Resistance measures how much a component opposes the flow of current. When resistors are connected in parallel, each one is joined across the same two points, so each feels the full voltage and carries its own current.
Adding another parallel path is like opening an extra lane on a road: for the same push, more charge can flow in total. Since the total current is larger for the same voltage, the effective resistance of the combination must be smaller, in fact smaller than the least resistor on its own.
This is why home wiring uses parallel connections: every appliance gets the full mains voltage, and switching one off does not cut the others. The trade-off is that the total current drawn rises as you add more devices.
In SPM you should reason about parallel resistors through the extra current paths and the shared voltage, and know the combined resistance falls.
Common misconceptions
- Adding more resistors always increases total resistance -> True in series, but in parallel each extra path lowers the total resistance.
- Resistors in parallel share the voltage unequally -> Each parallel resistor has the same voltage across it; they share the current instead.
- The combined parallel resistance is the sum of the resistors -> It is less than the smallest single resistor, not the sum.
The physics behind it
For resistors in parallel, the reciprocal of the total resistance equals the sum of the reciprocals of the separate resistances: 1/R = 1/R₁ + 1/R₂ +..., with resistance measured in ohms (Ω). Each resistor is joined across the same two points, so each has the full voltage across it and carries its own current; the branch currents add up to a larger total current.
For two 6 Ω resistors in parallel, 1/R = 1/6 Ω⁻¹ + 1/6 Ω⁻¹ = 2/6 Ω⁻¹, so R = 6/2 Ω = 3 Ω. The combined resistance, 3 Ω, is smaller than either resistor alone.
This always happens: adding a parallel path can only increase the total current for a fixed voltage, and by R = V/I a larger current means a smaller resistance. In fact the total is always less than the smallest individual resistor.
For unequal values, say 2 Ω and 3 Ω, 1/R = 1/2 + 1/3 = 5/6 Ω⁻¹, giving R = 6/5 Ω = 1.2 Ω, again below the smaller value.
See it in daily life
Home wiring is the clearest example. Every socket and light in a house is wired in parallel across the same mains supply, so each appliance gets the full supply voltage and you can switch one off without cutting the rest.
Because each device you switch on opens another current path, the total resistance of the house falls and the total current drawn from the supply rises.
That is exactly why plugging too many high-power appliances into one circuit can blow a fuse or trip a circuit breaker: the combined resistance drops so low that the current climbs past the safe limit. The same logic explains why a single extension board running a kettle, a heater and an iron together draws a heavy current, while any one of them alone draws less.
Parallel wiring gives convenience and full voltage to every device, but the falling total resistance is the trade-off you must respect for safety.
How this comes up in SPM
This is part of the Form 5 Electricity chapter on series and parallel circuits. Paper 2 commonly asks you to Calculate the effective resistance of resistors in parallel using 1/R = 1/R₁ + 1/R₂, so keeping units and showing the reciprocal step clearly is important.
You may be asked to Compare the total resistance of the same resistors in series and in parallel, or to Explain why the parallel combination is smaller than the smallest resistor.
Circuit questions require you to Determine branch currents and the total current, often combining V = IR with the parallel rule. You might also Explain why household appliances are connected in parallel, linking full voltage and independent switching.
Neighbouring standards include potential difference, current, Ohm's law and series circuits, where resistances simply add. Command words such as Calculate, Compare, Determine and Explain recur.
A frequent slip is adding parallel resistances directly like series ones, so always use the reciprocal relationship.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)