Formula for Parallel Resistors

The result for the effective resistance for a parallel connection is different, but we can start from the same two places:

(Think of water flowing over two waterfalls in parallel.) V=I*R, or R = rL/A .

For parallel, both resistors are across the same voltage, so Vtotal = V1 = V2 . The current can go through either resistor, so:

Itotal = I1 + I2 . Since the I’s are in the denominator, we have:

1/Reff = 1/R1 + 1/R2.

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