Kirchhoff loop rule

  • Thread starter PainterGuy
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  • #1
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hi everyone

this is about the example 26.4 in attached pdf. i'm having problem in conceptual understanding of kirchhoff's loop rule. i'm total beginner in physics. in the example 12v battery is used to feed the entire circuit, primarily to recharge the run-down battery. please see the highlighted part. the equation could be written as: 12v = (3A)r + 2A(3 ohm). this is the equation for loop labelled 1 in the diagram. this equation is saying that all the emf of 12v is being used in loop no. 1. r is 2 ohm - which means 12v = 3x2 + 2x3 = 12v. but "in reality" this is not the case because some of volts out of total emf of 12 are being used to push the electrons (or current) from point "b" to point "a". what i'm saying is that not all the volts are being used to traverse the loop labelled 1 (as the equation assumes). if this really was the case then there won't be any volts left to push the electrons through run-down battery. please help me to grasp the conceptual understanding of this aspect of kirchhoff loop rule. any help would be welcome.

edit: i just uploaded the pdf on googledoc:---
https://docs.google.com/viewer?a=v&...EyNjQtOGZkZjFmN2U0OWIy&hl=en&authkey=CPPhp64O
 

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  • #2
tiny-tim
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hi painterguy! :smile:
… but "in reality" this is not the case because some of volts out of total emf of 12 are being used to push the electrons (or current) from point "b" to point "a".

what i'm saying is that not all the volts are being used to traverse the loop labelled 1 (as the equation assumes). if this really was the case then there won't be any volts left to push the electrons through run-down battery.
no, you're fundamentally misunderstanding what https://www.physicsforums.com/library.php?do=view_item&itemid=301" is …

voltage is a difference, the difference in electric potential between two points

the difference in potential between points a and b in the diagram is the same whether you go straight across or via the 3Ω

current splits up at a junction (there's only so much of it), but potential doesn't

imagine tilting the page, left-side-up …

instead of wires there are water-pipes, and the 12 V battery is now a water pump pumping upwards …

the gravitational potential difference down the middle section is the same as down the 3Ω section …

the 12 V pump gets the water up to the top level (the LHS in the diagram), but once it's there, the gravity makes it go down (through either path), not the pump :wink:
edit: i just uploaded the pdf on googledoc:---
ooh, i've not seen that before … that's cool! :approve:
 
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  • #3
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The whole idea is that electric fields are conservative, the integral of E*dl (same as potential) is zero around any closed path.
 
  • #4
can u help me in my project in physics about kirchoff loop rule and junction rule and can you provide me also with some examples tnx
 
  • #5
tiny-tim
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welcome to pf!

hi cr0wncl0wn25! welcome to pf! :wink:

please start a new thread in the physics homework forum

as always, it helps if you tell us how far you've got, and give a specific problem (about Kirchhoff's rules) that you're having trouble with :smile:
 

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