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Homework Help: Electrical Engineering: Circuit Problem

  1. Feb 4, 2012 #1
    1. The problem statement, all variables and given/known data
    Consider the circuit shown in Figure Pl.64. Use repeated applications of Ohm's law, KVL, and KCL to eventually find Vx.

    3. The attempt at a solution
    I know how to solve the problem (almost). I'm having difficulty understanding the polarity at the left most resistor (5 ohms). I looked at the solution and it says that the positive terminal is on the left and the negative of the right of the resistor. I thought it was the opposite. The diagram of the circuit is attached.

    Attached Files:

  2. jcsd
  3. Feb 5, 2012 #2
    One way to look at it:

    When current enters a resistor, it loses some voltage (potential). The + sign on the left of the 5ohm resistor simply means that the current had more potential before entering it than when it exits the resistor. If the polarity was inverted, it would mean the resistor would be producing energy instead of absorbing it, which is impossible (as far as I know).
  4. Feb 6, 2012 #3

    rude man

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    Homework Helper
    Gold Member

    Resistors do produce thermal noise energy.
  5. Feb 6, 2012 #4
    Why did you think that?
    1A through 5Ohm = 5V across right most resistor
    Hence 5V across the middle 5 Ohm resistor, so current through middle one = 1A
    Similarly, 5V across 10Ohm resistor so current through it = 0.5Ohm
    Thus total current through leftmost 5Ohm resistor = 2.5A
    Voltage drop across 5Ohm = 5*2.5 = 12.5V
    Thus Vx = 17.5V
    Note that the current flows through leftmost 5V from left to right, Current always flows from higher potential to lower potential (opposite of direction of electron flow which is -ve to +ve) so left side of the 5Ohm resistor should be at higher potential than right side, or, the positive side is on the left and negative on the right
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