C. Matter and Energy · Electrical Phenomena, Energy, and Devices

Series and Parallel Circuits

C2.7 A1.3 A3.1About 45 minutes

Learning goals

I can describe a series circuit and a parallel circuit, and draw a diagram of each.
I can predict what happens when a bulb is removed from a series or a parallel circuit.
I can identify common uses of series and parallel circuits.
I can design a circuit for a purpose and explain my choice.

Words to know

series circuit
A circuit with only one path. The current flows through every part, one after another.

parallel circuit
A circuit with two or more separate paths (branches) for the current.

branch
One of the separate paths in a parallel circuit.

electrician
A skilled tradesperson who installs, repairs and maintains electrical wiring and equipment.

When one goes out...

Imagine your kitchen light burns out. Do the fridge, the TV and every other light in the house go off too? Of course not! But some old holiday light strings worked differently: when one little bulb burned out, the whole string went dark, and your family had to test each bulb to find the bad one.

The difference is the way the circuit is connected: in series or in parallel.

A snow-covered evergreen tree wrapped in strings of small warm white lights in front of a house at night. — Most modern light strings are wired so that one burnt-out bulb doesn't make the whole string go dark. AI-generated image

Series circuits: one path

In a series circuit, all the parts are connected one after another in a single loop. There is only one path for the current.

The same current flows through every part.
If one bulb is removed or burns out, the path is broken, so every bulb goes out.
When you add more bulbs in series to the same battery, each bulb gets dimmer, because they share the battery's push.

A series circuit: a battery and two bulbs in one loop. The current must go through bulb 1 and bulb 2 to get back to the battery.

Parallel circuits: more than one path

In a parallel circuit, the current has two or more separate paths, called branches. Each load is on its own branch.

If one bulb is removed or burns out, the other branches keep working.
Each bulb in parallel shines about as brightly as one bulb alone would, because each branch gets the battery's full push.
Each branch can have its own switch, so you can control each load separately.
The battery has to supply current to every branch, so it runs down faster.

A parallel circuit: two bulbs, each on its own branch. The dots show where the wires split and join. If bulb 1 is removed, current still flows through bulb 2's branch.

	Series circuit	Parallel circuit
Number of paths	One	Two or more (branches)
If one bulb goes out	All the bulbs go out	The others stay on
Brightness with more bulbs	Each bulb gets dimmer	Each bulb stays about as bright
Control	One switch controls everything	Each branch can have its own switch
Common uses	A switch and the light it controls; the batteries lined up end to end in a flashlight or TV remote; fuses and circuit breakers, which must be in series to cut the power; some older holiday light strings	The wiring in homes and schools (lights and outlets); car headlights; the many lights in an arena or a store

Comparing series and parallel circuits.

Homes use both

The lights and outlets in your home are wired in parallel, so you can use the toaster without turning on the dryer. But each switch is wired in series with the light it controls, and each circuit breaker is in series with the branch it protects. Real circuits often combine both types.

Worked example

Let's predict: series or parallel?

Two flashlight bulbs are connected to the same battery. When Sam unscrews bulb 1, bulb 2 stays on. Are the bulbs in series or parallel? What would happen in the other kind of circuit?

Bulb 2 stays on when bulb 1 is removed. That means bulb 2 still has a complete path to the battery.
A path that doesn't go through bulb 1 means there are separate paths.
Separate paths are branches, so this is a parallel circuit.
In a series circuit, there's only one path. Removing bulb 1 would leave a gap, so bulb 2 would go out too.

The bulbs are in parallel. In a series circuit, removing one bulb would turn the other one off.

Just for fun

Why did the old string of holiday lights get in trouble?

Because when one bulb went out, they all went out! (That's a series circuit for you.)

Hands-on activity

Circuit detective and design challenge

Part 1: Parallel detective (no tools needed). Walk around your home. Turn one lamp off. Do the other lights stay on? Is the fridge still running? Write what you find out about how your home is wired, and why that's useful.

Part 2: Series batteries. Open a flashlight or a TV remote that holds 2 AA or AAA batteries. Notice that the batteries are lined up so the + of one connects to the − of the next: they're in series, so their pushes add up (1.5 V + 1.5 V = 3 V). Close it up again.

Part 3: Design challenge (A1.3). A family wants to light a two-room model playhouse (a cardboard box with a wall in the middle). Criteria:

Each room has one bulb.
Each room's light can be turned on and off by itself.
If one bulb burns out, the other room stays lit.
Use one battery pack.

Plan: decide series or parallel. Draw a circuit diagram that meets every criterion.
Build (if you can): if you have a science kit with two bulb holders, or two small flashlight bulbs, build it with foil wires, 2 AA batteries and two paper-clip switches (made the same way as in the last lesson, with cardboard and paper fasteners). If not, build a model of the wiring with string or yarn on cardboard, and label each part.
Test: unscrew one bulb (or remove one string branch). Does the other room still work? Does each switch control only its own room?
Improve: what would you change? Share your diagram and explain your choice.

What to look for: a parallel circuit with two branches, each branch with its own bulb and its own switch, both branches connected to the battery.

Safety: use only AA batteries. Check that no foil wire connects the battery's + and − ends without going through a bulb (a short circuit makes the foil and batteries hot). Disconnect when finished.

Who works with circuits? (A3.1)

Electricians install and repair the wiring in homes, schools and businesses. They read and draw circuit diagrams, decide which parts need parallel branches, and install switches, outlets and breakers. In Ontario, electricians train through an apprenticeship (learning on the job while taking classes) and must be licensed. It's a skilled trade.
Powerline technicians build and repair the power lines that bring electricity to communities, including after ice storms.
Electrical engineers design electrical systems, from tiny circuits in phones to the power grid.

Your turn

What is a series circuit?

What is a parallel circuit?

Three bulbs are in series. One burns out. What happens to the other two? Why?

Three bulbs are in parallel. One burns out. What happens to the other two? Why?

Are the lights and outlets in your home wired in series or in parallel? How do you know?

Give one common use of a series circuit.

What happens to the brightness of each bulb when you add more bulbs in series to the same battery?

More practice: warm-up

How many paths does a series circuit have?

What is each separate path in a parallel circuit called?

Series or parallel? Car headlights: if one burns out, the other still works.

Series or parallel? An old string of lights that all go out when one bulb breaks.

Name one skilled trade that works with circuits.

More practice: on your own

Why are the batteries in a flashlight connected in series?

Why must a circuit breaker be connected in series, not in parallel, with the circuit it protects?

Two identical bulbs are connected to the same battery, once in series and once in parallel. In which circuit do the bulbs shine more brightly?

A parallel circuit with 3 bulbs drains its battery faster than a circuit with 1 bulb. Why?

A hallway has three lights that all turn on and off together with one switch, but if one bulb burns out the others stay on. Describe how it is wired.

Challenge

Draw (or describe) a circuit with one battery and two bulbs, where switch A controls only bulb 1 and switch B controls only bulb 2.

Jun builds a circuit with 2 bulbs. When he removes bulb 1, bulb 2 goes out. When he puts bulb 1 back and removes bulb 2, bulb 1 goes out. Which type of circuit is it? How could he change it so each bulb works on its own?

An electrician is wiring a new kitchen. Why would she put the fridge, the microwave and the lights on parallel branches rather than in one series loop? Give two reasons.

Check your understanding

Quiz

A circuit with only one path for the current is a...

In a parallel circuit, if one bulb burns out...

The lights and outlets in a home are wired mostly in...

What happens when you add more bulbs in series to the same battery?

Which is a common use of a series connection?

Which skilled trade installs and repairs wiring in homes and schools?

For parents and teachers

Big idea (C2.7): distinguish series circuits (one path) from parallel circuits (two or more branches), predict what happens when a component is removed, and identify common uses of each. The playhouse challenge uses the engineering design process (A1.3), and the careers section links to skilled trades (A3.1).

How to help: do the "parallel detective" walk together. If you have a hallway or staircase light, talk about how the switch is in series with the light. Science kits with bulb holders make series and parallel circuits much easier to build; foil wires work but need patience.

Common difficulties:

Thinking a drawing that "looks" spread out is parallel. Trace the path with a finger: if there's only one way around, it's series, no matter how it's drawn.
Thinking parallel bulbs are dimmer. Each parallel branch gets the full push; series bulbs share it and are dimmer.
Thinking homes are all one or the other. Homes use parallel branches, with switches and breakers in series.

Safety: only AA batteries; no short circuits (the foil and batteries get hot). Never open outlets, switch plates or light fixtures: home wiring is a job for licensed electricians.