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# Combined Series-Parallel Circuits

## Representing most real world circuits, these circuits are connected in series as well as in parallel.

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Lights in Parallel

### Lights in Parallel

Credit: Laura Guerin
Source: CK-12 Foundation
License: CC BY-NC 3.0

When a few batteries are hooked in parallel with light bulbs, they light up. If you continue to add more light bulbs in parallel, do the lights dim or do some of the light bulbs go out?

#### News You Can Use

• Parallel circuits are circuits in which the current in the system has more than one path for the current to flow though. In a parallel circuit, any resistors that are added in parallel decrease the total resistance of the circuit according to the following rule:

\begin{align*}\frac{1}{R_{eq}}=\sum\limits_{i} \frac{1}{R_i}= \frac{1}{R_1}+ \frac{1}{R_2}+ \frac{1}{R_3}+ \cdots\end{align*}

• When the current flows through the three light bulbs in the image above, the current through each light bulb is different, but the potential drop across each path must be the same. Because of this, you are able to remove one of the light bulbs completely from the system without the other light bulbs being affected.
• Watch a demonstration of light bulbs in parallel being lit up:

#### Explore More

Using the information provided above, answer the following questions.

1. As more light bulbs are added in parallel, what happens to the overall brightness of the light bulbs?
2. If more light bulbs are added in parallel to the image above, what should happen to the brightness of the indicator bulb?
3. If one of the light bulbs in the circuit burn out, what happens to the other light bulbs? Why?

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