Two questions. 180 degrees out of phase.

[Evil Bunny]

Question #1:
Can you have current in a wire without voltage? I was having a discussion with someone today and I said that it's impossible to have current with no voltage... He said, "oh yeah, well when I short this battery out, you will measure zero volts across it, but you've got all kinds of current flowing..." I wasn't real sure how to respond to that. Although I just had a thought as I was typing this... It has to do with the meter, doesn't it? Current needs to flow through the meter to get a reading. With a dead short, nothing will go through the meter. There is a voltage across the wire but our run-of-the-mill voltmeter can't measure it, right?

Question #2:
What happens when you combine two AC voltage sources together that have identical magnitudes and frequencies, but are exactly 180 degrees out of phase with each other (for example 120 volts at 60 Hz)? Would you get a flat-line 0 volts? And by "combine" I mean we wired them together (pole A of gen 1 to pole A of gen 2 and the same with the B poles of the generators). We will measure the voltage from pole A to Pole B (of either/both generators, doesn't matter). I think you get 0 Volts. Now... If we connected a wire across these two points... we have just created a dead short for both generators, which would be bad... but yet, we didn't have a potential difference between the two points, did we? What happens here? Are we melting wires and destroying generators?

More than two questions, I know... sorry.

 

[skeptic2]

Question #1:
The battery has internal resistance and the wire has resistance. The voltage you will see across the terminals of the battery when you put a wire directly across it will be Vb * Rw / (Rb + Rw) where Vb = battery voltage, Rw is the resistance of the wire and Rb is the internal resistance of the battery.

Question #2
This is really no different than question #1. Yes you will get 0 volts. And if you put a dead short across the point where there is zero volts, nothing will happen. The wires you might melt are the ones connecting the two generators, not the short you placed across the 0 voltage point.

 

[Evil Bunny]

The wires you might melt are the ones connecting the two generators...

Really? Current will flow from pole A of generator one to pole A of generator two (or vice versa)? Would this happen if we disconnected the wire connecting the B poles?

 

[skeptic2]

No, then it would not happen because it would be an open circuit.

 

[Evil Bunny]

So the current would flow through the wires and through the generators (in the original scenario)... even though they were 180 degrees out of phase with each other?

 

[Evil Bunny]

The voltage you will see across the terminals of the battery when you put a wire directly across it will be Vb * Rw / (Rb + Rw) where Vb = battery voltage, Rw is the resistance of the wire and Rb is the internal resistance of the battery.

Oh... and that looks like an equation to find voltage across Rw if it was wired in series with Rb across the voltage source. The scenario I laid out in question #1 would be a parallel circuit, wouldn't it?

 

[Evil Bunny]

Why would this be different than hooking up the + poles of two batteries and the - poles of the same two batteries?

No current flows, right?

Edit: It would be like hooking the + of one battery to the - of the other battery and completing the circuit by doing the same on the other poles of the battery... creating a circuit with 2 voltage sources in series (adding the voltages) with two loads hooked up in parallel... So what would happen in the original scenario is that we would be taking the internal resistance of the generators into account and figuring out current flow from there?