Question

I'm having trouble understanding how the following circuit diagram is correct or reasonable; it's the polarities that are confusing me. Posted below in a moment.

Answer 1

What I don't understand is that if these are both independent voltage sources, wouldn't they just sort of be...pushing electrons at one another? An instructor verified this as correct and said that they are both in parallel and have proper polarity-both of those statements confusing me-but, yeah, I don't get it.

Answer 2

To me, in the context of two identical circuit elements, having the same "polarity" means having the same orientation in terms of whether you encounter the positive or negative sign on its circuit diagram traveling in one direction, e.g., you run into one side of a battery on a diagram, hit the positive terminal, go through the battery, go out the negative terminal; keep going down that wire and hit another battery in series, enter through the positive terminal, go out the negative terminal. Why is this wrong?

And could somebody explain to me the bit about parallel, in particular? How the hell is that circuit a parallel circuit?

Answer 3

As the circuit is presented in the figure the two voltage sources are defacto connected in series such as to oppose one another with the net result that no current is flowing. It is not an invalid circuit just a useless circuit.

Now if something is connect between points "a" and "b" the batteries are now connect in parallel both supplying current at the same voltage to whatever is connected between the two points.

Answer 4

Yeah, that makes complete sense to me, what you just said; But why is the other situation, in which two voltage sources are connected effectively in series, with two terminals between them, where they are facing in the following way:

_______________________ + - ________________ + - ______________

Why is this "wrong"?

Answer 5

@gleem

Answer 6

I don't know what you mean by "wrong".

Answer 7

Sorry, web browser crashed, one moment.

Answer 8

Strongly implying that if not both of those conditions are simultaneously met-e.g. same voltage and same polarity, the circuit interconnection is "invalid".

If I had a circuit interconnection between two batteries in series sharing two terminals where the polarity was as posted above, unless it's just incredibly poor wording, the book seems to be implying that that interconnection is invalid.

Answer 9

I don't understand. The book says the connection is "valid" !

Answer 10

Yes, the connection demonstrated *in the figure* is valid, the connection demonstrated in what I wrote, you know, the long line, prior to the post, the thing that I wrote, *not* the picture, is heavily implied as invalid.

Answer 11

_______________________ + - ________________ + - ______________

(Assume there's the rest of a wire beneath it, forming a completed circuit)

^Strongly implied as invalid, because it doesn't meet the two criterion stated in the book

http://assets.openstudy.com/updates/attachments/54b3f361e4b0bdcb3ccdbc67-mendicant_bias-1421081451861-2015011211_42_55electriccircuits9theditionfoxitreader.png

^Valid.

First one invalid, second one valid, does that make sense?

Answer 12

what are the two criteria?

Answer 13

"This requires that each source supply the same voltage with the same polarity, which they do."

http://assets.openstudy.com/updates/attachments/54b3f361e4b0bdcb3ccdbc67-mendicant_bias-1421081451861-2015011211_42_55electriccircuits9theditionfoxitreader.png

Assume the same voltage for my ASCII circuit; they have different polarity, in that, if you go through the wire in only one direction, you encounter it like this:

___________________ ++ -- ______*_______ ++ -- ________________

As opposed to, apparently, the "same" polarity as in the picture,

______________ -- ++ _____*______ ++ -- ______________________

Where the star is one of the terminals between them.

Answer 14

The criteria are quoted directly from the solution statement.

Answer 15

(IMO this book is incredibly poorly written in general, I normally don't have these kinds of issues in reading texts on this and other solutions also have some odd issues where they omit a significant part of an explanation or a simultaneously valid explanation in reaching the answer)

Answer 16

My point is that the book makes it sound incorrect if you had two batteries, in series, going

++ -- ++ --

as if the world would implode or something....when you just have....a battery bank.

Answer 17

If you connect two batteries + to- in a series circuit the voltage both work in the same direction to cause current to flow reinforcing one another. When you connect them + to + and - to - then they oppose one another in the series connection.

If the battery circuit has something connect between a and b. as I said before when connected + to + etc they both provide the same voltage to what is connected between a and b. But if they where connect + to - etc. they would supply a voltage equal to the difference of the two battery voltages if they where different. As they are the same they would not provide any voltage between a and b.

Answer 18

Alright, that entirely makes sense to me. I think the way the book went about it was just totally poor; you're saying that, if the polarities were flipped, e.g., if the batteries looked like a battery bank spread out across a parallel circuit, and you had a component connected with a new, third wire, between a and b, no voltage would be generated, right?

That 100% makes sense to me. Sure would have helped in the book to, you know...put that third wire and component there, so it isn't just seen as a battery bank circuit.

Thank you.

Answer 19

glad to have helped