Series and parallel circuits · Grade 8
In a series circuit everything is in one loop, so the same current flows everywhere and the supply’s p.d. is shared. In a parallel circuit there are branches: each branch gets the full p.d., and the branch currents add up to the total. Homes are wired in parallel so each appliance works on its own.
Key facts
- Series: same current everywhere; p.d.s add up to the supply; R_total = R₁ + R₂ + …
- Parallel: each branch has the full supply p.d.; currents add: I = I₁ + I₂ + …
- Parallel resistors: 1/R = 1/(R₁) + 1/(R₂) + … (for two: product ÷ sum). The total is less than the smallest.
- In series, the bigger resistor gets the bigger share of the p.d.
- If one bulb breaks in series, all go out; in parallel the others stay on.
- Combined networks: replace each parallel group with its single equivalent, then add the series parts.
Worked examples
Example 1
The ammeters in the branches read 0.4 A and 0.7 A. What does the ammeter by the battery read?
Answer: 1.1 A
Example 2
Find the total resistance and the battery current for this circuit.
Answer: 7 Ω; about 1.71 A