When you disconnect a resistor from a parallel circuit, the total current falls if the supply voltage is fixed. Whether the current in the remaining branch changes depends on what else is in the circuit. Start by finding the voltage across that branch before and after the change.
The SEC 2027 G3 Physics syllabus (K323) includes current, voltage and effective resistance in series and parallel circuits. The examples below use ideal fixed-voltage supplies and resistors with constant resistance.
- Identify which components are connected across the same two points.
- Calculate total current separately from branch current.
- Look for a resistor shared by both branches.
Worked example | Two resistors directly across a battery
A 6 Ω resistor and a 3 Ω resistor are connected in parallel across a 12 V supply. Each branch has a potential difference of 12 V. The 6 Ω branch carries 12 ÷ 6 = 2 A and the 3 Ω branch carries 12 ÷ 3 = 4 A. The source supplies 6 A in total.
Disconnect the 3 Ω branch without altering the rest of the circuit. The 6 Ω resistor still has 12 V across it and carries 2 A. The source now supplies 2 A. The effective resistance increases from 12 ÷ 6 = 2 Ω to 6 Ω.
The branch that remains has the same voltage and resistance as before. Its current stays the same under the conditions given.
Add a resistor before the branches
Now put a 2 Ω resistor in series with the original parallel pair. The pair has effective resistance 2 Ω, so the circuit has total resistance 4 Ω and draws 3 A. The shared resistor drops 3 × 2 = 6 V, leaving 6 V across the pair. The 6 Ω branch carries 1 A.
After removing the 3 Ω branch, the circuit has 2 Ω + 6 Ω = 8 Ω and draws 1.5 A. The shared resistor now drops 3 V, leaving 9 V across the remaining branch. The 6 Ω branch carries 1.5 A. In this circuit, its current rises even as the total current falls.
Check which voltage stays fixed
In both examples the source remains at 12 V. In the second, part of that voltage is across the shared resistor, and its share depends on the total current. Draw the circuit before choosing a value to substitute in V = IR.
Try removing the 6 Ω branch instead from the second setup. The remaining circuit has 5 Ω in total and draws 2.4 A. The 3 Ω resistor has 7.2 V across it. Check that the shared resistor’s 4.8 V drop and the 7.2 V drop add to 12 V.
Frequently asked questions
Are currents always equal in parallel branches?
They are equal when the branch resistances are equal and the potential differences across them are the same.
Why does the remaining branch have less than 12 V in the second example?
The shared series resistor takes part of the supply voltage.
Official information
SEAB’s 2027 SEC G3 Physics syllabus (K323), D.C. Circuits. Checked 9 November 2026.