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.

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

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 across the Arduino's 5 V pin.

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

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 across 5 V lets about 22.7 mA flow. A standard LED is happy at 20 mA — so this resistor would let an LED glow brightly, safely.

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. An LED would be so dim you would not see it.

Step 5 — write it up in your notebook

A neat one-line summary of both calculations side-by-side:

• V = 5 V (from the Arduino 5V pin)
• 220 Ω: I = 5 ÷ 220 = 22.7 mA → bright LED
• 10 kΩ: I = 5 ÷ 10 000 = 0.5 mA → invisible LED

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.