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Simple Circuit Help.

  1. Sep 18, 2007 #1
    Hello everyone. I have some brief questions about a simple circuits problem in my homework.

    First off, here is the image of the circuit (sorry..drawn in MSPaint.) http://img211.imageshack.us/img211/3271/circuitgh9.jpg

    The Current Source = 0.5A
    R1 = 5 Ohms
    R2 = 25 Ohms
    R3 = 10 Ohms
    R4 = 50 Ohms
    R5 = 5 Ohms

    It asks to find the total resistance of resistors 2, 3, 4, and 5. I've done this by counting R2 as part of a series circuit, and figuring R3,R4, and R5 as a parallel.

    So, 25 + (1/((1/10)+(1/50)+(1/5))) = 28.13 Ohms (assuming this is correct..)

    Now it asks to find the Voltage drop across the current source..I'm not sure how to figure this since I don't have distance..

    Next it asks to find voltage drop across resistor 1, How much power is dissipated by this resistor (V=I^2*R)...But once again...voltage drop?

    And finally it asks for the voltage drop across R2,3,4,5...and once again, voltage drop?

    I've read and re-read all of the notes taken in class, and have no idea how to figure voltage drop for this type of circuit, I checked out an Ugly book and the NEC book, but everything there is to do with voltage drop requires distance.

    Thanks in advance,

  2. jcsd
  3. Sep 18, 2007 #2


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    Staff: Mentor

    Your initial resistance calculation is not correct. To simplify the resistive network, use several steps of combining series and parallel resistances. For example, what do the resistors R3 and R4 combine to make? What do R2 and R5 combine to make? And how do you combine those two resistive combinations in the next step?
  4. Sep 18, 2007 #3
    Hello, you are making some mistakes in combining the resistors. To see why this is we need to examine some basic terminology. First we need to know what an element is, basically a battery, resistor, capacitor, etc are all elements, so basically any circuit component is an element. Now a node is a place where two elements meet. This is where your problem lies as the definition of being in parallel means that two elements are connected to the same two nodes. The resistors you have described above aren't in parallel yet, but we can get them to that point.

    The first thing we need to recognize is that R2 and R5 are in series and so are R3 and R4 so we can directly add them to get equivalent resistors of 30 ohms and 60 ohms respectively. Now these two equivalent resistors are in parallel, because they share the same two nodes, visually this means that between where you had R3 and R4 you have an equivalent resistor (obviously you take the two resistors you combined to form these new resistors out of the drawing) of 60 ohms and in the spot of R5 is an equivalent resistor having 30 ohms. The nodes they share are the entire length of the bottom (the current source is attached to it too you see) and the other node is the point where the wires containing R1 and the two parallel resistors meet.

    Now combine the two equivalent resistors you just created using the formula for parallel resistance and you get a new equivalent resistor of 20 ohms which is in series with the last remaining resistor, R1. Now you can obviously add the two to get the final equivalent resistance of 25 ohms.

    I don't have time to help you with the rest of your questions, or post what I've done graphically so I apologize if the above visual description wasn't clear, but now that you have the equivalent resistance you can now find the voltage drop across the entire circuit using Ohm's law and you should be able to use what you know physically about the circuit to answer the rest of your questions, good luck!
    Last edited: Sep 19, 2007
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