Not given in the exam, recall it.
What it is for
Resistors in series is used in Electricity (SPM Physics Form 5). Keep the SI units in every line of working. Electricity
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| R | total resistance | Ω |
| R₁ | first resistance | Ω |
| R₂ | second resistance | Ω |
Rearrangements
R₁ = R − R₂R₂ = R − R₁
Worked example
Common trap
Understanding resistors in series
The equation R = R₁ + R₂ + … gives the total (effective) resistance of resistors joined end to end. Here R is the total resistance, and R₁, R₂ and any others are the individual resistances, all in ohms (Ω).
In series there is only one path for the current, so the same current flows through every resistor. Each resistor uses part of the supply voltage, and these add up, so the resistances simply add.
The total is always larger than any single resistor.
- To find one resistor: R₁ = R − R₂
- Or: R₂ = R − R₁
Add all the series resistances first, keeping every value in ohms before you combine them.
A worked example, step by step
Three resistors R₁ = 2 Ω, R₂ = 4 Ω and R₃ = 6 Ω are connected in series. Calculate the total resistance.
For a series arrangement, add the resistances:
- R = R₁ + R₂ + R₃
- R = 2 Ω + 4 Ω + 6 Ω
- R = 12 Ω
The total resistance is 12 Ω. As expected, it is larger than the biggest single resistor (6 Ω).
If this string were connected to a 24 V supply, the current would be I = V / R = 24 V / 12 Ω = 2.0 A, the same through all three resistors because there is only one path.
Common mistakes and how this is tested
The commonest error is treating a series total like a parallel one, do not take reciprocals here. In series you simply add.
- Assuming the voltage is the same across each resistor; in series it is the current that is shared equally, while the voltages add.
- Mixing units, for example adding a value in kΩ to one in Ω without converting.
- Overlooking a resistor drawn further along the same loop.
This is tested under calculate (find the total, or a missing resistor) and explain (why current is the same throughout). In circuit questions, first identify whether each part is in series or in parallel before combining.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)