An 8 Ω resistor and a 12 Ω resistor in series behave like one 20 Ω resistor. You can use that rule already. But why do the resistances add, rather than average, or anything else?
A series circuit has only one path. Every bit of charge that goes around must pass through the 8 Ω resistor and then through the 12 Ω resistor.
The 8 Ω resistor opposes the current. Then the 12 Ω resistor opposes it again. The charge cannot dodge either one: the two oppositions stack along the single path.
Stacked oppositions make the total: 8 Ω + 12 Ω = 20 Ω.
Add a second resistor after a first, and the same current has two oppositions to pass through instead of one.
Two resistors sit in series. Why do their resistances add?
Mia says the current only feels the first resistor it reaches, so the total resistance equals the first resistor's. Is Mia correct?
A third resistor is wired into a series loop that already holds two. Why does the total resistance rise?
Write down the fact:
a series circuit has only one path
Apply it:
The current must now pass through three oppositions in turn instead of two.
Each opposition adds to the total.
The loop still has one path, so the current must pass through the new opposition too. The total resistance rises.
A 5 Ω resistor in a series loop is swapped for a 25 Ω resistor. What happens to the loop's total resistance?
A series loop holds a bulb and a resistor. Which statement explains why their resistances add to give the total?
Circuit X holds one 40 Ω resistor. Circuit Y holds a 40 Ω resistor and a 15 Ω resistor in series. Which circuit has the bigger total resistance?
Which feature of a series circuit makes its resistances add?