Ohm's Law for Kids: Voltage, Current and Resistance

Learning Goals 5 min

By the end of this lesson you will be able to:

1. Name the three quantities in Ohm's Law — voltage, current and resistance — and the unit each one is measured in.
2. Rearrange V = I × R to calculate any one of the three when you know the other two.
3. Predict how much current would flow if the Arduino's 5V pin was connected to GND through a 220 Ω resistor, a 1 kΩ resistor and a 10 kΩ resistor — and explain which value would make an LED glow brightly.

Warm-Up 10 min

Last lesson we wired the Arduino's 5V and GND pins to the breadboard's power rails. Today we learn the maths that decides what we plug between them.

Quick-fire puzzle

Imagine a long garden hose connected to a tap in Petaling Jaya. The tap is open and water is rushing through. Now answer these without peeking:

1. You squeeze the hose tightly halfway down. What happens to how fast water comes out the other end?
2. You squeeze the hose much harder. What happens now?
3. What part of an electrical circuit acts the same way as squeezing the hose?

New Concept 20 min

The big idea — water in a hose

Every electrical circuit has three properties that work like a water hose:

• Voltage (symbol V, measured in volts) is the water pressure. How hard the electricity is being pushed.
• Current (symbol I, measured in amperes or milliamps) is the water flow. How much electricity actually moves per second.
• Resistance (symbol R, measured in ohms) is the narrowness of the hose. How much the flow is restricted.

Open the tap wider, or squeeze the hose harder, and see which of the three moves. The picture below is the same circuit written as water — every control has an electrical name underneath it.

Read the "where this picture stops being true" list at the bottom of the lab before you go on. Every model breaks somewhere, and knowing where is part of using it safely.

Ohm's Law in three forms

These three quantities are tied together by a famous formula called Ohm's Law:

V = I × R

That single equation can be rearranged three ways depending on which quantity you are trying to find:

• Find voltage: V = I × R
• Find current: I = V ÷ R — the one you will use most.
• Find resistance: R = V ÷ I

Quantities and units

Everyday voltages

Which way does the current actually go?

Current has a direction, and there are two answers to what that direction is. Every schematic in this course — and every one you will ever read — draws current leaving + and arriving at −. That is called conventional current. The electrons themselves are negatively charged, so they physically drift the other way.

Both pictures give the same answers to every calculation in this lesson. Switch between them below and watch what changes and what does not.

Use conventional current for everything you draw. The electron picture matters when you meet semiconductors later; it never changes an Ohm's Law sum.

How you'll write down a calculation

For the rest of this lesson, work each Ohm's Law problem out on paper or in your notebook — exactly the way you'd lay out a maths sum. Show every step. That way you can spot where a calculation went wrong, and your teacher can give credit for correct working even if the final number is off.

Example: a 220 Ω resistor on its own between the Arduino's 5 V pin and GND — no LED, nothing else in the path.

• Given: V = 5 V, R = 220 Ω
• Find: I
• Formula: I = V ÷ R
• Working: I = 5 ÷ 220 = 0.0227 A = 22.7 mA

Notice the words on its own. The whole 5 V is pushed across the resistor because nothing else in the loop takes any of it. The moment you add an LED — which you will in L01-07 — the LED keeps about 2 V for itself and the sum changes. We come back to that in the Worked Example.

Why it matters

Without Ohm's Law you would either burn out components (too much current melts LEDs and chips) or have them be invisible (too little current makes nothing happen). Every electronic component in this course has a current limit. Ohm's Law tells you exactly what resistor to use to stay safely under it.

Worked Example 20 min

Goal: predict how much current flows when the Arduino's 5V pin is connected through different resistors to GND.

Step 1 — write down what you know

• Arduino's 5V pin produces 5 V.
• Resistor value: 220 Ω.
• We want to find the current (I).

Step 2 — pick the right form of Ohm's Law

We know V and R, we want I. So use:

I = V ÷ R

Step 3 — plug in the numbers

• I = 5 ÷ 220 = 0.02272… amperes
• Convert to milliamps: 0.02272 × 1000 = 22.7 mA

So a 220 Ω resistor on its own across 5 V lets about 22.7 mA flow.

Step 4 — compare to a bigger resistor

What if we swapped to a 10 kΩ resistor instead?

• I = 5 ÷ 10000 = 0.0005 amperes = 0.5 mA
• That is 45 times less current — far too little to light anything.

Step 5 — now put an LED in the loop

This is the step that trips people up, so do it slowly. An LED is not a resistor: it keeps about 2 V for itself no matter what else is in the circuit. That 2 V is called its forward voltage. Only what is left over gets pushed across the resistor.

• Voltage the resistor actually sees: 5 − 2 = 3 V
• Formula: I = (V_pin − V_F) ÷ R
• Working: I = 3 ÷ 220 = 0.0136 A = 13.6 mA

So the same 220 Ω resistor passes 22.7 mA on its own and 13.6 mA with an LED in series. Both numbers are right; they are answers to different questions. A standard LED is rated for about 20 mA, so 13.6 mA is bright and safe — which is exactly why 220 Ω is the resistor in your kit.

Step 6 — push on it

Drag the two sliders. The number, the speed of the current dots and the brightness of the LED all move together, because they are all the same fact. Take the resistor below about 150 Ω and watch what happens to the LED.

Step 7 — write it up in your notebook

A neat summary of all three calculations side by side:

• V = 5 V (from the Arduino 5V pin)
• 220 Ω alone: I = 5 ÷ 220 = 22.7 mA
• 10 kΩ alone: I = 5 ÷ 10 000 = 0.5 mA
• 220 Ω + red LED: I = (5 − 2) ÷ 220 = 13.6 mA → bright and safe

What changed since last lesson? The hardware is exactly the same as L01-04. The new skill is sitting at your desk and being able to predict what would happen before you wire anything up.

Try It Yourself 20 min

Work each problem out in your notebook. Show your formula, your numbers and your answer with units.

Mini-Challenge 15 min

Plan the LED circuit

Combine today's Ohm's Law skill with the breadboard layout from ARD-L01-04. You will not build this yet — that happens in two lessons' time — but you should be able to plan it on paper.

Your task:

1. On a sheet of paper, draw the Arduino UNO and a breadboard side by side.
2. Pick a resistor value from the kit — 220 Ω, 470 Ω, 1 kΩ or 10 kΩ — that will let about 15 mA flow through an LED. Assume the LED drops 2 V, so the resistor sees 3 V.
3. Use Ohm's Law to prove your resistor choice gives a current close to 15 mA. Write the calculation next to your drawing.
4. Mark on the drawing exactly which breadboard holes the resistor and LED would plug into, and which wires go where.

It works if a partner can read your drawing and your calculation and agree on three things:

• Your resistor maths gives a current between 5 mA and 25 mA (safe and visible).
• The resistor and the LED are in series — they sit one after the other in the path from 5V to GND, not side by side.
• Nothing in your drawing connects 5V directly to GND without going through the resistor and LED.

Recap 5 min

Voltage is the push, current is the flow, resistance is the squeeze. They are tied together by V = I × R, which you can rearrange three ways to find any quantity from the other two. Bigger resistor → smaller current → dimmer LED.

Voltage (V)

Electrical pressure, measured in volts. The Arduino's 5V pin gives 5 V.

Current (I)

How much electricity flows per second, measured in amperes (A) or milliamps (mA). 1 A = 1000 mA.

Resistance (R)

How hard a circuit squeezes the flow, measured in ohms (Ω) or kilohms (kΩ). 1 kΩ = 1000 Ω.

Ohm's Law

The formula V = I × R. The single most useful equation in electronics.

Short circuit

Connecting 5V directly to GND with no resistance in between. Don't.

Homework 5 min

Three calculations. Answer all three in your notebook. For each one show the formula, your numbers and your answer with units.

1. V = 5 V, R = 470 Ω. Find I in milliamps.
2. V = 9 V (a 9 V battery), I = 20 mA. Find R in ohms.
3. You want exactly 10 mA to flow from 5V through a resistor (no LED, just the resistor) to GND. What value of R do you need?

Bring back next class: your notebook page or a phone photo of the page, with all three calculations worked out neatly.