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Series Circuits

A closed circuit in which the current follows through one path.

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Lights in Series

Lights in Series

Credit: Laura Guerin
Source: CK-12 Foundation

When three light bulbs hooked in series are attached to a battery, they light up. If you continue to add more light bulbs in series, do the lights dim, get brighter or do some of the light bulbs go out?

News You Can Use

• A series circuit is a circuit in which the current in the system has only one path for the current to flow though. For a series circuit, any resistors that are added along the path changes the total resistance of the circuit according to the following rule:

\begin{align*}R_{eq}=\sum\limits_{i}R_i=R_1+R_2+R_3+ \cdots\end{align*}

• When the current flows through the 3 light bulbs in the image above, the current through each light bulb is the same but the voltage across each bulb is different. According to Kirchoff's rule which states that the sum of the voltage drops in a closed loop must be equal to 0, the above situation can be written as

\begin{align*}V_{battery}-(IR_1)_{bulb \ \# 1}-(IR_1)_{bulb \ \# 2}-(IR_1)_{bulb \ \# 3}=0\end{align*}

• (If you factor out \begin{align*}I\end{align*}, from each of the terms in parenthesis and divide by the current \begin{align*}I\end{align*}, you get the first equation above.)
• Watch a demonstration of light bulbs in series being lit up:

Explore More

Using the information provided above, answer the following questions.

1. As more light bulbs are added in series, what happens to the overall brightness of the light bulbs? Why?
2. What is Kirchoff's loop rule say for a system that only has a battery and 1 light bulb attached?
3. If one of the light bulbs in the series burns out, what happens to the other light bulbs? Why?

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