.
  7. Oct 19, 2015 #6
    Thanks! Got it
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted