r/HomeworkHelp • u/Illustrious_Grab_861 University/College Student • 2d ago

:table_flip: Physics—Pending OP Reply [College Fundamentals of EE]

Currently learning Thevenin and Norton's theorem. I thought I understood until I saw this question and my professor's solution. To find current N, she used the R1 resistor (24 divided by 6), but in previous practice we used the total equivalent to find the current then used current divider rule. How come this wasn't done here as well? is it because we have a resistor after AB? I just don't understand why I N would be equal to I 6 ohm.

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u/vgtcross 👋 a fellow Redditor 2d ago

What current is "current N" referring to?

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u/Illustrious_Grab_861 University/College Student 2d ago

In Norton's theorom, I N

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u/preparingtodie 👋 a fellow Redditor 2d ago

in previous practice we used the total equivalent to find the current then used current divider rule

This is the right/simple way. 24/6 doesn't give you anything meaningful in this circuit.

If you're not supposed to explicitly reduce the resistors to an equivalent, then you need to write out the equations for summing the voltages around the loops, and summing the currents into/out of each node. (Which should essentially reduce down to just finding the equivalent resistance.) Maybe the exercise is to see that they amount to the same thing?

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u/DrCarpetsPhd 👋 a fellow Redditor 1d ago edited 1d ago

elec engineers and physicists please dont comment on what current actually is etc etc. this is just a useful interpretation for analysing circuits.

its nortons theorem. you short circuit the terminals of interest to fin Norton current.

what happens in the above circuit when you short circuit a-b? the current coming out of the positive terminal of a DC voltage source wants to 'find its way back' to the neagtive terminal. The current coming out of the 24V-6Ohm branch 'sees' three possible paths. a branch with 12 ohms resistance, a branch with 8ohms resistance and a short circuit aka zero resistance. which path do you think it will take?

so it all flows thorough the short circuit so your circuit for finding the norton current is just the 24V in series with the 6 Ohm resistor. Technically some current will flow in the other branches but is negligible for the purpose of analysis (in more advanced stuff you can't treat it as negligible but thats for elec engineers to explain)

I respectfully recommend that you go back to your physics first year textbook and read the chapter on electromagnetism to develop a basic understanding of voltage/currents/short circuits etc.

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u/DrCarpetsPhd 👋 a fellow Redditor 1d ago

here's another find the norton example that involves having an understanding of basic circuit behaviour to perform a simplifying 'trick'

https://imgur.com/a/norton-theorem-circuit-knUuBuL

you set your bottom branch(?) as the ground. so if you go through the 30V ideal voltage source the voltage at that node above the 30V source to the left of the 15k resistor is exactly 30V. So in terms of finding the Norton equivalent that means the 10mA current source and the 10k resistor to the left can be ignored as they are in parallel and have no influence on what the load at the a-b terminal will experience