Lesson 0.2 · Module 0: Electricity Basics for Beginners

Series vs Parallel Circuits Explained

Two resistors connected in a line (series) or side by side (parallel). These two patterns are the building blocks of every circuit diagram.

Think of it like this: Series is like one pipe with two narrow bits in a row: all the water passes through both. Parallel is like two pipes side by side: the flow splits and joins up again.

The live, interactive simulation for this lesson opens here when JavaScript is on.

Key ideas

In series, the same current flows through every part. It is not used up — what goes in comes out.
What the resistors use up is energy: the voltage "drops" across each one, and the drops add up to the battery voltage.
In parallel, each part gets the full voltage and the current splits. The branch currents add back up to the total (Kirchhoff’s current law).

Try this in the simulation

In series, hover the wire before R1 and the wire after R2: the current is exactly the same.
Switch the layout to parallel. Make R2 smaller and see it take a bigger share of the current.
Compare the total current in series and in parallel for the same resistors. Which one draws more from the battery?

What is the difference between series and parallel circuits?

In a series circuit the parts sit in one line, so the same current passes through every one of them and their resistances add up. In a parallel circuit the parts sit side by side, so each one gets the full voltage and the current splits between them. Putting a resistor in series makes the total resistance bigger, while putting one in parallel makes it smaller.

What you’ll learn

How to tell series from parallel by following the path of the current.
How to find the total resistance of resistors in series and in parallel.
Why current is the same all the way round a series loop.
Why the branch currents of a parallel circuit add up to the total.

The formula, letter by letter

\begin{aligned}\htmlData{sym=R_total}{R_{\mathrm{total}}} &= \htmlData{sym=R_1}{R_1} + \htmlData{sym=R_2}{R_2} = \htmlData{funit=kΩ,v=1000}{\text{1.00 kΩ}} + \htmlData{funit=kΩ,v=2200}{\text{2.20 kΩ}} = \htmlData{funit=kΩ,v=3200}{\text{3.20 kΩ}}\\ \htmlClass{mv mv-i_tot}{\htmlStyle{font-size:1.00em}{\htmlData{sym=I}{I}}} &= \frac{\htmlData{sym=U}{U}}{\htmlData{sym=R_total}{R_{\mathrm{total}}}} = \htmlData{funit=mA,v=0.001875}{\text{1.88 mA}} = \htmlClass{mv mv-i1}{\htmlStyle{font-size:1.00em}{\htmlData{sym=I_1}{I_1}}} = \htmlClass{mv mv-i2}{\htmlStyle{font-size:1.00em}{\htmlData{sym=I_2}{I_2}}}\end{aligned}

Shown with this lesson’s starting values. In the simulation the numbers change as you move the sliders.

$R_{\mathrm{total}}$: Total resistance (Ω · ohm)
$R_1$: Resistance of R1 (Ω · ohm) : How strongly something opposes current.
$R_2$: Resistance of R2 (Ω · ohm) : How strongly something opposes current.
$I$: Current (A · ampere) : How much charge flows each second.
$U$: Voltage (V · volt) : The electrical push between two points.
$I_1$: Current in branch 1 (A · ampere) : How much charge flows each second.
$I_2$: Current in branch 2 (A · ampere) : How much charge flows each second.

Common questions

Why is the total resistance of resistors in parallel smaller than any one of them?

Every extra branch gives the current another path to take. With the same voltage, more current flows in total, and more current for the same voltage means a lower total resistance.

Are the outlets in a house wired in series or in parallel?

In parallel. Every outlet gets the full supply voltage, and switching one lamp off does not turn the others off.

Does the first resistor in a series chain use up some of the current?

No. The current is the same before and after each resistor. Each resistor drops some of the voltage, and the drops add up to the battery voltage.

Keep going

Next lesson: 1.1 Ohm's Law, Power & Why Resistors Get Hot
Related components: Resistor

Words used in this lesson

Parts in this lesson

Resistor

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