Need help

Discussion in 'Homework Help' started by sidk47, Sep 5, 2013.

  1. sidk47

    Thread Starter New Member

    Jul 1, 2013
    13
    0
    Hi guys,

    I am trying to learn electronics. I would like to know how to solve for Vab in the circuit given below. There is a 12V voltage source. There is a 1K and a 4K resistor.

    I don't know how to solve for Vab. Help would be most appreciated.

    Thanks.
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    Q1) What is the current in the two resistors?

    Q2) How much voltage is dropped across each resistor?

    Q3) As you go from Pt b, how much voltage do you change by as you go across the 4kΩ resistor?

    Q4) How much more do you change by as you go across the 1kΩ resistor?

    Q5) How much more do you change by as you go across the 12V source and arrive at Pt a?

    Q6) What is the total change in voltage as you went from Pt b to Pt a?
     
  3. sidk47

    Thread Starter New Member

    Jul 1, 2013
    13
    0
    2.4mA?


    9.6 V in the 4K resistor and 2.4V in the 1K resistor?

    9.6V

    2.4V
    -12V
    0V

    So is Vab zero? Please correct me if I am wrong.

    Thanks a lot.
     
  4. sidk47

    Thread Starter New Member

    Jul 1, 2013
    13
    0
    0A


    0V

    0V

    0V
    -12V
    -12V

    Please correct me if I am wrong.
     
  5. sidk47

    Thread Starter New Member

    Jul 1, 2013
    13
    0
    Thanks for helping me to learn. I think I have it figured out. If I went from A to B would I get +12V? So is Vab +12V
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    I think you got it.

    Yes, Vab = 12V because there is no current flowing in the resistors and, hence, no voltage drop across them.

    As a minor point, going from Pt B to Pt A is a change of +12V, not -12V. Think of it like if you were asked what your change in elevation is when going from the 6th floor to the 9th floor in a building with 10ft floors. It would be +30ft. If you go from a lower potential to a higher potential, the change is a positive voltage.

    So you can get Vab two equivalent ways: Start at Pt b and sum up the voltage increases until you get to Pt a, or start at Pt a and sum up the voltage drops until you get to Pt b.
     
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