2-4 Series and Parallel Circuits

What Do Those Words Mean?

First, the word series: have you ever watched a TV show that has one episode after another, and the plot builds during these episodes? This is sometimes called a TV series.

You may have seen the word parallel in math class. Parallel lines don't meet:

We write symbols like the following on parallel lines to show that they are, in fact, parallel:

We do this in science sometimes: we use words that have meanings in other places. If you see a new term in science class, there's a chance you can figure out what it means by thinking about what the words that make it up mean.

Series Circuits

A series circuit contains all the loads, one after another -- just like episodes in a TV series. Here's a circuit that contains three light bulbs in a series:

Things get a little clearer when we label the light bulbs and show the direction of electron flow.

Going clockwise... the current comes out of the source and through the switch (assume it's closed; it's shown open here for clarity). Then it goes into bulb #1, out of bulb #1, into bulb #2, out of bulb #2, into bulb #3, and out of bulb #3. After the last load, it follows the wire back to the source to pick up more energy.

Here's a picture of a series circuit that someone built. The light bulbs are labelled, the direction of electron flow is shown, and the negative and positive terminal of the battery are indicated.

Question: What would happen if someone removed bulb #2, or if bulb #2 burned out?

Answer: All of the bulbs would go out. There is only one path for the current to flow, and if there is a break in that path, all the current stops flowing.

Parallel Circuits

A parallel circuit contains loads which are beside each other, in some way. Here's a circuit that shows two light bulbs, arranged in parallel with each other:

Again, we can label the current flow and number the bulbs:

Two other points are labelled, X and Y. These are called junctions, and they are places where wires meet and current can either split apart or join together. These are very important places in a circuit, so we need to look at them more closely.

Current comes out of the source, through the switch, and hits the junction at point X. Now it has the choice: turn down the wire and go through bulb #1, or keep going on the same wire and go through bulb #2? If both bulbs are identical, equal numbers of electrons will go down both branches of this parallel circuit.

So, some current goes through bulb #1 and gives it energy. The rest of the current goes through bulb #2 and gives it energy. But since the current keeps going, it will all meet back up at the junction at point Y, to return back to the source.

Here's a picture of a parallel circuit that someone built, with three light bulbs all in parallel. The four junctions are also labelled, A through D. Do you notice anything about bulb #2?

Bulb #2 is out -- maybe the filament in the light bulb has broken, or maybe there's a break in the wire between B and C. But, even though bulb #2 is out, there is still a path for the current to flow through bulbs #1 and #3, and still get back to the source.

Circuits in your Home

Here's a picture of a kitchen counter, showing four appliances which all plug into the wall.

Each of these appliances has a switch inside them, which controls whether they are on or off.

Question: Are these appliances in series or parallel with each other?

Think about the series circuit shown in the picture above: for any one light bulb to be on, all of them need to be on. So, if the appliances were in series with each other, all the appliances would have to be switched on at the same time for any of them to work.

In the parallel circuit shown in the picture above, all of the light bulbs could work indepenently of each other. (You could also put a switch into the circuit beside each bulb, to control each bulb separately.) This is how the appliances work: for example, you can have the kettle turned on without the dishwasher also being on.

The first minute and a half of this contains some really clear explanations about the difference between series and parallel. The animation is very clear, and you can see how they both work. You can go beyond that point, but it gets into some math that we haven't looked at yet (and will mostly be saved for Grade 11 or 12 Physics).

Short Circuits

A short circuit is a type of circuit which, usually by mistake, has a path for current to flow which has no load at all. Here's a circuit diagram showing a short circuit, with current flow shown and the two junctions labelled P and Q:

The current comes out of the source and hits junction P. It now has two choices: go down the branch that has all the loads, or go straight down the wire with no loads. Since the loads are more difficult to go through than a simple wire, virtually all the current goes through the wire and none through the loads.

This is a dangerous circuit. The reason for this is that a lot of current will flow down the short -- the wire that by-passes the loads -- and this will make the wire heat up a lot. If this happens to the wiring inside the walls of a building, that can cause an electrical fire.

Fortunately, there are safety devices that detect when current flow is too high, called fuses, which cut current off before things get too dangerous. A common type of fuse called a circuit breaker is found in most homes built in the past few decades; these are usually organized in what's called a breaker panel and can be found in a place like a basement.

This breaker panel contains about 25 individual circuit breakers, which control different rooms in a house. If there's a short circuit, the circuit breaker detects this, switches itself off, and then the wiring or the device that caused the short circuit can be fixed.

Practice

The Basics

1. Describe the difference between series and parallel circuits, in terms of how the loads are arranged.

2. Draw a circuit diagram showing a 2-cell battery, no switch, and two light bulbs in series with each other.

3. Draw a circuit diagram showing a 3-cell battery, no switch, and four light bulbs in parallel with each other.

Extensions

1. In the circuit diagram above which had junctions X and Y labelled, think about the amount of current flowing into, and out of, junction X. How does the current flowing in compare to the total current flowing out of it, with both branches (down and right) added together?

2. In the same circuit, think about the voltage drop through bulb #1 and bulb #2. Is one bigger than the other? Are they the same?