Series Circuits have multiple resistors wired in a single path.
To solve a series circuit, we can think of an equivalent simple circuit with an equivalent resistance, REQ.
Add all of the resistance values together that are in series, and that is equivalent to a simple circuit with just one resistor with resistance REQ.
For example, let’s say we have two resistors wired in series,
with R1 = 5 Ω and R2 = 20 Ω.
The equivalent resistance of the circuit is 25 &Omega. You can use this value to calculate the current in the circuit if you know the voltage. (Use Ohm’s Law)
Parallel Circuits have multiple paths, and each path contains one resistor.
Having multiple paths actually makes it easier for electricity to flow through a circuit, so the equivalent resistance of this type of circuit is less than it would be with any one of the individual resistors in a simple circuit.
For example, let’s say we have two resistors wired in parallel,
with R1 = 5 Ω and R2 = 20 Ω.
In this case:
1/REQ = 1/5 + 1/20 = 0.25
REQ = 4.0 Ω
Notice that you have to take the reciprocal of both sides of the equation after you add up the fractions to get the correct answer.
Also notice that 4.0 Ω is less than the resistance of either of the other resistors.
Even though the same resistors were used in the series and parallel examples, the overall resistance is different. That also means that the overall current flow in the circuit would also be different.