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.

 

[kyle8921]

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The sign for R3 in this circuit is determined by the direction of the current flowing through it. Since the current is flowing from the positive terminal of the battery, through R1 and R2, and then through R3 back to the negative terminal, the current direction through R3 is from + to -. This means that R3 is experiencing a voltage drop, which is why it is labeled as - in the junction rule. The positive charge is indeed carried from E2 to R3, but the direction of the current flow determines the voltage drop and therefore the sign of R3 in the junction rule.