Know that the current at every point in a series circuit is the same

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – Series and Parallel Circuits

4.3.2 Series and Parallel Circuits

Learning Objective

Know that the current at every point in a series circuit is the same.

Key Concepts

  • A series circuit is a single path for charge flow. All components are connected end‑to‑end.

  • In a series circuit the same current flows through each component because there is only one continuous path.

  • The total resistance of a series circuit is the sum of the individual resistances:

    \$R{\text{total}} = R1 + R2 + R3 + \dots\$

  • The voltage supplied by the source is divided among the components according to Ohm’s law:

    \$V{\text{total}} = V1 + V2 + V3 + \dots\$

  • Ohm’s law for each component:

    \$Vn = I \, Rn\$

    where the current \$I\$ is the same for all \$n\$.

Why the Current Is the Same

Electric charge cannot accumulate at any point in a steady‑state circuit. If more charge entered a junction than left, charge would build up, creating an increasing electric field that would oppose further flow. In a series circuit there are no junctions; therefore the same amount of charge that leaves the source must pass through each component per unit time, giving a uniform current \$I\$ throughout.

Comparison: Series vs Parallel

FeatureSeries CircuitParallel Circuit
Path for currentSingle continuous pathMultiple independent paths
Current through each componentSame everywhere: \$I{\text{total}} = I1 = I_2 = \dots\$Different in each branch: \$I{\text{total}} = I1 + I_2 + \dots\$
Voltage across each componentDivides: \$V{\text{total}} = V1 + V_2 + \dots\$Same across each branch: \$V{\text{total}} = V1 = V_2 = \dots\$
Total resistance\$R{\text{total}} = R1 + R_2 + \dots\$ (adds)\$\frac{1}{R{\text{total}}}= \frac{1}{R1}+ \frac{1}{R_2}+ \dots\$ (decreases)

Worked Example

Problem: A 12 V battery is connected to three resistors in series: \$R1 = 2\;\Omega\$, \$R2 = 3\;\Omega\$, and \$R_3 = 5\;\Omega\$. Find the current flowing through the circuit and the voltage across each resistor.

  1. Calculate the total resistance:

    \$R_{\text{total}} = 2\;\Omega + 3\;\Omega + 5\;\Omega = 10\;\Omega\$

  2. Use Ohm’s law for the whole circuit to find the current:

    \$I = \frac{V{\text{total}}}{R{\text{total}}} = \frac{12\;\text{V}}{10\;\Omega} = 1.2\;\text{A}\$

  3. Since the current is the same through each resistor, find the voltage drop across each:

    • \$V1 = I R1 = 1.2\;\text{A} \times 2\;\Omega = 2.4\;\text{V}\$

    • \$V2 = I R2 = 1.2\;\text{A} \times 3\;\Omega = 3.6\;\text{V}\$

    • \$V3 = I R3 = 1.2\;\text{A} \times 5\;\Omega = 6.0\;\text{V}\$

  4. Check: \$V1 + V2 + V_3 = 2.4\;\text{V} + 3.6\;\text{V} + 6.0\;\text{V} = 12\;\text{V}\$, which matches the source voltage.

Common Misconceptions

  • “Current splits in a series circuit.” – In a series circuit there is no branching point; the current cannot split.

  • “All components have the same voltage in series.” – Only in parallel circuits is the voltage the same across each component.

  • “Adding more resistors in series reduces current.” – This is true, but the reason is the increase in total resistance, not a change in current distribution.

Quick Revision Checklist

  1. Identify whether a circuit is series or parallel.

  2. Remember: Series – same current, voltage divides.

  3. Calculate total resistance by summing individual resistances.

  4. Use \$I = V/R_{\text{total}}\$ to find the uniform current.

  5. Apply \$Vn = I Rn\$ to find voltage across each component.

Suggested diagram: A simple series circuit showing a battery connected to three resistors in a single line, with arrows indicating the direction of current flow.