Self-contradictory combinations

Discussion in 'Homework Help' started by Teszla, Jun 11, 2013.

  1. Teszla

    Thread Starter Member

    Jun 7, 2013
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    0
    Let's analyse these:

    Seems legit, and I imagine it could be connected in several ways.

    Why is this contradictory? Wouldn't these two current sources in series be equivalent to one single current source of 5 A?

    This sounds contradictory to me, but apparently it's not. I wonder why.

    Why is this contradictory? Although of course no current will go through an open circuit, but let's say there is a current source in series with a closed circuit (which I assume is not contradictory), why would it suddenly be contradictory if the circuit is opened?

    This sounds contradictory to me, and apparently it is. But I don't see why this is contradictory when c. is not.

    Some help / clarification of this would be appreciated. :)
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    The first thing understand is that a source guarantees its value at its terminals.

    What this means is that a voltage source will generate a voltage of X volts, with the positive terminal being X volts above the negative terminal.

    Likewise, a current source guarantees that X amps are flowing through its terminals, X amps in, X amps out.

    So, let's look at your questions:
    a.) The 12V source says there is 12V across its terminals, where the current source says there are 2A through it. Neither of these definitions causes one to not be able to make its guarantee.
    b.)One current source guarantees 2A moving through its terminals, but the other guarantees 3A, where does this extra amp come from?

    c.) The source guarantees 2A through it, does a short circuit make that impossible? A short circuit can maintain infinite current, but cannot have a voltage impressed on it, nor does it produce one.

    d.) The source guarantees 2A through a open circuit, but can current flow through something wise definition prevents it from doing so?

    e.) The source is guaranteeing a voltage across its terminals, but a short is defined as something through which infinite current can flow, but no voltage may exist.
     
  3. Teszla

    Thread Starter Member

    Jun 7, 2013
    43
    0
    Thank you! This explains a lot. So a source ALWAYS guarantees a certain current or voltage.

    I wonder how one is to prioritize, if a circuit contains both a current source and a voltage source. If they point in different directions, will this create something like a voltage charger?
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    A what!?

    Any current may flow through a voltage source, likewise, any voltage can exist across a current source.
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,792
    If you connect ideal sources in a contradictory way, they simply create unrealizable circuits.

    Does this mean that it is physically impossible to connect a 12V battery in parallel to a 24V battery? No. You can do that. But you can't treat them as ideal sources in order to determine what will happen.

    Keep in mind that ideal current and voltage sources do not physically exist. However, they are "good enough" models for real current and voltage sources for the vast majority of circuits such that we can use them and the answers we get on paper, using simple circuits, are "good enough" for the much more complex circuits that we physically have.

    But when you push the limits of the model, then the model is no longer "good enough" and your paper calculations don't mean diddley.

    The key mantra in much of engineering is: If it's "good enough" then use it and move on, otherwise use something better.
     
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