dumb questions from an outsider

The current will be the same throughout the loop. It will be very, very high, because the only thing limiting it is the internal resistance of the batteries themselves.

Don't do this, it's very dangerous.

Hope This Helps! Rich

They sure are, they are fundemental concepts. So fundamental and "obvious" in fact that it is common not to even mention that you are using Kirchhoff's law when proving something or doing calculations etc

The Kirchhoff's law exam questions you get are usually much more complicated than you get in real practical work though. It's not often you actually get 10 resistors in some weird grid configuration connected to two batteries for instance.

Dave :)

Absolutely. It's fundamental that all the currents entering a node add up to zero. It's like bookkeeping, dollars in must equal dollars out.

Ideal batteries connected this way generate infinite current. Lots of insane things happen if you play (in simulation) with ideal sources and components. In real life, a lot of current would flow at first, and taper off as the batteries polarize and then eventually discharge. They'll get warm or hot, too. Try it with two 9-volt batteries.

John

I think Enron forgot that bit !

Graham

On Fri, 14 Jul 2006 03:17:28 +0100, in message , Eeyore scribed:

Damn your eyes! I was just about to suggest that! Safety Nazi!! Always going on about ways to prevent death and dismemberment. Hmph.

what does this follow from? the way i understand kirkhoff's axioms they don't determine a current in that situation. am i wrong about that?

sean

Kirchoff's laws don't actually determine any currents - we still use Ohm's law for that ;)

An ideal battery (voltage source) has zero internal resistance. So I = V/R = infinity (well, indeterminate to the anal-retentive^W mathematicians ;)

Kirchoff's laws are (reminder, and possibly paraphrased, but accurate)

The algebraic sum of currents into a junction is zero The algebraic sum of voltages around a closed loop is zero

Nothing in there about the magnitudes of any currents or voltages.

Cheers

PeteS

they are... eg when it's time to bias a transistor, or if you're designing a DC-coupled amplifier....

it turns into one big short-circuited battery.

if you'ure using ideal batteries with zero resistance an infinite current will flow.

in real life batteries and connecting wires have resistance so a less-than-infinite current flows the batteries and/or wires get hot and are possibly damaged. (usually one or the other or both)

And, of course, they're not true. Maxwell trumps Kirchoff.

John

Well, they're true at DC with no external interferences - to a first approximation, as physicists like to say. Of course, to a first approximation, all species are extinct, too ;)

If the OP is confused about this, I wonder how he'll feel when introduced to Maxwell's equations, transmission lines, self inductance

- all those practical matters :)

Cheers

PeteS

are you saying i should just forget the whole thing? and if yes, why do you think so?

sean

you sre right... it's ohm's law tht determines the theoretically infinite current.

Bye. Jasen

I did not say that :)

Indeed, I am glad you are investigating the subject. My point (such as it is) is to be aware that there are subtleties to watch out for

Cheers

PeteS

No, he's saying you "should" study what has already been done before, to try to understand the principles involved.

Try googling for "Kirkhoff's law" or "Ohm's law", and see what you can learn.

Good Luck! Rich

On 13 Jul 2006 16:47:22 -0700, in message , "Sean McIlroy" scribed:

For the two batteries in that configuration, you do the test of "the sum of all the voltages around the node must equal zero." So, since you have, let's say, 0 = +9 +9, you have to come up with -18 somewhere, and that somewhere will be in the connection between the batteries. What it ends up being is 18V dropped across a resistance approaching zero, which according to Ohm's law will approach infinity. Or more simply, zzzzzzzzt - SNAP!

About practical application, I can add one more to what's already been said. Software simulation and test design. I used Kirchoff's laws repeatedly when I was grinding out software to test electronic circuitry. It paid pretty well. :-)