Why Electrical Circuits are Like the Checkout Line

Jul. 31, 2014


Circuits – Series vs. Parallel – an analogy

Key Terms:

Circuit: A complete loop that connects a positive end of a power source to the negative end.

Current: the flow of charge from the positive end to the negative end.

Series Circuit: When the current only has one pathway to get to the negative end.

Parallel Circuit: When the current has many paths to get to the negative end.Resistor: An electrical device that slows down the current.

Electrical circuits can be much like trying to get out of the checkout line at the supermarket. A small resistor is like going through the express lane with only one item. This person can check out a bunch of people very quickly.

Now, imagine you got behind this lady in the checkout line. It’s going to take a long time for the checkout person (our resistor) to get her out and on her way and everyone else is going to get backed up behind her. She creates a high resistance to the flow of traffic (our current). 

OK, now we have an idea of how resistance can affect the flow of traffic through the checkout line. This is very similar to the current through a circuit.  So what happens as we put a bunch of these low resistance checkout lines one after the other as we have in a series circuit? Let’s imagine that to buy our one pink bottle, we had to ask someone to get it, that slows us down a little. Then we have to go through a line to pay for it, slowing us down more. Then before we can leave the store, we need to have our receipt checked by someone, slowing us down even more.

This is a series circuit. We do not have a choice of how we get out of the store and each step slows us down more. That’s why the resistance increases in a series circuit as we add more resistors. Each step causes a back log.

Now let’s see how a parallel circuit might decrease the overall resistance.  In a parallel circuit, there is more than one pathway, or using our analogy, more than one checkout line. Now if we are trying to get out of the store, we can choose which line to get into, just like the current can choose which resistor to go through. Having this choice will help us get checked out faster, we will just go through the checkout stand that gets done first. If we had a choice between the express lane with everyone having 1 item which is now 20 people deep and the regular checkout line with one overflowing basket, does it really matter which line any ONE person gets into? More people (like more current) will go through the express line because it has a smaller resistance, but once it gets backed up, it doesn’t really matter which line you get into. Either way, we will get out at the same time, but MORE people altogether will get out, so an overall decrease in resistance to letting people getting out of the store.

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