Circuits - Power absorbed / supplied

Discussion in 'Homework Help' started by J.live, Sep 10, 2010.

  1. J.live

    Thread Starter Member

    Sep 10, 2010
    35
    0
    I need to solve the circuit. I have trouble comprehending absorbed/supplied power concept.


    Attempt:

    So, if power is being supplied by Vx then the current is passing out of the negative terminal towards the dependent source 4x and the element A?

    Vx * 4x= W?

    Second senario. The power is supplied by the dependent source 4x so again the current is passing out of negative terminal and being delivered to element A and source Vx?

    4Vx * 16= W?

    Here is the Circuit:
     
  2. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    You can calculate all the voltages and currents rather easily.
    Once you have those numbers you can note the polarities and it will tell you what is providing/absorbing what power.

    So, for example, the current is calculated like this:

    I = 4 Vx (given)
    Vx = 4 (given)

    Therefore I = 4*4 = 16A with the indicated polarity.
    The polarity is that positive current is leaving the positive terminal - this means power is being provided, or P = -16A * 16V = -256W

    That current source is providing 256W.
     
  3. J.live

    Thread Starter Member

    Sep 10, 2010
    35
    0
    The power is leaving positive terminal and entering the negative terminal.

    How is 16A negative? I am confused with polarities.

    The absorbing/supplying 's polarities can vary in every situation?

    Edit:

    So, when the power is leaving (+) it is providing and when entering (+) it is absorbing ?

    Still dont understand why 16A is negative :/
     
    Last edited: Sep 10, 2010
  4. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    It's about polarity conventions. Polarities are arbitrary, it's your job to pick a convention and stick with it and interpret it correctly.
    You can draw arrows and plus signs however you want and then solve for the numbers. A positive number means your arrow is correct, a negative sign means the actual value is opposite of what you drew.

    The 'passive sign convention' is the most common convention: Positive current entering positive (voltage) terminal gives positive power, where positive power means absorbed power.
    http://en.wikiversity.org/wiki/Passive_sign_convention

    Flip either the current or the voltage and you get negative power.

    It's not the 16A that's negative, it's the power of the current source that's negative.

    Look at the equation of the current source:
    I = +4 Vx

    Vx is given as +4V, therefore I = +16A.
    The positive number means it's going in the direction of the arrow. If Vx = -10V then I = -40A, meaning it's 40A in the direction opposite the arrow.

    Power does not enter or leave terminals, it merely exists in the device. Only current enters or leaves terminals. Voltage is across terminals.
     
  5. J.live

    Thread Starter Member

    Sep 10, 2010
    35
    0
    Wow, this makes a lot more sense now.

    Thanks a lot Ghar. I really appreciate your help.
     
    Last edited: Sep 10, 2010
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