# Assigning voltage polarity over a resistor

#### fdsa

Joined Aug 16, 2011
9
When I have a circuit like the one below how do I go about assigning the polarities of the voltage over R & L? I know that the equation for this is $$Ri(t)+L\times \frac{di}{dt}=V$$ (i(t) clockwise) but not shure i understand why. What if i reversed the polarity of V? or assume that we replaced L with a voltage source for simplicity how would the voltages of the two sources impact the voltage and polarity over the resistor?

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#### wmodavis

Joined Oct 23, 2010
739
Remember that the sum of the voltage drops around a loop MUST equal zero.

So the polarity of the voltage drops of the components must sum to be equal and opposing polarity to voltage source.