Explain the advantages of connecting lamps in parallel in a lighting circuit

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – Series and Parallel Circuits

4.3.2 Series and Parallel Circuits

Objective

Explain the advantages of connecting lamps in parallel in a lighting circuit.

Key Concepts

  • In a parallel circuit each component is connected across the same two points of the circuit.

  • The voltage across each parallel branch is the same and equal to the source voltage.

  • The total current supplied by the source is the sum of the currents through each branch:

    \$I{\text{total}} = \sum{k=1}^{n} I_k\$

  • The equivalent resistance of \$n\$ parallel resistors is given by:

    \$\frac{1}{R{\text{eq}}}= \sum{k=1}^{n}\frac{1}{R_k}\$

Why Parallel Wiring Is Preferred for Lighting

  1. Uniform Brightness

    Each lamp receives the full supply voltage, so all lamps glow with the same intensity. In a series circuit the voltage is divided, causing dimmer lamps further from the source.

  2. Independent Operation

    If one lamp fails (opens), the other lamps continue to operate because each has its own branch. In series, a single failure interrupts the whole circuit.

  3. Control Flexibility

    Switches or dimmers can be placed on individual branches without affecting other lamps.

  4. Safety and Energy Efficiency

    Lower equivalent resistance reduces the overall current drawn for a given power requirement, minimising heating in conductors.

Comparison: Series vs Parallel Lighting Circuits

FeatureSeries ConnectionParallel Connection
Voltage across each lampDivides among lamps (\$V{\text{lamp}} = V{\text{total}}/n\$)Same as source voltage (\$V{\text{lamp}} = V{\text{source}}\$)
Current through each lampSame current flows through all lampsCurrent varies with lamp resistance; total current is sum of branch currents
Effect of a lamp failureAll lamps go out (open circuit)Only the failed lamp goes out; others stay lit
Brightness uniformityOften uneven; lamps farther from source may be dimmerUniform brightness for all lamps
Equivalent resistance\$R_{\text{eq}} = nR\$ (higher)\$\displaystyle\frac{1}{R{\text{eq}}}= \sum\frac{1}{Rk}\$ (lower)

Practical Example

Consider three identical 60 W, 240 V lamps.

Each lamp has a resistance \$R = \dfrac{V^2}{P} = \dfrac{240^2}{60} = 960\ \Omega\$.

Series connection:

  • Total resistance \$R_{\text{eq}} = 3 \times 960 = 2880\ \Omega\$.

  • Total current \$I = \dfrac{V}{R_{\text{eq}}}= \dfrac{240}{2880}=0.083\ \text{A}\$.

  • Power per lamp \$P_{\text{lamp}} = I^2 R = (0.083)^2 \times 960 \approx 6.6\ \text{W}\$ (very dim).

Parallel connection:

  • \$\displaystyle\frac{1}{R{\text{eq}}}= \frac{1}{960}+\frac{1}{960}+\frac{1}{960}= \frac{3}{960}\$ → \$R{\text{eq}} = 320\ \Omega\$.

  • Total current \$I = \dfrac{240}{320}=0.75\ \text{A}\$.

  • Current per lamp \$I_{\text{lamp}} = \dfrac{240}{960}=0.25\ \text{A}\$.

  • Power per lamp \$P{\text{lamp}} = V I{\text{lamp}} = 240 \times 0.25 = 60\ \text{W}\$ (full brightness).

Suggested diagram: Schematic showing three identical lamps wired in series on the left and in parallel on the right, with the same voltage source.

Summary

  • Parallel wiring ensures each lamp receives the full supply voltage, giving uniform brightness.

  • A fault in one lamp does not affect the others, enhancing reliability.

  • Parallel circuits allow independent control of individual lamps.

  • The lower equivalent resistance of a parallel arrangement reduces overall current draw for the same total power, improving safety.