About Kirchhoff's Voltage Law.

Hi all.

Why Voltmeter shows -32V and not 25V.

http://sub.allaboutcircuits.com/images/00117.png

Thank you.

25V is the voltage between points 5 and 9. Between the points 3 and 4, you must add the 12V (voltage drop between 4 and 9) and the 20V (voltage drop between 3 and 8 ).

The voltage at the node of points 7, 8, 9, and 10 we'll consider common, the reference point.

So the voltage at node 3 is +20V with respect to common. But the voltage at at node 4 is -12V with respect to common.

So there is 32V difference between nodes 3 and 4. However, you have the red lead of the meter, positive, connected to node 4. So the meter will read a negative voltage, therefore it says -32V.

You didn't apply Kirchoff's laws to all the relevant parts of the circuit - in general case you get N equations in N unknowns and you have to solve them. Often there are simplifications you can make due to circuit topology and symmetry. Here each half of the circuit can be treated independently and the voltages summed.

Beware of getting signs right - it often helps to do a sanity check via rough rules of thumb.

Oh yes, draw your supplies +ve at the top, -ve at the bottom, consistently!

Sorry guys, mistyped. But i learnt another things from your answers. Thank you.

I should have typed "why its not 28V". Because 15V (2-3) and 13V (4-5) above the Voltmeter probes.

But now i realized that there is not a wire between 2 and 5.

So i have another question, what would Voltmeter shows if there is a line between 2 and 5?

Thanks.

beingobserver: Sorry guys, mistyped. But i learnt another things from your answers. Thank you.

I should have typed "why its not 28V". Because 15V (2-3) and 13V (4-5) above the Voltmeter probes.

But now i realized that there is not a wire between 2 and 5.

So i have another question, what would Voltmeter shows if there is a line between 2 and 5?

Thanks.

In that case, the power sources would be shorted together in series (look at the outer loop, bearing in mind the polarity of the power sources), and the tool you would be using on it is a fire extingisher, not a volt meter.

beingobserver: (...) So i have another question, what would Voltmeter shows if there is a line between 2 and 5?

I think that is impossible to know with the data that you give. You need to know the current or the value of the resistors.

Thank you.

So i have another question, what would Voltmeter shows if there is a line between 2 and 5?

It would be difficult to read (at first) because of all the smoke. When the smoke clears, it should read 0V.

The amount of smoke you will make will be largely dependent on the current capacity of the batteries. For example, if the batteries were small and were not able to sustain a large current flow for any significant time, you may not see anything, maybe some warmth in the wires if the wire gauge was small.

Now, if the current capacity of the batteries was larger, and able to supply more current, then you will see one of two things, either a battery fritzing or exploding and or the wiring in the loop melt and potentially burn up.

Then, if you had really large capacity batteries, say like 400Ahr lithium cells, then you will only see a flash as the wire will simply act as a fuse and create a quick bit of smoke and then go open circuit.

You can see it will be just a short circuit of the whole battery system.

My advice is to not even try :grin:


Paul

Yes, i realized that there would be a short after DrAzzy's answer.

It takes time to see the details :)

Thank you guys.

rockwallaby:
The amount of smoke you will make will be largely dependent on the current capacity of the batteries.
For example, if the batteries were small and were not able to sustain a large current flow for any significant time, you may not see anything, maybe some warmth in the wires if the wire gauge was small.

Now, if the current capacity of the batteries was larger, and able to supply more current, then you will see one of two things, either a battery fritzing or exploding and or the wiring in the loop melt and potentially burn up.

Then, if you had really large capacity batteries, say like 400Ahr lithium cells, then you will only see a flash as the wire will simply act as a fuse and create a quick bit of smoke and then go open circuit.

You can see it will be just a short circuit of the whole battery system.

My advice is to not even try :grin:


Paul

the 600 amp hour battery in my dad’s car and the 600 amp hour battery in my car with the proper heavy wire sizes and connection only made one wire glow red.

glad to say that there was no explosion or smoke released.

of course when you realized that a copper wire the thickness of a pencil was glowing red, you know there was some serious power involved.

600A, not 600Ah. Most car batteries are in the range 30 to 60Ah. That 600A is the cold-cranking current, ie the current at which the terminals falls to 8V (or is it 6V) for brief period discharge. So think 1000A for shorting out a car battery, or put another way 2 milliohms per cell of internal resistance.

Good point Mark, a 12 volt 600Ahr battery simply does not fit in the engine bay of a car, even if the engine were out :)

Dave, the correct term would be 600CCA, where CCA is Cold Cranking Amps.

My use of 400Ahr is actually what I mean, a 400AHr lithium cell pack at 24Volts, what I am about to buy for my off-grid place.


Paul

I stand corrected. it would be CCA, not A/hr.

but to go with the old concept of ignorance gets you into trouble..... and pride keeps you there. : )

I have found quite a few sites in a very brief search, that talks about car batteries and amp hours. but, going with #1 above and not #2, I am not going to try to use Instructables as a reference.