# Parallel Circuits: Definition & Concepts

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• 0:03 Circuits Are Pathways
• 1:03 Branched Pathways
• 2:09 Circuit Resistance
• 3:58 Ohm's Law
• 5:14 Lesson Summary

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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

People moving through an airport are similar to electrons moving through a circuit. Parallel circuits are one type of simple circuit, and they get their name from the multiple, parallel paths they provide for electron flow.

## Circuits Are Pathways

Going to the airport instills a bit of dread in people, and for good reason! There are long lines, multiple security checkpoints and endless hallways that seem to go on forever. Sometimes you move through the airport effortlessly, while other times it takes forever just to get from one point to another.

A pathway in an airport is much like a circuit, which is a path that electrons can flow through. Circuits are very useful - for example, they are the pathways that provide electricity to the appliances, lights and other things in your house.

Circuits come in two basic forms. The first is a series circuit, which connects devices in series. This type of circuit provides a single pathway for electron movement. The second type is a parallel circuit, which connects devices along branched pathways. This type of circuit provides separate paths for the electrons to flow. We will explore series circuits in detail in another lesson. For now, let's focus on how parallel circuits work to power devices.

## Branched Pathways

Parallel circuits get their name because the devices along the circuit are connected in parallel. This is like having multiple X-ray screening stations in the same airport terminal. The line initially begins as single-file, but then breaks into multiple, separate lines as you go through your X-ray machine of choice. Others behind you may choose to go through a different X-ray machine, and the number of lines that form depends on how many machines are open. After going through the X-ray machine, everyone merges back together into the same line and continues along the path to their gate.

Just as each X-ray machine line is independent of the others, the devices in a parallel circuit are also independent of each other. Think about it: if the next line over moves more slowly than yours, it has no effect on how quickly or slowly your lines moves. The same is true for parallel circuit branches. And because of this independence, while the total current in the circuit is divided among the parallel branches, the amount of current in each branch is specifically related to the amount of resistance in that branch.

## Circuit Resistance

The resistance comes from the device itself and is an opposition to electron movement through the device. Because resistance opposes the movement of current, the amount of current in each branch is inversely proportional to the resistance of that branch.

This makes sense if you think about it. Perhaps the line next to you is moving more slowly because the security officers are manually searching through each individual's carry-on luggage. This resistance slows the movement of the line, making it more difficult for people to move through the screening point and on to their next destination. The same is true for branches in a parallel circuit. The more resistance in an individual branch, the more opposition the current faces as it travels through.

But what's really interesting is that even though each branch is independent, the overall resistance of the circuit depends on the total number of branches present. In fact, as the number of branches is increased, the overall resistance of the circuit is decreased.

Let's go back to the airport to see why this is so. If there were only one X-ray machine open, there would be one long line and everyone would have to move through it. Overall, this would be very inefficient and provide a lot of resistance to the total movement of the people through the security station. But if another X-ray machine opens up, then the line can split into two, and everyone will move through much more quickly. Add yet another X-ray machine, and the 'resistance' decreases even more because people have yet another path they can move through.

The same is true for a parallel circuit. Adding branches is like opening up more X-ray stations in the circuit. The more branches there are, the more pathways for the current to travel through, and this decreases the overall circuit resistance.

## Ohm's Law

We can construct our own simple parallel circuit of two branches, each with a light bulb, connected to the same two points. The battery is also connected to these two points. If each branch is closed, meaning that there are no breaks in the circuit path, current will flow from the battery, along each branch, through each device and then back to the battery.

Similar to how a heart pumps blood through your body, the battery produces voltage that 'pumps' current through the circuit. Therefore, more voltage means more current. We also know that like a clogged artery, resistance opposes current, so more resistance means less current.

In fact, this relationship between voltage, current and resistance is summed up in Ohm's Law, which states that the current is directly proportional to the voltage across the circuit and inversely proportional to the resistance.

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