Electronics

Fundamentals — voltage, current, resistance, and how circuits work

Three quantities and one equation explain most of what happens in a circuit. Everything else in this notebook — every sensor, every protocol, every peripheral — is these three things arranged in different shapes.

Electric charge and current direction

Everything here starts with electric charge — a property of electrons (negative) and protons (positive). Current is charge in motion. There's one famous historical wrinkle worth knowing by name: conventional current is defined as flowing from + to − through a circuit, but the electrons actually doing the moving flow the opposite way, − to +. This convention was fixed before anyone knew what an electron was, and it stuck — every schematic, every formula, every datasheet you'll ever read uses conventional current (+ to −), so that's what this page uses too.

Voltage

Voltage is the difference in electrical potential between two points — it's what pushes charge through a circuit, and it's always measured between two points, never at just one.

Symbol

V (sometimes E)

Unit

Volt (V)

Measured

Across a component, in parallel with it

Current

Current is the rate at which charge flows past a point — how much charge per second.

Symbol

I

Unit

Ampere / "amp" (A)

Measured

Through a component, in series with it (the circuit has to be broken to insert a meter)

Resistance and Ohm's Law

Resistance is a material's opposition to current flow. The relationship between all three quantities is Ohm's Law, the single most-used equation in electronics:

V = I × R
I = V / R
R = V / I

The water analogy

The most common — and genuinely useful — way to build intuition for all three at once:

Power

Power is the rate energy is used or dissipated — usually as heat in a resistive component. It matters for battery life, heat dissipation, and component ratings (a resistor rated for less power than it actually dissipates will overheat and fail).

P = V × I
P = I² × R
P = V² / R

A complete circuit

Current only flows in a closed loop — a complete path from a power source's positive terminal, through components, back to its negative terminal. Break the loop anywhere and current stops everywhere, not just past the break.

Series vs. parallel circuits

The resistor-and-LED loop above is a series circuit — there's only one path, so the same current flows through both the resistor and the LED.

AC vs. DC

Real-world applications

• Current-limiting resistors — the LED circuit above is the single most common first circuit anyone builds, and the resistor's job — protecting the LED from too much current — shows up constantly wherever a component needs protecting.
• Voltage dividers — two resistors in series (see series circuits above) are the standard way to scale down a voltage for a microcontroller's ADC input.
• Battery life estimation — P = V × I directly gives the current draw a battery has to supply, and from there, roughly how long a battery of known capacity will last.
• PCB trace sizing — P = I² × R is exactly why high-current traces need to be wider — too narrow, and I²R heating can damage the board.

Gotchas