### Author Topic: Kirchoffs law.  (Read 3068 times)

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#### Dave327

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##### Kirchoffs law.
« on: March 17, 2017, 11:06:12 pm »
I am new to electronics. In a class that using that law and for the life of me I can't understand it. Need help please

#### tpowell1830

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##### Re: Kirchoffs law.
« Reply #1 on: March 17, 2017, 11:23:11 pm »
I am new to electronics. In a class that using that law and for the life of me I can't understand it. Need help please

Hi Dave327, welcome to the forum.

I am afraid you will need to be more specific than that to get help. For instance,What part of Kirchoff do you not get?
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#### Ratch

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##### Re: Kirchoffs law.
« Reply #2 on: March 17, 2017, 11:30:32 pm »
I am new to electronics. In a class that using that law and for the life of me I can't understand it. Need help please

Kirchoff has a current law and a voltage law.  To which one are you referring?

Ratch
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#### Dave327

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##### Re: Kirchoffs law.
« Reply #3 on: March 17, 2017, 11:46:59 pm »
Working on some problems.  I know ohms law and watts law. I know what goes in come out to 0.  Can't figure out how to get amp to get the power drop for each resistors.  The problem shows 24v to r1 330ohm on first loop. 10v to r2 200 ohm on 2nd loop.  In middle r3 100 ohms. Need to know what volt and amps that are going thru each resistors. Like to know how to use the law to get amps and volt.

#### rx8pilot

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##### Re: Kirchoffs law.
« Reply #4 on: March 17, 2017, 11:49:22 pm »
I would suggest posting a diagram of some sort. Also, are you looking for an answer or an understanding?
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#### tpowell1830

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##### Re: Kirchoffs law.
« Reply #5 on: March 17, 2017, 11:52:00 pm »
Working on some problems.  I know ohms law and watts law. I know what goes in come out to 0.  Can't figure out how to get amp to get the power drop for each resistors.  The problem shows 24v to r1 330ohm on first loop. 10v to r2 200 ohm on 2nd loop.  In middle r3 100 ohms. Need to know what volt and amps that are going thru each resistors. Like to know how to use the law to get amps and volt.

Professor Lewin of MIT has a unique way of explaining it.

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#### rstofer

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##### Re: Kirchoffs law.
« Reply #6 on: March 17, 2017, 11:55:48 pm »
It's pretty easy!  Kirchoff's current law simply says that current can't pile up at a node and spill onto the floor.  The current going in is exactly equal to the current going out.  The voltage law law simply states that the voltage gains and losses around a loop equal zero.  Imagine a 9V battery feeding a resistor and an LED in a loop.  Put the battery on the left with the + at the top.  Start at the lower left corner, pass through the battery gaining 9V, pass through the resistor losing some voltage and then pass through the LED losing the rest of the voltage so that you are back where you started in the lower left corner.

To put numbers, assume the LED has a voltage drop of 2.2V at 20 mA and from this we see we need to drop 6.8V in the resistor.  6.8 / 0.02 = 3400 ohms.  So, starting again, we gain 9V at the battery we drop 6.8V at the resistor (20 mA * 3400 ohms) and then we drop the last 2.2V at the LED.

There are better explanations on Google.

#### helius

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##### Re: Kirchoffs law.
« Reply #7 on: March 17, 2017, 11:59:07 pm »
KVL is the Kirchhoff law for loops: it says that adding the voltage across each component of a loop sums to 0 V when you look all around the loop. All quantities it deals with are volts.
This is just formalizing the intuition that voltage is conserved around a circuit. If you end up in the same place you started, you would think that you would also be at the same place in the electric field.

KCL is the Kirchhoff law for branches: it says that the current entering a node is equal to the current leaving it (or equivalently, that adding the currents on each branch to a node sums to 0 A). All quantities it deals with are amps.
This is just formalizing the intuition that charge is conserved at each point in a circuit. We don't expect charges to build up anywhere, but simply to flow in an orderly way.

#### rstofer

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##### Re: Kirchoffs law.
« Reply #8 on: March 18, 2017, 04:10:02 am »
I have attached the wxMaxima code and results for both mesh and nodal analysis.  Mesh takes 2 equations and nodal takes 1.
Va is the node voltage where the 3 resistors join.  From that voltage you can get the current through R3 and compare it with the sum of i1 and i2 which are the mesh current.

You should be able to follow the mesh equations around a clockwise loop on the left and a counterclockwise loop on the right.

To verify the solution, take the node voltage and divide by R3 and compare to the sum of i1 and i2.  They should be equal.

I don't have a way to make sketches and I am not interested in doing the solutions by hand.   Hence the wxMaxima solution.

#### TimFox

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##### Re: Kirchoffs law.
« Reply #9 on: March 19, 2017, 08:12:05 pm »
Note one important detail about KVL:  the sum of emf's around the loop equals the emf due to time-changing magnetic flux linking the loop.  Absent such flux, the sum is zero.  Also note that this law does not assume a linear circuit (e.g., only resistors).  Furthermore, in a non-linear AC circuit, Kirchoff's laws are satisfied separately at each harmonic of the fundamental frequency.
« Last Edit: March 19, 2017, 08:14:06 pm by TimFox »

#### Howardlong

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##### Re: Kirchoffs law.
« Reply #10 on: March 19, 2017, 08:34:22 pm »
Also one bit of advice: try not to get too fixated by one particular facet of your course.

In the real world, Kirchhoffs' laws become common sense, and only very rarely does an engineer need to get pen and paper out to solve a Kirchhoff problem in the classical way. When I do, I don't even think Kirchoff, most real world examples tend to be quite simple and straight forward, not the comtrived examples all too often presented in class.

Not electronics related, but a few years ago while revising for an exam (meteorology for pilots FWIW) I found myself becoming fixated on two particular points, and spent days trying to understand. In the end I never understood those issues fully, but I still achieved 100% in the exam, the first time in my life that's ever happened.

#### Nerull

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##### Re: Kirchoffs law.
« Reply #11 on: March 19, 2017, 10:14:44 pm »
Someone struggling to use ohms law and Kirchhoffs' laws in a DC circuit really doesn't need to deal with magnetic flux analysis.

You have to walk before you can run, bombarding someone with information they won't understand doesn't do any good, as much as everyone likes showing off.

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#### Benta

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##### Re: Kirchoffs law.
« Reply #12 on: March 19, 2017, 10:36:14 pm »
^ +1

#### rrinker

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##### Re: Kirchoffs law.
« Reply #13 on: March 20, 2017, 12:35:32 am »
Dave has a good video on node and mesh analysis that may help as well. Forget exactly which one it is, but it was a ways back.

#### Brumby

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##### Re: Kirchoffs law.
« Reply #14 on: March 20, 2017, 04:18:28 am »
Not that far back...

#### CatalinaWOW

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##### Re: Kirchoffs law.
« Reply #15 on: March 20, 2017, 05:13:32 am »
Perhaps the OP is missing the point that each node and mesh is an opportunity to write an equation in some or all of the unknowns.  In order to solve for the unknowns you need to have as many independent equations as you have unknowns.

After writing the equations for each node you should have enough independent equations.  The same is true if you write each mesh equation, though it is sometimes easy to select meshes that are not actually independent. You can even mix node and mesh equations, but it is even trickier to assure independence.   Once you have them written standard methods for solution work.  Which you will use depends on what you have in your tool box.  You can use a calculator, excel, the method of determinants, MAPLE or one of the other symbolic algebra programs, MATLAB or even (heaven forbid), a pencil and paper.

Smf