Ohm's law is an important mathematical formula that electricians and physicists use to determine certain measurements in a given circuit. The formula is V = I x R where V is the voltage, measured in volts, I is the amount of current measured in amps or amperage and R is the resistance, measured in ohms. Resistors impede the electron flow within a circuit and, depending on their material, offer more resistance than others. The voltage in a circuit is nothing more than "a source of electric potential," within that circuit.
Circuit in Series
Determine the total amperage in the circuit. If you had a circuit and you found that it carried a total current of 6 amps, you should use this as the amperage in the circuit. Remember that in a circuit the total amperage is everywhere equal.
Determine the total number of resistance in the circuit. You measure resistance in ohms, which is expressed using the Greek letter omega. If you measure that there is a resistor with 3 ohms of resistance in this circuit and another with 2 ohms of resistance, that means that the circuit has a total resistance of 5 ohms.
Find voltage output by multiplying the amperage by the total number of resistance in the circuit. In the examples above, we know that the amperage is 6 amps and the total resistance is 5 ohms. Therefore, the voltage output for this circuit is 6 amps x 5 ohms = 30 volts.
Circuits in Parallel
Determine the total current in the circuit. Just as it is in a series circuit, the current or amperage is everywhere the same. Using the same example, we'll say that the total amperage is 6 amps.
Find the total resistance in the circuit. The total resistance in a parallel circuit differs from a series circuit. In the series circuit, we obtain the total resistance by simply adding each individual resistance in the circuit; however, in a parallel circuit, we need to find the total resistance by using the formula: 1/ 1/R1 + 1/R2 +...+1/Rn. That is, one divided by the sum of the reciprocals of all the resistors in the parallel circuit. Using the same example we will say that the resistors have a resistance of 2 ohms and 3 ohms. Therefore the total resistance in this parallel series is 1/ 1/2 + 1/3 = 1.2 ohms.
Find the voltage the same way you found the voltage in the series circuit. We know that the total amperage for the circuit is 6 amps and the total resistance is 1.2 ohms. Therefore, the total voltage output for this parallel circuit is 6 amps x 1.2 ohms = 7.2 volts.
TL;DR (Too Long; Didn't Read)
If you're using a calculator to find the total resistance in a parallel circuit, don't forget to put parentheses around the bottom fraction. For example when you calculated the total resistance in a parallel circuit you got 1/ 5/6. In a calculator this is different than 1/ (5/6).