power supplied/ absorbed in electric circuits

Discussion in 'Homework Help' started by Ramiel, Oct 4, 2018.

  1. Ramiel

    Thread Starter Member

    Feb 19, 2018
    65
    0
    Hi guys,
    I am currently taking circuit theory 1 and I have trouble in identifying power.
    At first all I know is that I should use passive sign conv. and that if positive current enters a positive terminal then power is positive and it is absorbed and negative/supplied if negative current enters positive terminal or positive current enter negative terminal.
    Now in this example I posted, it says that power for 30 ma is 5.4 w supplied, 9 ohm is 3.6 w, 6 ohms is 0.6 w, 12 ohm is 1.2 watt all absorbed.
    couple of things i dont understand. How were they able to find the power? I know p equals vi, vi^2, or v^2/r. I also know how to find current by kcl and voltage by kvl but I donw understand how to assign the signs in the calcuation and when to know which element absorb or supplies power. Like do all resisters absorb and all current supply or does it depend on some factors. Guys I am lost. I have been thinking about this for the past 5 hours and my mind is going to explode. Can someone please help me. I am the type of guy who cant sleep until I understand something. please help.

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    Last edited: Oct 4, 2018
  2. geekoftheweek

    Active Member

    Oct 6, 2013
    237
    29
    Your confusion is confusing...

    I'm thinking you have a constant current power source that is set at 30 mA. That would be the only thing that supplies power.

    I don't remember the names of the laws but it sounds like you can follow. Find the parallel resistance and use that to find your voltage of your power source at 30 mA.

    Once you have the voltage of the source you can calculate the voltage and current "absorbed" by the resistors.
    You can then use the volts and amps of each resistor to calculate the power of each resistor.
    P = VI (watts = volts x amps)
     
  3. KeithWalker

    Member

    Jul 10, 2017
    381
    124
    I think you need to get a better understanding of the functions of the basic electrical elements. This will explain what resistors are and what they do:
    https://en.wikipedia.org/wiki/Resistor
     
  4. Ramiel

    Thread Starter Member

    Feb 19, 2018
    65
    0
    Okay so you said that the current supplies power. This my confusion. How did you know that it supplied power? Do all current sources supply power or does it depend on certain factors?
     
  5. Ramiel

    Thread Starter Member

    Feb 19, 2018
    65
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    Okay, so I guess it is clear now that resistors absorb power and not supply, but what about current sources? Do they always supply? Or does it depend?
     
  6. MrChips

    Moderator

    Oct 2, 2009
    19,135
    6,150
    A constant current source, by definition, provides a constant current, not power. Power is absorbed by the load attached to the source.
    By the law of conservation of energy, the power absorbed by the load must be provided by the source. Hence the power supplied by the source will be equal to the power consumed by the load.
     
  7. WBahn

    Moderator

    Mar 31, 2012
    24,562
    7,700
    The three power formulas you are trying to identify are

    P = V·I = V²/R = I²·R

    Yes, resistors will always absorb power, but the same can not be said of sources. Consider a battery being charged -- it is absorbing power.

    In a circuit like this, since three of the four elements can only absorb power, you know that the fourth component, the current source, has to be supplying power. But if we add a second source to that circuit, now all bets are off. While we can confidently state that at least one of the sources will be supplying power, we don't know whether both of them are and, if only one of them is, which one it is, until we analyze the circuit.

    As for the passive sign convention, what this basically means is that you can go around your circuit and blindly assign either a voltage polarity across it or a current direction through it. Flip a coin for each component if you want. But then when you assign the other quantity, you choose to assign it's polarity such that if the current for THAT device is positive, then it is flowing into the positive voltage terminal for THAT device.

    Now you analyze the circuit using whatever technique you want and calculate the power for each device. If the power is positive, that device is absorbing power. If it is negative, that device is supplying power.
     
    Ramiel likes this.
  8. Ramiel

    Thread Starter Member

    Feb 19, 2018
    65
    0
    Thanks for the explanations. This makes sense now.
     
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