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: Expression for voltage across resistor in circuit

  1. Jan 13, 2013 #1
    1. The problem statement, all variables and given/known data
    Write an expression for the voltage across R3 in terms of V0, R1, R2, and R3. Under what condition is the voltage across R3 approximately independent of the value of R3?

    2. Relevant equations
    I uploaded a picture.
    I think Req = R1 + R2...
    and 1/Req = 1/R1 + 1/R2...
    Are what I need.

    3. The attempt at a solution
    Best thing I could think of doing was adding R2 and R3, and then add R1 to get this:

    1/R2+R3 + R1, but I don't know how to include the voltage.

    And I have no idea what it means by what condition is the voltage across R3 approximately independent of the value of R3.

    Attached Files:

  2. jcsd
  3. Jan 13, 2013 #2
    There seems to be a confusion, common among many people, in your calculation of Req: Identifying which resistors are in series and which are in parallel. What helped me was watching these two videos: http://www.youtube.com/watch?v=0vqmQuo03Ss&list=PL4F8106B5158CB89E&index=13

    In fact, the entire playlist was helpful on the whole subject of circuit analysis!

    While finding Req first and finding the voltage drop across R3 from there is a valid method (and I suggest you work it out for learning purposes), also consider the application of Kirchoff's Voltage Law

    Hmmm... I'm not exactly sure what this means either. However, I would suggest to consider how the behavior of voltages and currents differ in purely series and purely parallel circuits.
  4. Jan 13, 2013 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    What would happen if R1 << R3 or R2 << R3?
  5. Jan 13, 2013 #4


    User Avatar

    Staff: Mentor

    Without R3, you can see R1 and R2 form a potential divider, dividing that potential V0. R3 could represent a load that you wish to connect, to be powered by that smaller potential set by R1 and R2 and the voltage source.

    As you know, the fact of connecting R3 will change the potential divider so the equation for the voltage across R2 must now involve a modification to include the value of R3.

    If we denote the voltage across R2 as Vx, then your first task is to start with the basic potential divider equation:
    Vx = R2/(R1+R2).V0

    and modify it to include R3, since R3 appears in parallel with R2. (At this stage, you have made no simplifying assumptions or approximations.)

    Your second task then becomes more of a mathematical one ....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook