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Circuits analysis question...

  1. Sep 30, 2015 #1
    i have just started taking electric and electronic circuits course and i have a question about a basic concept in KVL, regarding the loop direction that you assume in any circuit : does the loop direction that you assume is just telling that you will subtract the voltage of the near points from the voltage of the far points ? for example if you are moving with the direction you assumed between 2 points a and b like this : a ----> b, does writing the voltage of this part as +V tells that b has voltage higher than a (Vb - Va = +V) ?
     
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  3. Sep 30, 2015 #2
    KVL is basically an expression of the principle of conservation of energy - wherever energy is consumed in a circuit, it had to be supplied somewhere else. If your power supply is providing you +4 V and you have four resistors, each causing a drop of 1 V, the total voltage when all summed together is 0 (+4 - 1 -1 -1 -1 = 0). Consistency is important, so if you assume that your supply has a positive voltage, make sure that is carried through all your calculations (e.g. don't assume it is negative elsewhere in your analysis). To provide more specific help, can you provide more specifics on the problem you are working on?
     
  4. Sep 30, 2015 #3

    sophiecentaur

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    KVL is not about "near points" and "far points". It's about IR drops and emfs around loops in a circuit. The directions that you use will follow once you have chosen one of the emfs in the circuit. It will work either was as long as you are consistent. (Note that the word Consistent has been used above too.
     
  5. Sep 30, 2015 #4
    making the question more clear : when you want to solve specific loop using KVL you specify a loop direction. so that the element with increasing voltage throughout the assumed loop direction (- to +) you write its volt as +V and the element with decreasing voltage throughout the assumed loop direction (+ to -) you write its volt as -V. the question is: what does the loop direction that we assume actually represent ?
     
  6. Sep 30, 2015 #5

    phinds

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    You can use the convention that you just stated or you can use the opposite convention. It doesn't matter at all. What MATTERS is that as everyone keeps telling you, you are consistent. Personally, I always draw my loops clockwise and count as positive voltage those elements for which I encounter the higher potential first. When you have an unknown, just assign it plus or minus and if you've assigned it backwards, your answer for it will come out negative.
    I don't really understand the question. The loop itself represents total voltage drops in a closed loop, which have to add up to zero. As has also been said already, the direction is just an arbitrary choice for how you are going to label things.
     
  7. Sep 30, 2015 #6
    alright i just wanted to make sure that the direction is not more than reference and that using the opposite convention is ok also
    thanks all
     
  8. Sep 30, 2015 #7
    by the way I want to know something else, when I get the current as negative value this means that I assumed its direction reversed, what about when I get the voltage as negative value, what does it mean ?
     
    Last edited: Sep 30, 2015
  9. Sep 30, 2015 #8

    phinds

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    Why would you think a negative value on an assumed direction for a voltage would be any different than a negative value for an assumed direction on a current?
     
  10. Sep 30, 2015 #9
    assumed direction for a voltage ? do you mean like if i got the volt of voltage source as -V therefore the actual terminals (+ and -) of this source are reversed ?
     
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