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Passive Sign Convention

  1. May 19, 2014 #1
    1. The problem statement, all variables and given/known data

    I am having trouble understanding PSC. Please help on this question. Teach me like I am 5. My tutor couldn't break it to me.

    (SEE ATTACHED) and find whether the power is being given or taken

    2. Relevant equations

    p=vi
    p=-vi

    3. The attempt at a solution

    I cant realize if this current is leaving positive to get into negative or negative to positive. Also, the relation to negative and positive current is really messing me up. I got:

    Pa: a) 1250w b) 1200w c) -5760 d) 16500

    but my book says they are opposite.

    Thank you!
     

    Attached Files:

  2. jcsd
  3. May 19, 2014 #2
    Chegg has this. This because the power A > 0 so it is being delivered power to A and it flows from negative to positive?

    Is my assumption correct?

    thank you
     

    Attached Files:

  4. May 19, 2014 #3

    NascentOxygen

    User Avatar

    Staff: Mentor

    Consider when a resistor is hidden inside a box. Whatever end of the resistor is positive is also the end that current flows into that resistor. Ohm's Law.

    Current in the second interconnecting wire will flow in the direction opposite to that in the first. (It may help you to picture what is happening if you draw both of these current arrows.)
     
  5. May 19, 2014 #4
    so this being said since the top connection is to the left the bottom is to the right?

    & I guess what I am Really confused at is which one is positive. The tutor explained to me both the connected terminals where positive on top and negative on the bottom. I am more lost on determining which point (node, black dot, terminal, etc) is pos/neg. I would think the top right (black dot) is where the "positive" comes from into box A which is "negative" on the top left black dot.
     
  6. May 19, 2014 #5

    NascentOxygen

    User Avatar

    Staff: Mentor

    Are you talking about current direction?

    The positive terminal has the "+" sign. Alternatively, the positive terminal is pointed to by the arrow head of the voltage arrow. If an arrow is shown but + and ─ are not already marked in, you can write them in lightly as a reminder.

    +
     ↑_

    What might get you confused is where a voltage arrow is shown and you are told it is negative volts. In this case, you can redraw the diagram with the arrow pointing in the opposite direction and then say the voltage is a + voltage.

    A similar thing applies where a current arrow is drawn but labelled ─. You can reverse it and say it's positive.
     
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