Understanding Junction Rule: Slide 10 Explanation

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http://online.physics.uiuc.edu/courses/phys102/fall06/print_files/lect06_handout.ppt#10

Slide 10


How come the sign for R3 is not the opposite, shouldn't the postive charge be carried from E2 up to R3 making it + then -

 

[Astronuc]

The way the currents are written, I₁ + I₂ = I₃. If E₂ is sufficiently strong, the current I₃ may be negative, indicating it is in the opposite direction as that which is written.

If anything the current I₂ should have been written in the opposite direction and it's polarity reversed, such that I₁ = I₂ + I₃. By convention, currents are usually written in the clockwise direction of a loop. The + sign simply indicates a higher potential.