Problem — Christmas lights in series

Problem

Christmas lights connected in series: if one bulb blows, all the others go out. Explain why. What happens if they are in parallel? Which scheme is used in houses and why?

Solution

Why in series they all go out. In a series connection there is a single path for the current: the charge must pass through one bulb after another. If one blows, its filament breaks and the circuit becomes open: the current can no longer flow anywhere in the chain, so all the bulbs go out at the same time. (Many modern Christmas light strings add a small internal shunt that short-circuits the blown bulb precisely to avoid this.)

In parallel. Each bulb is an independent branch between the same two supply conductors, and sees the same voltage. If one blows, its branch opens but the others remain closed: the other bulbs keep working normally.

Scheme used in houses. Domestic wiring uses the parallel scheme. The reasons:

• each appliance receives the same rated voltage ($230\ \text{V}$), independently of the others;
• you can switch on and off each appliance without affecting the others;
• the failure of one appliance does not cut off the whole house.

$\ev{\text{series: single path} \to \text{all off; parallel: independent branches} \to \text{houses use the parallel scheme}}$

Links

Topics: Circuiti elettrici Concepts: Resistenze in serie e parallelo