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!

The reading of the ammeter and the reading of the voltmeter.

Tags:
  1. Jun 8, 2017 #1
    1. The problem statement, all variables and given/known data
    In this circuit
    bYQ3m.jpg
    What will be the reading of the ammeter and that of the voltmeter when:
    1. Key S1 and key S2 are open.
    2. Key S1 and key S2 are closed.
    3. Key S1 is closed and key S2 is open.
    (Neglect the internal resistance of the source)
    2. Relevant equations


    3. The attempt at a solution
    That might be an easy question, but my book doesn't give the solution, and I want to amke sure of my work.

    Case 1.
    i = 0
    V = 2V

    Case 2.
    i = V/R =2/3 A
    V = Vb - iR = 2 - ((2/3) * 3 = 0

    Case 3.
    Rt = 5+3 = 8 ohm
    i = V/Rt = 2/8 = 0.25 A

    V = iR = 0.25 * 5 = 1.25 V

    So, is my answer correct.
     
  2. jcsd
  3. Jun 8, 2017 #2

    andrevdh

    User Avatar
    Homework Helper

    Yes, I think you have got all 3 right.
     
  4. Jun 8, 2017 #3
    Ok, but I can't understand why in case two the voltage will be zero? I understand the maths but I don't understand the physics explanation.
     
  5. Jun 8, 2017 #4

    cnh1995

    User Avatar
    Homework Helper

    What does S2 do to the 5 ohm resistor?
     
  6. Jun 8, 2017 #5
    S2 cancels the resistance 5 ohm.
    But we do still have the resistance 3 ohm.
     
  7. Jun 8, 2017 #6

    cnh1995

    User Avatar
    Homework Helper

    In technical terms, S2 short-circuits the 5 ohm resistor. So the 5 ohm resistor is ineffective.
     
  8. Jun 8, 2017 #7
    OK, and then what? more explanation please!
     
  9. Jun 8, 2017 #8

    cnh1995

    User Avatar
    Homework Helper

    What is the resistance of S2 and the wire that shorts the 5 ohm resistance?
     
  10. Jun 8, 2017 #9
    It is zero so the voltage is zero.
    Ok, I got it. But what's your opinion about the whole solution?
     
  11. Jun 8, 2017 #10

    cnh1995

    User Avatar
    Homework Helper

     
  12. Jun 9, 2017 #11

    andrevdh

    User Avatar
    Homework Helper

    the current calculation (correctly) assumes the resistance of the parallel combination is zero and for the voltage it subtracts the voltage drop over the 3 ohm resistor from the power supply's voltage to obtain the voltage over the combination (the remaining part of the circuit) - a bit of a merry go round way of doing the calculation if you ask me. Words would have done better to provide the answer.
     
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: The reading of the ammeter and the reading of the voltmeter.
  1. Reading of a Voltmeter (Replies: 1)

  2. Reading of voltmeter (Replies: 13)

  3. Ammeter reading (Replies: 2)

Loading...