What is Electricity? Voltage, Current and Resistance Explained Simply

The three fundamentals every electronics engineer must understand before touching a circuit

Introduction

You’ve used electricity your whole life — charging your phone, switching on lights, listening to music. But what actually IS electricity? What is flowing through those wires, and why does your phone charger output 5V and not 10V or 100V? These aren’t academic questions. Every electronics project, from a single LED to an industrial motor controller, is built on three fundamentals: Voltage, Current, and Resistance.

This post goes deep on all three from first principles, building the mental model you’ll rely on for the rest of this series — and adds visual diagrams so the abstract becomes concrete.

Part 1: What is Electricity, Really?

The Atom — Where It All Starts

Everything physical is made of atoms: a nucleus of protons (positive) and neutrons (neutral), orbited by electrons (negative). In insulators (rubber, plastic, glass, wood) electrons are tightly bound and cannot move. In conductors — copper, aluminium, gold — the outermost electrons are loosely bound and drift freely between atoms. That looseness is precisely what makes metals useful for wires.

Figure 1 — Nucleus with orbiting electrons. Only the outermost (loosely bound) electrons are free to drift in a conductor.

The Water Analogy

The single most useful mental model for electricity is the water-pipe analogy. It isn’t perfect, but it gets you 90% of the way instantly: an elevated tank creates pressure (voltage); a pipe lets water flow (current); a narrow restriction in the pipe resists that flow (resistance).

Figure 2 — Tank height = voltage, pipe restriction = resistance, flow rate = current.

Electrical Concept	Water Analogy	Unit
Voltage (V)	Water pressure in the pipe	Volts (V)
Current (I)	Flow rate of water (litres/min)	Amperes / Amps (A)
Resistance (R)	Pipe width restriction	Ohms (Ω)
Power (P)	How much work the water does	Watts (W)
Charge (Q)	Volume of water	Coulombs (C)

The critical insight: increase pressure (voltage) and more water flows (current increases, if resistance stays fixed). Add restriction (resistance) and less flows. This exact relationship is Ohm’s Law, covered fully in Post 2.

Part 2: Voltage — The Electrical Pressure

Voltage is the potential difference between two points — the ‘push’ that drives electrons through a circuit. Voltage does NOT flow; it exists as a difference between two points. You never say ‘the voltage at point A’ — you say ‘the voltage from point A to point B.’ That’s why a multimeter always uses two probes.

Voltage	Source	What it Powers
1.5V	AA/AAA battery	Small toys, remote controls
3.3V	Modern MCU logic level	ESP32, nRF52, Raspberry Pi GPIO
5V	USB port, Arduino	Arduino Uno, most sensors, basic circuits
9V	PP3 battery, some adapters	Arduino barrel jack input
12V	Car battery, PSU	Relays, solenoids, some motors
230V	Indian mains power	Everything plugged into the wall
415V	Three-phase industrial	Heavy motors, industrial equipment

Figure 3 — Common voltage levels shown on a logarithmic scale. Notice how mains voltage dwarfs microcontroller logic levels.

DC vs AC Voltage

• DC (Direct Current): constant voltage, fixed direction. Batteries, USB, Arduino power pins. Your circuits will almost always use DC.
• AC (Alternating Current): voltage alternates direction 50 times per second (50Hz in India) — this is mains electricity, more efficient to transmit over long distances.

Part 3: Current — The Flow of Electrons

Current is the rate of flow of electric charge, measured in Amperes (A). One Ampere = one Coulomb of charge flowing past a point every second — and one Coulomb is roughly 6.24 × 10¹⁸ electrons. Even a modest 1A current involves an almost incomprehensible number of electrons moving every second.

Figure 4 — Typical current draw across common devices, log scale (milliamps).

Current	What it does
1 mA (0.001A)	Barely visible LED glow, some sensor signals
10-20 mA	Bright LED at full brightness
100-500 mA	Raspberry Pi Zero, small Arduino projects
1-2 A	Raspberry Pi 4, smartphone charging
5-10 A	Small motors, 3D printer heated bed
20-100 A	Car starter motor, EV charging
10,000+ A	Lightning bolt (for reference!)

Conventional vs Electron Current

Historically, scientists assumed current flowed positive-to-negative (conventional current). Later we discovered electrons actually flow negative-to-positive — the opposite direction. In electronics we still use conventional current direction in every diagram and calculation. The maths works either way, so this quirk doesn’t affect practice — just be aware it exists.

Part 4: Resistance — The Opposition to Flow

Resistance opposes the flow of current, measured in Ohms (Ω). Every material, component, and wire has some resistance. A resistor converts electrical energy into heat — sounds wasteful, but this is incredibly useful: it lets us control current, protect components, and build voltage dividers.

What Determines Resistance?

• Material: copper has very low resistance (good conductor); nichrome wire has high resistance (used in heaters).
• Length: longer wire = more resistance (electrons travel further).
• Cross-section: thicker wire = less resistance (more paths for electrons).
• Temperature: most metals increase resistance as they heat up.

⚡  REAL WORLD

The tungsten filament in an incandescent bulb has very high resistance.

Current flowing through it heats it to ~2700°C — hot enough to glow.

This is resistance deliberately used to produce light and heat.

Type	Resistance	Examples	Use in Electronics
Conductor	Very low (<1Ω/m)	Copper, aluminium, gold	Wires, PCB traces, connectors
Semiconductor	Variable (controlled)	Silicon, germanium	Transistors, diodes, ICs, solar cells
Insulator	Extremely high	Rubber, plastic, glass	Wire coating, PCB substrate, enclosures

🔑  KEY CONCEPT

Semiconductors are the foundation of all modern electronics.

Their resistance can be controlled by applying voltage or current — this is how transistors work, the building block of every computer chip.

Part 5: How They Work Together

• More voltage (more pressure) → more current flows (if resistance is fixed)
• More resistance → less current flows (if voltage is fixed)
• Keep voltage the same and want less current → increase resistance
• Keep resistance the same and want more current → increase voltage

A Practical Example: LED Circuit

An LED is damaged if too much current flows through it. A typical red LED wants ~20mA and has a forward voltage of 2V. Connect it directly to a 5V Arduino pin and it tries to draw unlimited current, burning out instantly. Adding a resistor limits the current to a safe level: R = (5 − 2) ÷ 0.02 = 150Ω. The nearest standard value, 220Ω, is used to stay safely under that limit.

Part 6: Open, Short, and Closed Circuits

Circuit Type	Description	Result
Closed	Complete path from source, through components, back to source	Electrons flow, circuit works
Open	A break in the path (disconnected wire, blown fuse, off switch)	Current = 0, nothing happens
Short	Direct connection between + and − with near-zero resistance	Huge current, extreme heat, possible fire

⚠️  WARNING

Never connect the positive and negative terminals of a battery or power supply directly.

Always have components (resistance) in the circuit to limit current.

Even a AA battery can get dangerously hot if short-circuited with a wire.

Frequently Asked Questions

Q: Is voltage the same thing as current?

A: No. Voltage is the pressure that pushes electrons; current is the actual rate of electron flow that results. You can have high voltage with almost no current (a static shock) or low voltage with high current (a car battery cranking a starter motor).

Q: Why does my phone charger say 5V but my wall socket is 230V?

A: The charger contains a switching power supply that converts the higher AC mains voltage down to a safe, steady 5V DC suitable for charging electronics. We cover exactly how in Post 12.

Q: Can current flow without voltage?

A: No. Current only flows when there is a voltage difference (a potential difference) across a conductor. Zero voltage difference means zero current, regardless of how good a conductor the wire is.

Q: Is a higher resistance always safer?

A: Generally yes for limiting current to protect components like LEDs, but too much resistance can also starve a circuit of the current it needs to function. The goal is the right resistance for the job, not simply the highest one.

Q: Why is 10mA through the heart dangerous but LEDs run at 20mA safely?

A: Current path matters enormously. An LED’s 20mA flows through a tiny semiconductor junction designed for it. The same current through the human heart — a path it was never designed for — can disrupt its electrical rhythm. Voltage level and skin resistance also change how much current actually enters the body.

Q: What’s the difference between a conductor and a semiconductor?

A: A conductor like copper always conducts well. A semiconductor like silicon can be made to conduct or block current depending on how it’s doped and what voltage is applied — this controllability is what makes transistors and diodes possible.