TextBook Vol 1 DC Notation question

Discussion in 'Homework Help' started by mboensel, Dec 30, 2015.

  1. mboensel

    Thread Starter New Member

    Dec 27, 2015
    It comes from section 10.4. I can't figure out what's going on with his polarities. I thought that the Current Source when converting from a Voltage Source. I thought it was supposed to be Conventional notation (see other screen shot). This seems to be pointing the direction of electron flow...

  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
  3. MrAl

    Distinguished Member

    Jun 17, 2014

    The interpretation depends partly on if there was a mistake in the text or not.

    In the first diagram, the circuit to the right seems to have a current source with current directed downward, and since the polarity + and - voltage is also marked it appears that they are using electron current flow. Then what you see to the far right "=5A" would still be correct because the current is shown as unsigned, which is normal.
    So if there was no mistake then it would still be correct, except that it is more common to use conventional current flow, especially when in another part of the text they use conventional current flow.
    It is also not common at all to see a passive current flow depicted as an actual current source, or at least drawn as such. That implies energy input where really there is none. The exception is when one source is replaced by another source, but then there are still the same number of sources in the circuit, and then the new source is modified to reflect the Thevenin equivalent for example.
    If they are trying to show the equivalent Thevenin source though that would be fine, except for the sign unless they are using electron current flow. In other words, a voltage source in series with a resistor is equivalent to a current source in parallel with a resistor. That would be indicated somewhere in the text also.

    If instead the current arrow is meant to show the algebraic conventional current flow (which would be implied from other places in the whole text), that means that it should be signed, so what you see on the far right "=5A" would be incorrect as it should read "=-5A".

    I tend to think they meant electron current flow, but i believe it is better to use conventional current flow in circuit analysis that does not depend on explicit analysis of actual physical objects that dont usually fall into the usual category of common circuit elements such as resistors and capacitors and inductors that are defined by their external electrical properties only.
    A counter example would be found in electroplating, where electron flow better describes what is actually happening, although sometimes we would not have to make a distinction there either.
    Last edited: Dec 31, 2015