Branches: the same p.d. across each, currents that add at a junction, and a total resistance smaller than any branch.
What you will learn
- State the rules for current, p.d. and resistance in parallel
- Calculate branch currents and the total current
- Explain why adding a branch lowers the total resistance
The three parallel rules
In a parallel circuit each component has its own branch, connected straight across the supply.
- Potential difference: each branch has the full supply p.d.
- Current: the branch currents add up to the total: A = A₁ + A₂.
- Resistance: the total is less than the smallest branch resistance.
- 6.0 V supply, R₁ = 3 Ω, R₂ = 6 Ω in parallel
- Each branch has 6.0 V across it
- A₁: I = 6.0 ÷ 3 = 2.0 A
- A₂: I = 6.0 ÷ 6 = 1.0 A
- A: 2.0 + 1.0 = 3.0 A
At a junction the current splits, and the branch currents add up to the current in the main circuit. To find a missing branch current, subtract: 0.50 A in the main circuit and 0.20 A in one branch leaves 0.50 − 0.20 = 0.30 A in the other. Identical lamps in parallel each take the same current, so four lamps take four times the current of one.
The rest of this lesson and its 7 practice questions are locked.
In this lesson
- The three parallel rules
- Why the total resistance falls
- Series or parallel for lights
- Try it: Series against parallel in the calculator
- Try it: The same p.d. on every branch
7 practice questions, marked as you go; about 20 minutes in all.
Key terms
Full, Add up, Less than the smallest
In this unit: Electric circuits
Circuit symbols, charge and current, V = IR, I-V graphs, series and parallel circuits, and sensing circuits with LDRs and thermistors.