Lesson 0.1 · Module 0: Electricity Basics for Beginners

Your First Circuit: Voltage, Current & Resistance

The simplest useful circuit there is: a battery, a switch, a resistor and a light (an LED). Every torch and every power light on a gadget works this way.

Think of it like this: Think of water in a closed loop of pipes. The battery is a pump that creates pressure (voltage). Water flowing round is the current. The resistor is a narrow bit of pipe that limits the flow.

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

Key ideas

voltage is the push. current is the flow that the push causes. resistance limits that flow.
Current only flows round a complete loop — a circuit. Open the switch and the flow stops everywhere at once.
An LED needs a resistor in front of it. Without one, too much current flows and it burns out.

Try this in the simulation

Click the switch S1 in the drawing to open and close it. Watch the moving dots (the current) stop and start.
Drag the resistor slider down: less resistance means more current, and the LED gets brighter. Too far, and things start to smoke!
Hover any wire or part in the circuit to see its live voltage and current. Click to pin the readout.

What are voltage, current and resistance?

Voltage is the push that moves electric charge, current is how much charge flows past a point every second, and resistance is how hard a part makes that flow. In this lesson a battery supplies the push, a closed loop lets the charge travel, and a resistor limits how much flows so the LED stays safe. Ohm’s law links the three: current equals voltage divided by resistance.

What you’ll learn

What a volt (V), an amp (A) and an ohm (Ω) measure.
Why current only flows round a complete loop, and what a switch does to it.
Why an LED needs a resistor in series to limit its current.
How to read the live voltage and current on any wire.

The formula, letter by letter

\begin{aligned}\htmlClass{mv mv-i}{\htmlStyle{font-size:1.21em}{\htmlData{sym=I}{I}}} &= \frac{\htmlData{sym=U_bat}{U_{\mathrm{bat}}} - \htmlClass{mv mv-v_a}{\htmlStyle{font-size:1.00em}{\htmlData{sym=U_LED}{U_{\mathrm{LED}}}}}}{\htmlClass{mv mv-r}{\htmlStyle{font-size:1.13em}{\htmlData{sym=R}{R}}}} = \frac{\htmlData{funit=V,v=3}{\text{3.00 V}} - \htmlData{funit=V,v=1.6736425352928304}{\text{1.67 V}}}{\htmlData{funit=kΩ,v=1000}{\text{1.00 kΩ}}} = \htmlData{funit=mA,v=0.0013263574647071682}{\text{1.33 mA}}\\ \htmlData{sym=P_R}{P_{\mathrm{R}}} &= \htmlData{sym=I}{I}^2 \htmlData{sym=R}{R} = \htmlData{funit=mW,v=0.0017592241241844268}{\text{1.76 mW}}\end{aligned}

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

$I$: Current (A · ampere) : How much charge flows each second.
$U_{\mathrm{bat}}$: Battery voltage (V · volt)
$U_{\mathrm{LED}}$: Voltage across the LED (V · volt)
$R$: Resistance (Ω · ohm) : How strongly something opposes current.
$P_{\mathrm{R}}$: Power in the resistor (W · watt)

Common questions

What is the difference between voltage and current?

Voltage is the push and current is the flow that the push causes. Think of water in pipes: voltage is the pressure, current is how much water moves every second. A battery has voltage even when nothing is connected, but current only flows once there is a complete loop.

Why does an LED need a resistor?

Once an LED’s voltage passes about 2 V, its current rises extremely steeply. Nothing inside the LED stops that rise, so without a resistor the current climbs until the LED burns out. A resistor in series sets a safe current.

Is current used up as it goes round the circuit?

No. The same current that leaves the battery comes back to it. What gets used up is energy, which the resistor turns into heat and the LED turns into light.