Chapter 1 Intuition: What Sadiku Actually Wants You to Know

Voltage, Current, Resistance, and Power — Explained Without Memorization

I spent two weeks on Chapter 1 of Sadiku. Not because it was hard — because I refused to move on until every concept had a physical story behind it.

This article is that story. If you're learning circuits (or re-learning them), read this before you touch a single formula.

What Is Voltage, Really?

Sit with the discomfort of this question before reading on. "Potential difference" is not an answer — it's a name we gave to something we don't fully feel.

Here's the image: imagine voltage as water pressure in a pipe.

Water at the top of a hill has more "potential" to do work than water at the bottom. If you open a valve, it flows down. The difference in height creates pressure. That pressure is voltage.

Voltage = the "push" that makes charge want to move. Higher voltage = stronger push.

The unit is volts (V). One volt means one joule of energy per coulomb of charge. But forget that for now. Think pressure.

What Is Current, Then?

If voltage is pressure, current is the flow of water itself — the actual movement of charge through the pipe.

When you connect a battery (a pressure source) across a resistor (a narrow pipe), charge flows. That flow is current.

Unit: amperes (A). One ampere = one coulomb per second. Picture the water.

Resistance: The Pipe That Fights Back

Now put a narrow pipe in the water path. What happens? Less water flows for the same pressure. That's resistance.

Ohm's Law: V = I × R
In water terms: Pressure = Flow × Narrowness

To keep the same flow through a narrower pipe, you need more pressure. Same with electricity.

Resistance is measured in ohms (Ω). A wire has very low resistance (wide pipe). A tiny surface-mount resistor has higher resistance (narrow pipe). Simple.

Power: What's Actually Being "Used"

This is where most textbooks lose people. Power in a circuit is the energy being converted per second.

Think of a water wheel. The water pressure (voltage) pushes water through (current), and the wheel spins. Power = how fast the wheel spins × how hard the water pushes.

A resistor takes electrical energy and turns it into heat. A motor turns it into mechanical motion. Same equation (P = V × I), different result.

The Passive Sign Convention (Don't Trip Here)

This is the most tedious part of Chapter 1. Here's the trick:

• Passive element (resistor, capacitor): current enters positive terminal. Power = +VI (absorbing)
• Active element (battery): current leaves positive terminal. Power = −VI (supplying)

Just remember: passive elements eat power. Active elements cook it.

Why This Chapter Matters

Chapter 1 isn't the "easy" chapter. It's the foundation chapter. If you skip the intuition here, every future chapter will feel like memorizing words in a language you don't speak.

But if you nail it — if you feel voltage as pressure, current as flow, resistance as narrowness — then the rest of the book is just variations on the same story.

One pipe. One flow. One pressure. That's all circuits ever are.

I'm working through Alexander Sadiku's "Fundamentals of Electric Circuits" and writing down what I learn — in public, with intuition first.

Next: Chapter 2 — Basic Laws.

Originally published at https://cliovlsi.github.io/circuit-intuition/articles/sadiku-ch1-intuition.html