1. Not finding help here? Sign up for a free 30min 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!

Calculate the voltmeter reading

  1. Sep 28, 2016 #1
    1. The problem statement, all variables and given/known data
    The light-dependent resistor (LDR) in the circuit below is found to have resistance 800 Ω in moonlight and resistance 160 Ω in daylight. Calculate the voltmeter reading, Vm, in moonlight with the switch S open.

    df0febad6d98.jpg

    If the reading of the voltmeter in daylight with the switch S closed is also equal to Vm what is the value of the resistance R?

    Answers: 8.0 V, 50 Ω

    2. The attempt at a solution
    At first I would like to check whether I understand the scheme correctly. The switch S is a diode (open is a reverse-biased and closed is a forward-biased). The LDR is just a regular resistor, just need to take into account different values of resistance depending on the daytime. Should this be correct?

    I this representation correct?
    371fe9db422c.jpg
     
  2. jcsd
  3. Sep 28, 2016 #2

    gneill

    User Avatar

    Staff: Mentor

    A switch is not a diode. A switch will pass current in either direction with equal facility and does not exhibit a potential drop when current passes through it (at least for an ideal switch). Diodes pass current in one direction only, and except for ideal diodes cause a potential drop on the order of a half volt when they do.

    It's better to think of a switch as a zero resistance wire connection that can be inserted or removed manually.

    Your redrawn circuit is fine.
     
  4. Sep 28, 2016 #3
    We need to find the voltmeter reading during moonlight when the resistance is 800 Ω and the switch is open. If the switch is open, the resistor R is shut off. Therefore we have only two resistors 200 + 800 = 1000 Ω. I = V / R = 10 / 1000 = 0.01 A. Now we find the 200 Ω has V200 = I R200 = 0.01 * 200 = 2 V, and the LDR resistor, as a result has 8 V. Since the voltmeter is connected to LDR, its reading is 8 V. Correct logic?

    Update: for the second question we have: since the reading doesn't change, therefore V = I R and we can find I = V / R = 8 / 160 = 0.05 A of the whole circuit. Then, knowing V = 2 V and I = 0.05 A, we find the RParallel: R = V / I = 2 / 0.05 = 40 Ω. Then, by 1 / R = 1 / R + 1 / R → 1 / 40 = 1 / 200 + 1 / R → R = 50 Ω. Should be correct.
     
    Last edited: Sep 28, 2016
  5. Sep 28, 2016 #4

    gneill

    User Avatar

    Staff: Mentor

    That all looks good. Well done.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Calculate the voltmeter reading
  1. Reading of a Voltmeter (Replies: 1)

  2. Reading of voltmeter (Replies: 13)

Loading...