Circuit types
Series and Parallel Circuits
- Grades 3-8
- 20 minute activity
- NGSS 4-PS3-2
Quick answer
In a series circuit, every part sits on one single path, one after another. In a parallel circuit, each part gets its own separate path back to the power source. Series circuits share the energy, so lights glow dimmer and one break turns everything off. Parallel circuits keep each light bright, and if one stops working the others stay on.
Series vs. parallel at a glance
Series circuit
- One single path for the electricity
- Parts share the battery's voltage
- More LEDs means each one is dimmer
- If one part breaks or is removed, everything goes off
- Batteries last longer
Parallel circuit
- A separate path for each part
- Each part gets the full battery voltage
- Every LED stays bright
- If one part breaks or is removed, the others keep working
- Batteries run down faster
What happens when one LED comes out?
This is the easiest way to tell the two circuits apart. In a series circuit, the electricity has to pass through every LED to get back to the battery. Take one out and you open a gap in the only path, so the whole circuit goes dark.
In a parallel circuit, each LED is its own bridge between the same two pieces of dough. Take one out and electricity still has the other paths, so the rest keep shining.
Why do series lights get dimmer?
A battery holder with 4 AA batteries gives a push of about 6 volts. Each LED needs about 2 to 3 volts to light. In series, the LEDs have to share that 6 volts. One LED gets plenty. Two LEDs get about 3 volts each, so they glow dimmer. Three LEDs need more push than the battery holder has, so they may not light at all.
In parallel, every LED is connected straight across the two pieces of dough, so each one gets the full 6 volts. You can add LED after LED and they all stay bright.
There is a catch. Each parallel LED draws its own current from the battery, so lots of LEDs in parallel will run the batteries down faster. Red LEDs need a little less voltage than blue, green or white ones, so they keep glowing a bit longer in series.
Try it with dough
Build series and parallel circuits
Build both circuits with the same parts and compare how bright the LEDs are. Then pull one LED out of each and see what happens.
You need
- Battery holder with 4 AA batteries
- Conductive (salty) play dough
- 3 LEDs of the same color
Steps
- Series: make three pieces of dough and set them in a ring with a small gap between each piece.
- Push the red and black battery wires into two pieces that sit next to each other.
- Bridge each of the other two gaps with an LED. Point each long leg toward the red-wire side of the loop.
- Switch on and notice how bright the LEDs are. Then pull out one LED and watch the other.
- Parallel: roll two long logs of dough and lay them side by side with a gap between them.
- Push the red wire into one log and the black wire into the other.
- Bridge the gap with three LEDs, each long leg in the red-wire log. Switch on, compare the brightness, then pull one LED out.
What happens
The series LEDs glow dimmer and both go out when one is removed. The parallel LEDs glow brightly and the others keep shining when one is removed.
Not working?
A series LED will not light? Check that each long leg points toward the red wire. If two pieces of dough touch, they make a shortcut and some LEDs will go dark. See What Is a Short Circuit?.
Series and parallel in real life
- Your home is wired in parallel. That is why you can switch off one lamp without turning off the refrigerator.
- Batteries in a battery holder are in series. Four 1.5 volt AA batteries end to end add up to 6 volts.
- Some older holiday light strings were wired in series. One burned-out bulb could make the whole string go dark, and you had to test each bulb to find it.
- Car headlights are in parallel. If one headlight burns out, the other still works.
- A switch is usually in series with the parts it controls, so opening the switch breaks their path.
Mix them: one switch, three LEDs
Most real circuits use both. In this challenge, a switch sits in series with three LEDs that are wired in parallel. The switch controls all three at once, and each LED still gets its own path. Engineers draw circuits like this as a schematic. Learn the symbols in Circuit Symbols and Schematics.
Key words
- Series circuit
- A circuit with one single path, where the parts are connected one after another.
- Parallel circuit
- A circuit with more than one path, where each part has its own branch.
- Branch
- One of the separate paths in a parallel circuit.
- Voltage
- The push that moves electricity around a circuit, measured in volts.
- Current
- How much electricity flows through the circuit.
- Load
- A part that uses electrical energy, like an LED, buzzer or motor.
For teachers
This activity follows Lesson 2 of our free NGSS curricula and educator guide. Have students predict which circuit will be brighter and what will happen when an LED is removed, then test and explain their results using the words path, branch and voltage.
For older students: in series, the voltage is shared across the LEDs and the same current flows through each one. In parallel, each branch gets the full voltage and the currents of the branches add up, which is why parallel circuits drain batteries faster. Find more lessons on our education page.
Series and parallel questions
What is the difference between series and parallel circuits?
A series circuit has one path, so the parts share the energy and one break turns everything off. A parallel circuit has a separate path for each part, so each one gets the full voltage and keeps working if another one fails.
Which is better, series or parallel?
It depends on the job. Parallel is better when parts need to work on their own, like the lights and outlets in a house. Series is useful when you want one switch or one fuse to control everything, or when you stack batteries to get more voltage.
Why are houses wired in parallel?
So every light and appliance gets the full voltage and can be switched on and off on its own. If houses were wired in series, turning off one lamp would turn off everything else.
Why do LEDs in series get dimmer?
The LEDs share the battery's voltage. A 6 volt battery holder can light one LED brightly, but two LEDs get about 3 volts each and glow dimmer. Three LEDs need more voltage than 6 volts can give, so they may not light at all.
Do batteries last longer in series or parallel?
LEDs wired in series draw less current, so the batteries last longer, but the LEDs are dimmer. LEDs in parallel are brighter, but each one draws its own current, so the batteries run down faster.
Is a switch in series or parallel?
A switch is almost always wired in series with the parts it controls. When the switch opens, it breaks their path and they turn off. See How to Use the Switch.
Free NGSS educator guide (PDF). Lesson 2 covers series and parallel circuits
Next, try a project
Build the Rainbow Project A parallel circuit with a row of LEDs, with printable instructions.