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Finding terminal voltage without voltmeter

  1. Jan 22, 2015 #1
    << Mentor Note -- OP has been warned about not showing any effort toward a solution >>

    1.
    How can I find the terminal voltage across a 6v power supply, without using a voltmeter. The problem is the only equipment listed on the practical is an ammeter and a resistor with unspecified resistance...


    2. Relevant equations
    V=IR
    V=emf-Ir


    3. The attempt at a solution
    Not a clue
     
    Last edited by a moderator: Jan 22, 2015
  2. jcsd
  3. Jan 22, 2015 #2

    Stephen Tashi

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    Science Advisor

    What does the resistor look like? No color code? Any way to use only part of it in a circuit?
     
  4. Jan 22, 2015 #3
    No idea, I have only been provided with a list of equipment, but the practical is in about half an hour...
     
  5. Jan 22, 2015 #4
    http://www.ocr.org.uk/Images/67766-unit-g483-practical-skills-in-physics-1-quantitative-task-specimen.pdf [Broken]
    This is the practical I'm doing
     
    Last edited by a moderator: May 7, 2017
  6. Jan 22, 2015 #5
    I'm only confused on the first point, seeing as no internal resistance or resistance of any resistor has been stated... It simply asks to draw a circuit set-up to find the terminal voltage across the power supply
     
  7. Jan 22, 2015 #6

    Stephen Tashi

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    Science Advisor

    Assuming the protective resistor isn't color coded, the only idea that comes to my mind is to incorporate varying lengths of the wire in the circuit.

    R = unknown resistance of protective resistor
    Rw = unknown resistance of the wire
    V = unknown voltage

    Using the circuit in Fig. 1.1, assume incorporating 1/3 of the wire in the circuit puts a resisance Rw/3 in the circuit, etc
    Make measurements to establish simultaneous equations

    For example:

    V = I1 R + I1 Rw
    V = I2 R + I2 Rw/2
    V = I3 R + I3 Rw/3

    Gives 3 equations with currents I1, I2, I3 known by measurement to solve for unknowns V, R, Rw
     
    Last edited: Jan 22, 2015
  8. Jan 22, 2015 #7

    Stephen Tashi

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    Science Advisor

    Good luck. It's 4:30 AM here and I must go to bed.
     
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