# Cheat Sheet : Ohm’s law, Power law, Series and Parallel Circuits

##### Power law and Ohm’s law:

You can find below a table with every formula for the Power law and Ohm’s law. This table will be useful to solve problems and to understand examples.

##### Series Circuits:

-In series circuit, the voltage is divide between all the resistors. To calculate the voltage across a resistor in series: $Vx=Vin*\cfrac{R_x}{R_x+R_{n1}+R_{n2}+\dots}$

where Vx is the voltage across the resistor Rx and V is the voltage across all resistors in series.

-If you need to simplify a series circuit, you will need to calculate the equivalent resistor and current: $Req=R_1+R_2+R_n+\dots$ $I_{tot}=\cfrac{V_{source}}{Req}$

##### Parallel Circuits:

-The current in parallel circuit is divide between the resistors. Use Ohm’s law to calculate the current going through the resistor. The total current in the circuit is going to be equal to the sum of every currents flowing through the resistors. $I_{tot}=I_1+I_2+I_n+\dots$

where I1, I2 and In are the current flowing through the resistors R1, R2 and Rn.

-If you need to simplify a parallel circuit, you will need to calculate the equivalent resistor and current: $Req=\cfrac{1}{\cfrac{1}{R_1}+\cfrac{1}{R_2}+\cfrac{1}{R_n}+\dots}$ $I_{tot}=\cfrac{V_{source}}{Req}$