Resistors in series
When resistors are in series they are arranged in a chain, so the current has only one path to take and is therefore the same through each resistor.
The sum of the potential differences across each resistor equals the total potential difference across the whole chain.
The equivalent resistance of resistors in series is:
Req = R1 + R2 + R3 + ...
Resistors in parallel
When resistors are arranged in parallel, the current has multiple paths to take. The potential difference across each resistor is the same, and the currents add to equal the total current entering (and leaving) the parallel combination.
I = I1 + I2.
The equivalent resistance of resistors in parallel is:
| 1
|  |
| Req
|
|
= |
| 1
| |
| R1
|
|
+ |
| 1
| |
| R2
|
|
+ |
| 1
| |
| R3
|
|
+ ... |