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: Uni physics circuit problem

  1. Jan 19, 2010 #1
    The question appears not to complex! however im been on break (and still am haha) but I saw this in an old exam paper and its bugging me that I dont know how to do it!

    heres the question;
    http://img59.imageshack.us/img59/9868/dfdfmu.jpg [Broken]

    Kirchhoff voltage law comes to mind. However im confusing myself. I thought that the current across each resistor in one of the closed loops cant be the same? or can it?
    anywhos ignore my little rant. Id really appreciate if somone could explain to me the answer cheers.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jan 19, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    Is there a way to simplify the circuit to find the current from the battery? Look at how the resistors are arranged.
  4. Jan 19, 2010 #3
    I am new to this as well in my circuits class, so anyone correct me If I am wrong.
    At point A the voltage from the battery hasn't reached any electrical resistance yet, correct?
    So would not the voltage be the same since as it started 2 volts because it hasn't reached resistors yet?
    And solving for current would be just doing a little algebraic manipulation of one form of the equation for voltage

    [itex]V=IR[/itex] and resolving to find current [itex]I=\frac{V}{R}[/itex]


    I think I might be way off.... :(
    Last edited: Jan 19, 2010
  5. Jan 19, 2010 #4


    User Avatar
    Science Advisor

    Start from the right of the circuit. There are some resistors in series.

    What would be the size of one resistor that could replace these three resistors?


    Then, take this new resistor and put it in parallel with the 4 ohm resistor. What is the formula for two resistors in parallel?

    So, you could put one resistor in here that is equivalent to the 4 resistors.

    Now, you have 3 resistors in series. How do you get the equivalent resistance of these 3 resistors?

    So, now there is one resistor across a battery. Ohms Law tells you what the current will be.
    Last edited by a moderator: Apr 24, 2017
  6. Jan 20, 2010 #5
    hey i think u should use mesh analysis for solving this circuuit. consider seperate currents I1 and I2 for the two loops.now apply kirchoffs voltage law in both the loops sepreately.solve for I1 and I2.
    the value of I1 will give u the current at point A.
  7. Jan 20, 2010 #6
    No real need to calculate. The current has to pass the two 1 Ohm resistors on the left, and then at least one more resistor. Hence, the resistance is larger than 2 Ohms, this means the current must be smaller than 2V/2Ohm = 1 A. I only see one answer smaller than one.
  8. Jan 20, 2010 #7


    User Avatar
    Science Advisor

    It should be mental arithmetic anyway.

    1 + 2 + 1 = 4

    4 // 4 = 2

    1 + 2 + 1 = 4

    2 / 4 = 0.5
  9. Jan 25, 2010 #8
    hey sorry late reply guys, thanks so much for the help :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook