Series Circuits - Examples

Series Circuits

































Your Christmas tree lights are connected in series. Suddenly, all the lights go out! Probably only one bulb has blown, but you have to test each one in turn to find it. If, by chance, two bulbs went bad, then you may never figure it out!

Lightbulbs and many other devices break the current path when they fail. Hooking them up in series is a bad idea.


Sometimes a failure produces a low resistance path for the current --- a short circuit. For example, electric tea kettles often fail because the cord is bent back and forth until the insulation wears out and the two wires in the cord make direct contact.

Devices which fail only in this way should be connected in series. However such devices are not common.

Examples --- What this is an example of




































Four identical light bulbs are connected in series to a 120V supply. What is the potential difference or voltage drop across each bulb?

In this case all of the voltage drops are the same and they must add up to 120V.

4V = 120V or V = 30V.

Two light bulbs are connected in series to a 120V supply. The voltage drop across one is 100V. What is the voltage drop across the other?

Since they must add up to 120V, the other voltage drop must be 120V - 100V = 20V.

Examples --- What this is an example of




































A light bulb with a resistance of 3 is connected in series with a light bulb whose resistance is 2 . What is the total resistance of this combination?

Just add the resistances:

R = 3 + 2 = 5

Examples --- What this is an example of




































Suppose that three 40 light bulbs are connected to a 120V supply. How much current will flow through each bulb.

The same current flows through all and is given by

I = V/R = (120V)/( 40 + 40 + 40 )
= 120V/ 120 = 1A

Examples --- What this is an example of