An annoying voltage question

**Dadface** · Feb16-09, 09:58 AM

You have the following:
1.Two batteries one of emf 9V the other of emf 12V.The batteries are ideal(zero internal resistance)
2.Connecting wires of zero resistance
2.An ideal voltmeter(infinite resistance)

The batteries and voltmeter are all connected in parallel.What voltage will be measured when the batteries are connected positive terminal to positive terminal and negative terminal to negative terminal?What voltage will now be measured if one of the batteries is turned around to face the other way?

(Dont get too annoyed if you are unable to come up with an answer)

**redargon** · Feb16-09, 10:14 AM

i'm going for the obvious and probably wrong answer:
batteries ++ -- : (12v+9v)/2 therefore: 10.5v
batteries +- +- : 12v+9v = 21v

**Kittel Knight** · Feb16-09, 10:37 AM

0V and 0V
They have exploded.

**Dadface** · Feb16-09, 02:05 PM

Well done Kittel Knight.

**mikelepore** · Feb21-09, 03:45 PM

It's not only the batteries that have exploded. The zero internal resistance of the wire has dissipated a power of V^2/R = (12 V - 9 V)^2 / 0 ohms = infinity watts. That's enough heat to vaporize the universe.

You can get the same effect if you take an ideal current source, don't connect to anything, and turn it on. Pushing any required current through an infinite resistance, the power dissipated becomes infinity.

**Gokul43201** · Feb21-09, 04:42 PM

Originally Posted by mikelepore

It's not only the batteries that have exploded. The zero internal resistance of the wire has dissipated a power of V^2/R = (12 V - 9 V)^2 / 0 ohms = infinity watts. That's enough heat to vaporize the universe.

That's hardly dramatic. Most of the universe is vapor anyway!