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Arrows: Whats the POINT???

Discussion in 'Electronics Homework Help' started by Raven Luni, Mar 18, 2012.

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  1. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    Greetings,

    I've been doing the free MIT thingy and have come across the age old bugbear of 'teaching stupid **** for no discernable reason':

    Take this 'homework question':

    wtf.gif

    Looking at the series circuit, the values for voltage across / curent through the resistors etc. is what you would expect it to be.

    The correct answer for i(v) is -2A because (and only because) theyve got the arrow pointing the other way.

    The (supposedly) correct answer for the amount of power in the circuit is 0 Watts! WTF?? All because theyve got an arrow pointing the other way. If you created that circuit in real life I think it would use a bit more than 0 Watts - probably closer to 21.4W in a perfect world.

    I just dont get why they would consider this a valid method of teaching. Or am I missing something here?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    Power cannot be created nor destroyed...

    Some part of the circuit is supplying power, and some part of the circuit is consuming power.

    If you define supplying power as consuming some negative number of watts, and something dissipating power as consuming a positive number of watts, then what will the sum be?

    It's exactly the same a looking at currents flowing into a node, or the sum of voltages in a loop.

    Whilst you may not "like" the idea of the current flowing the opposite way to the arrow and thus being negative, it makes the math work.

    You are simply rebelling against a particular model which goes against (probably) all the stuff you've taught yourself.

    Relax, accept it, and you will be able to understand and solve whole sets of problems you couldn't solve previously.

    I know :D
     
  3. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    Can I argue that point next time my electricity bill comes in? :D
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    If you consume a negative amount of energy you can. ;)
     
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