Section 1 of 5Resistors in series
Layout
- Layout Components are connected end to end in a single loop, so there is only one path for the charge.
Current
- Current The same current passes through every resistor in the loop.
Potential difference
- Potential difference The supply p.d. is shared between the resistors, so the individual p.d.s add up to the total: Vtotal = V1 + V2 + …
Total resistance
- Total resistance Rtotal = R1 + R2 + …
- Adding another resistor in series always increases the total resistance, because charge has a longer obstructed path.
Section 2 of 5Resistors in parallel
Branches joined across the same two points
Potential difference
- Potential difference Each branch has the same p.d. across it, equal to the supply p.d.
Current
- Current The current from the supply splits between the branches and recombines afterwards, so the branch currents add up to the total.
Total resistance
- Total resistance The total resistance is always smaller than the smallest branch resistance.
- Adding a branch gives charge another route, so the total resistance falls.
Higher tier (HT only)
- Higher tier (HT only) For two or more branches, 1 ÷ Rtotal = 1 ÷ R1 + 1 ÷ R2 + … ; for two resistors this equals the product divided by the sum.What the symbols mean: RT = total resistance of resistors in parallel (Ω), always smaller than the smallest one · R1, R2 = the separate resistances (Ω)
Section 3 of 5Working out a series problem
1
Add the individual resistances to find the total, for example 4 Ω + 6 Ω = 10 Ω.
2
Use I = V ÷ R with the supply p.d. and the total resistance; for a 20 V supply this gives 2 A.
3
Because the current is the same everywhere, use V = I × R on each resistor: 8 V across the 4 Ω and 12 V across the 6 Ω.
4
Check that the p.d.s add to the supply: 8 V + 12 V = 20 V.
Section 4 of 5Key terms
| Term | Meaning |
|---|---|
| Series circuit | A circuit with a single loop and one path for current. |
| Parallel circuit | A circuit with branches that provide more than one path for current. |
| Total resistance | The single resistance that would have the same effect as all the resistors in the circuit. |
| Potential difference | The energy transferred per coulomb of charge, in volts. |
| Branch | One of the separate routes between two junctions in a parallel circuit. |
Section 5 of 5Exam technique
1
Adding resistances to get the total for a parallel circuit — the total is smaller than any branch.
2
Thinking the current is used up as it passes round a series circuit — it is the same everywhere.
3
Assuming the p.d. is shared between parallel branches; each branch gets the full supply p.d.
4
Forgetting to convert units, for example kΩ to Ω, before using V = I × R.