In a series circuit there is only one loop, so three rules apply. First, the current is the same at every point in the circuit. Second, the potential differences across the components add up to the potential difference of the supply. Third, the total resistance is the sum of the individual resistances: Rtotal = R1 + R2. These are combined with V = I × R, which can be used for the whole circuit or for a single component.
Worked example: an 18 V supply is connected across a 3 Ω resistor and a 6 Ω resistor in series. The total resistance is 3 + 6 = 9 Ω. The current is 18 ÷ 9 = 2 A, and it is the same through both resistors. The potential difference across the 3 Ω resistor is 2 × 3 = 6 V and across the 6 Ω resistor is 2 × 6 = 12 V. Check: 6 + 12 = 18 V, the supply potential difference.
A second example: a 12 V supply across a total resistance of 4 Ω gives a current of 12 ÷ 4 = 3 A. A 1 Ω resistor in that circuit then has 3 × 1 = 3 V across it, leaving 9 V for the rest of the circuit.