Lesson 1: Basics of Electricity

Indy Hendrickx21/08/2026About 15 min

Basics of Electricity

Every electronic device you use, from a simple LED to a sophisticated microcontroller, operates based on the same fundamental principles of electricity. Understanding these principles is not optional. It is foundational. You cannot design reliable circuits, debug hardware problems, or work with sensors without grasping what happens when electricity flows through a wire.

The good news: the core concepts are simple. This lesson covers the essential building blocks. Once you understand voltage, current, and resistance, everything else follows naturally.

What is electricity?

At its most basic, electricity is the flow of electrons through a conductor. An electron is a subatomic particle with a negative charge. When electrons move, they carry energy from one place to another. That movement is what we call current.

But before electrons can move, there must be a reason for them to move. That reason is an imbalance of charge: a place with an excess of electrons (negative) and a place with a deficit of electrons (positive). This imbalance creates an electric field, which pushes the electrons toward the positive side. We call this push voltage.

A battery creates an electrical imbalance: excess electrons on one terminal (negative), a deficit on the other (positive). This imbalance drives electron flow

The path that the electrons follow is called a circuit. A complete circuit has three things: a source of voltage (like a battery), a path for current to flow (like a wire), and something that uses that current (like a resistor, LED, or motor). Break any part of the circuit, and the current stops.

Understanding electricity boils down to understanding three quantities that describe what is happening in any circuit: voltage, current, and resistance. Let's define each one clearly.

The Three Core Quantities: Voltage, Current, and Resistance

Every circuit problem, no matter how complex, can be reduced to these three quantities. Understanding what they are, physically and mathematically, is the gateway to understanding electronics.

Voltage: The Push

Voltage is the electrical potential difference between two points in a circuit. Think of it as pressure. Just as water pressure pushes water through a pipe, voltage pushes electrons through a wire.

The unit of voltage is the volt (V). A volt is formally defined as the energy (in joules) per unit charge (in coulombs) needed to move that charge between two points. In practice, all you need to know is: voltage is the electrical "push" that drives current.

Common voltage sources you will encounter:

A typical AA battery: 1,5 V
A USB power supply: 5 V
The mains power in Europe: 230 V
A microcontroller logic level: 3,3 V or 5 V

An important insight: voltage is relative. It always exists between two points. When we say a battery is "9 volts," we mean there is a 9-volt difference between its positive and negative terminals. We call the negative terminal "ground" and use it as our reference point (0 V). Everything else is measured relative to ground.

Current: The Flow

Current is the rate at which electrons flow through a circuit. If voltage is the push, current is the actual movement of charge.

The unit of current is the ampere (A), often shortened to "amp." One ampere is defined as one coulomb of charge flowing past a point per second. Smaller currents are written in milliamps (mA: thousandths of an amp) or microamps (µA: millionths of an amp).

Common current values in IoT projects:

An LED: typically 10-20 mA
A microcontroller: typically 50-200 mA
A small motor: could be 500 mA to several amps
A sensor on I2C: a few milliamps

Current flows in one direction: from the positive terminal of a voltage source, through the circuit, back to the negative terminal. This direction is called conventional current flow. (Historically, scientists thought positive charges were moving, but electrons are actually negative and move in the opposite direction. We stick with conventional current anyway because it is the standard in all technical documentation.)

Resistance: The Obstacle

Resistance is the opposition to the flow of current. Some materials allow electrons to flow easily (conductors, like copper). Others resist the flow strongly (insulators, like rubber). Resistance quantifies how much a material opposes current.

The unit of resistance is the ohm (Ω). A one-ohm resistor means that one volt will push one amp of current through it. Larger resistances are written in kilohms (kΩ: thousands of ohms) or megohms (MΩ: millions of ohms).

Common resistance values in circuits:

Connecting wire: milliohms (nearly zero)
An LED current-limiting resistor: typically 150 Ω to 10 kΩ
A pull-up resistor: typically 1 kΩ to 100 kΩ
A light-dependent resistor (LDR) in bright light: hundreds of ohms
A light-dependent resistor (LDR) in darkness: megohms

The key insight is this: resistance is not a bad thing. It is a tool. By controlling resistance, we control how much current flows, and therefore how much power is dissipated. An LED needs a resistor in series with it, not because the resistor is wasting energy, but because the resistor protects the LED by limiting the current to a safe level.

Ohm's Law

Now we have three quantities. How do they relate to each other? The answer is Ohm's Law, one of the most fundamental equations in electronics:

Ohm's Law: Voltage equals Current times Resistance. U = I × R

U = I \times R

Where:

U is voltage (in volts)
I is current (in amperes)
R is resistance (in ohms)

Note: In Europe, we use the symbol U for voltage in formulas. You may also encounter V in some contexts, but U is the standard symbol in European electronics and electrical engineering. The unit remains "volt" (V).

This single equation unlocks almost everything. If you know any two of the three quantities, you can calculate the third. Rearranging:

I = \frac{U}{R}

R = \frac{U}{I}

Let's work through some examples to make this concrete.

Example 1: Calculating current through an LED

You have a 5 V power supply and a 220 Ω resistor. What current flows through the resistor?

Using Ohm's Law:

I = \frac{U}{R} = \frac{5 \text{ V}}{220 \text{ Ω}} = 0,0227 \text{ A} = 22,7 \text{ mA}

This is a safe current for a typical LED.

Example 2: Choosing a resistor

You want to limit current through an LED to 10 mA on a 3,3 V logic pin. What resistance do you need?

Using Ohm's Law:

R = \frac{U}{I} = \frac{3,3 \text{ V}}{0,010 \text{ A}} = 330 \text{ Ω}

A 330 Ω resistor would work. (You would use the next standard value available, which happens to be exactly 330 Ω.)

Example 3: Finding voltage across a component

You measure 50 mA flowing through a 100 Ω resistor. What is the voltage drop across it?

Using Ohm's Law:

U = I \times R = 0,050 \text{ A} \times 100 \text{ Ω} = 5 \text{ V}

A 50 mA current flowing through 100 Ω produces a 5 V drop.

Try it yourself!

For each scenario, use Ohm's Law to find the missing quantity.

Scenario 1: You have a 12 V supply and a 600 Ω resistor. Find the current.

Solution

I = \frac{U}{R} = \frac{12 \text{ V}}{600 \text{ Ω}} = 0,020 \text{ A} = \mathbf{20 \text{ mA}}

Scenario 2: You measure 0,5 A flowing through a 100 Ω resistor. Find the voltage across it.

Solution

U = I \times R = 0,5 \text{ A} \times 100 \text{ Ω} = \mathbf{50 \text{ V}}

Scenario 3: A 5 V supply powers a circuit with 25 mA flowing. Find the total resistance.

Solution

R = \frac{U}{I} = \frac{5 \text{ V}}{0,025 \text{ A}} = \mathbf{200 \text{ Ω}}

Power and Wattage

Current flowing through resistance produces heat. This happens because the electrons are colliding with atoms, transferring energy. The rate at which energy is dissipated is called power.

Power is measured in watts (W). One watt is one joule of energy consumed per second. Just as voltage, current, and resistance are related by Ohm's Law, power is related to them by another fundamental equation:

P = U \times I

Where:

P is power (in watts)
U is voltage (in volts)
I is current (in amperes)

You can also substitute Ohm's Law to get two alternative forms:

P = I^2 \times R

P = \frac{U^2}{R}

All three equations are correct and useful. Which one you use depends on which quantities you know.

Why power matters in IoT

In embedded systems, power consumption directly determines battery life. A device drawing 100 mA from a 2000 mAh battery lasts 20 hours. The same device drawing 500 mA lasts only 4 hours. Power is not abstract, it is a critical design constraint.

Power also determines how much heat a component generates. A resistor dissipating 1 W will become hot. A resistor dissipating 0,1 W will barely warm up. Exceeding a component's power rating causes it to fail.

Example 1: Power dissipated in a resistor

You have a 220 Ω resistor with 25 mA flowing through it. How much power is dissipated?

Using the $P = I^2 \times R$ formula:

P = (0,025)^2 \times 220 = 0,000625 \times 220 = 0,1375 \text{ W} = 137,5 \text{ mW}

This is well within the limits of a standard 0,25 W resistor (which can safely dissipate up to 250 mW).

Example 2: Power supplied by a battery

A 5 V power supply delivers 2 A to a circuit. How much power is supplied?

Using the $P = U \times I$ formula:

P = 5 \text{ V} \times 2 \text{ A} = 10 \text{ W}

The power supply must be rated for at least 10 W to safely handle this load.

Example 3: Power through a resistor via voltage

A 1 kΩ resistor is connected across a 12 V supply. How much power does it dissipate?

Using the $P = \frac{U^2}{R}$ formula:

P = \frac{(12)^2}{1000} = \frac{144}{1000} = 0,144 \text{ W} = 144 \text{ mW}

Try it yourself!

Question 1: A 5 V supply delivers 2 A to a circuit. How much power is supplied?

Solution

P = U \times I = 5 \text{ V} \times 2 \text{ A} = \mathbf{10 \text{ W}}

Question 2: A 100 mA current flows through a 100 Ω resistor. How much power is dissipated?

Solution

P = I^2 \times R = (0,1)^2 \times 100 = \mathbf{1 \text{ W}}

Question 3: A 1,5 kΩ resistor is connected across a 12 V supply. How much power does it dissipate?

Solution

P = \frac{U^2}{R} = \frac{(12)^2}{1500} = \frac{144}{1500} = \mathbf{0,096 \text{ W} = 96 \text{ mW}}

Series and Parallel Circuits

Up until now, we have looked at simple circuits with one resistor. Real circuits often have multiple components. The way components are connected determines how voltage, current, and power are distributed.

There are two fundamental ways to connect components: in series and in parallel. Many circuits use a combination of both.

Series Circuits

In a series circuit, components are connected one after another in a single path. Current has only one way to flow.

In series, all components share the same current. The total voltage is divided among them

Key properties of series circuits:

Current is the same everywhere. The same current flows through every component. If 1A enters the first resistor, 1A exits it, flows through the next resistor, and so on.
Voltages add up. The voltage drops across each resistor add up to the total voltage supplied.
Resistances add up. The total resistance is the sum of all individual resistances.

R_{\text{total}} = R_1 + R_2 + R_3 + \ldots

Series example: Three resistors

Suppose you have three resistors in series with a 12 V supply:

R1 = 100 Ω
R2 = 200 Ω
R3 = 300 Ω

Three resistors in series circuit

Find the total resistance:

R_{\text{total}} = 100 + 200 + 300 = 600 \text{ Ω}

We can simplify our circuit:

Simplified circuit

Find the current:

I = \frac{U}{R_{\text{total}}} = \frac{12}{600} = 0,02 \text{ A} = 20 \text{ mA}

Find the voltage across each resistor:

U_1 = I \times R_1 = 0,02 \times 100 = 2 \text{ V}

U_2 = I \times R_2 = 0,02 \times 200 = 4 \text{ V}

U_3 = I \times R_3 = 0,02 \times 300 = 6 \text{ V}

Notice that $U_1 + U_2 + U_3 = 2 + 4 + 6 = 12$ V. The voltages add up to the supply voltage.

Also notice that larger resistors get larger voltage drops. This is because the current is the same through all of them, and U = I × R. A larger R means a larger U drop.

Parallel Circuits

In a parallel circuit, all components are connected across the same two points. There are multiple paths for current to flow.

Three resistors in parallel circuit

Key properties of parallel circuits:

Voltage is the same everywhere. Every component experiences the full supply voltage.
Currents add up. The currents through each branch add up to the total current from the supply.
Resistances combine differently. The formula for total resistance is:

\frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \ldots

Or equivalently:

R_{\text{total}} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \ldots}

An important observation: the total resistance is always less than the smallest individual resistance. This is because parallel paths give current more ways to flow, reducing overall opposition.

Parallel example: Three resistors

Suppose you have three resistors in parallel with a 9 V supply:

R1 = 100 Ω
R2 = 200 Ω
R3 = 300 Ω

Three resistors in parallel circuit

Find the total resistance:

\frac{1}{R_{\text{total}}} = \frac{1}{100} + \frac{1}{200} + \frac{1}{300} = 0,01 + 0,005 + 0,00333 = 0,01833

R_{\text{total}} = \frac{1}{0,01833} = 54,5 \text{ Ω}

Simplified circuit:

Simplified circuit

Find the current through each resistor:

Since all resistors experience 9 V:

I_1 = \frac{U}{R_1} = \frac{9}{100} = 0,09 \text{ A} = 90 \text{ mA}

I_2 = \frac{U}{R_2} = \frac{9}{200} = 0,045 \text{ A} = 45 \text{ mA}

I_3 = \frac{U}{R_3} = \frac{9}{300} = 0,03 \text{ A} = 30 \text{ mA}

Find the total current:

I_{\text{total}} = I_1 + I_2 + I_3 = 90 + 45 + 30 = 165 \text{ mA}

Verify using Ohm's Law:

I_{\text{total}} = \frac{U}{R_{\text{total}}} = \frac{9}{54,5} = 0,165 \text{ A} = 165 \text{ mA}

Notice that the resistor with the smallest resistance carries the most current. This makes sense: less resistance means easier path for current to flow.

Try it yourself!

Series circuit: A 9 V battery powers three resistors in series: 47 Ω, 100 Ω, and 150 Ω.

Three resistors in series - try it yourself

Question 1: What is the total resistance?

Solution

R_{\text{total}} = 47 + 100 + 150 = \mathbf{297 \text{ Ω}}

Question 2: What is the current through the circuit?

Solution

I = \frac{U}{R_{\text{total}}} = \frac{9 \text{ V}}{297 \text{ Ω}} = \mathbf{0,0303 \text{ A} = 30,3 \text{ mA}}

Question 3: What is the voltage across the 100 Ω resistor?

Solution

U = I \times R = 0,0303 \text{ A} \times 100 \text{ Ω} = \mathbf{3,03 \text{ V}}

Parallel circuit: A 12 V battery powers two resistors in parallel: 1 kΩ and 2 kΩ.

Two resistors in parallel - try it yourself

Question 1: What is the total resistance?

Solution

\frac{1}{R_{\text{total}}} = \frac{1}{1000} + \frac{1}{2000} = 0,0015

R_{\text{total}} = \mathbf{666,67 \text{ Ω}}

Question 2: What is the current through the 1 kΩ resistor?

Solution

I_1 = \frac{U}{R_1} = \frac{12 \text{ V}}{1000 \text{ Ω}} = \mathbf{0,012 \text{ A} = 12 \text{ mA}}

Question 3: What is the total current?

Solution

I_{\text{total}} = \frac{U}{R_{\text{total}}} = \frac{12 \text{ V}}{666,67 \text{ Ω}} = \mathbf{0,018 \text{ A} = 18 \text{ mA}}

Or: $I_{\text{total}} = I_1 + I_2 = 12 + 6 = 18$ mA

Mixed (Series-Parallel) Circuits

Real circuits rarely use only series or only parallel. They mix both topologies. For example, you might have two parallel branches, each containing resistors in series.

Example 1 of a mixed circuit

Example 2 of a mixed circuit

The strategy for solving mixed circuits is to break them into smaller chunks:

Identify series and parallel groups.
Simplify one group at a time, working from the simplest sub-circuits outward.
Use Ohm's Law and power equations once you have simplified the circuit.

Mixed circuit example

Consider a circuit with a 12 V supply and two parallel branches:

Branch 1: 100 Ω and 200 Ω in series
Branch 2: 300 Ω and 600 Ω in series

Example of a mixed circuit

Step 1: Find the resistance of each branch.

Branch 1 resistance: $R_1+_2 = 100 + 200 = 300$ Ω

Branch 2 resistance: $R_3+_4 = 300 + 600 = 900$ Ω

Simplified circuit:

Simplified circuit

Step 2: Find the equivalent resistance of the two parallel branches.

\frac{1}{R_{\text{total}}} = \frac{1}{300} + \frac{1}{900} = 0,00333 + 0,00111 = 0,00444

R_{\text{total}} = 225 \text{ Ω}

Simplified circuit:

Simplified circuit

Step 3: Find the total current from the supply.

I_{\text{total}} = \frac{U}{R_{\text{total}}} = \frac{12 \text{ V}}{225 \text{ Ω}} = 0,0533 \text{ A} = 53,3 \text{ mA}

Step 4: Find the voltage across each branch (they are the same in parallel).

Both branches experience the full 12 V.

Step 5: Find the current through each branch.

I_1 = \frac{U}{R_1} = \frac{12 \text{ V}}{300 \text{ Ω}} = 0,04 \text{ A} = 40 \text{ mA}

I_2 = \frac{U}{R_2} = \frac{12 \text{ V}}{900 \text{ Ω}} = 0,0133 \text{ A} = 13,3 \text{ mA}

Verify: $I_1 + I_2 = 40 + 13,3 = 53,3$ mA

Reading Resistor Color Codes

A resistor is a physical component that you will see countless times in electronics projects. But how do you know its resistance value? It is marked with colored bands.

Resistor manufacturers use colored bands as a standardized way to label resistance values. This system dates back to the 1920s, before digital displays were common. It persists today because it works reliably and the labels are visible from any angle.

The color code system

Standard resistors have four bands (or sometimes five for precision components). The first three bands encode the value; the fourth band indicates tolerance (accuracy).

Four color bands on a resistor: first band (tens), second band (ones), third band (multiplier), fourth band (tolerance)

The color codes are:

Color	Digit	Multiplier
Black	0	10⁰ = 1
Brown	1	10¹ = 10
Red	2	10² = 100
Orange	3	10³ = 1,000
Yellow	4	10⁴ = 10,000
Green	5	10⁵ = 100,000
Blue	6	10⁶ = 1,000,000
Violet	7	10⁷ = 10,000,000
Grey	8	-
White	9	-

The tolerance band (fourth band) is typically gold (±5%), silver (±10%), or brown (±1%).

Reading a resistor: the memory aid

To memorize the color code, use this Dutch phrase:

"Zij bracht rozen op Gerrits graf bij vies grijs weer"

Breaking it down:

Zij → Zwart (Black) = 0
bracht → Bruin (Brown) = 1
rozen → Rood (Red) = 2
op → Oranje (Orange) = 3
Gerrits → Geel (Yellow) = 4
graf → Groen (Green) = 5
bij → Blauw (Blue) = 6
vies → Violet (Violet) = 7
grijs → Grijs (Grey) = 8
weer → Wit (White) = 9

(If you prefer English: "Bad Boys Respect Our Great Big Beautiful Women." But the Dutch phrase is more elegant.)

Example 1: A four-band resistor

Consider a resistor with bands: Brown, Black, Red, Gold

First band (Brown): 1
Second band (Black): 0
Third band (Red): multiplier of 100
Fourth band (Gold): ±5% tolerance

Value: 10 × 100 = 1000 Ω = 1 kΩ (±5%)

Example 2: Another four-band resistor

Consider a resistor with bands: Yellow, Violet, Orange, Brown

First band (Yellow): 4
Second band (Violet): 7
Third band (Orange): multiplier of 1,000
Fourth band (Brown): ±1% tolerance

Value: 47 × 1,000 = 47,000 Ω = 47 kΩ (±1%)

Example 3: Reading a physical resistor

You have a resistor with bands: Orange, Orange, Brown, Gold

First band (Orange): 3
Second band (Orange): 3
Third band (Brown): multiplier of 10
Fourth band (Gold): ±5% tolerance

Value: 33 × 10 = 330 Ω (±5%)

This is an extremely common value, the resistor used to limit current through LEDs on 5 V supplies.

Try it yourself!

Resistor 1: Bands are Brown, Black, Brown, Gold. What is the value?

Solution

First band (Brown): 1
Second band (Black): 0
Third band (Brown): multiplier of 10
Value: 10 × 10 = 100 Ω (±5%)

Resistor 2: Bands are Red, Red, Orange, Brown. What is the value?

Solution

First band (Red): 2
Second band (Red): 2
Third band (Orange): multiplier of 1,000
Value: 22 × 1,000 = 22 kΩ (±1%)

Resistor 3: Bands are Yellow, Purple, Yellow, Gold. What is the value?

Solution

First band (Yellow): 4
Second band (Purple): 7
Third band (Yellow): multiplier of 10,000
Value: 47 × 10,000 = 470 kΩ (±5%)

Resistor 4: Bands are Brown, Green, Red, Brown. What is the value?

Solution

First band (Brown): 1
Second band (Green): 5
Third band (Red): multiplier of 100
Value: 15 × 100 = 1,5 kΩ (±1%)

Five-band resistors

Some precision resistors have five bands instead of four. The first three bands represent the digits (instead of two), and the fourth band is the multiplier. The fifth band is the tolerance.

For example, a resistor with bands Brown, Black, Brown, Brown, Brown would be:

First band: 1
Second band: 0
Third band: 1
Fourth band: multiplier of 10
Fifth band: ±1% tolerance

Value: 101 × 10 = 1,010 Ω (±1%)

The logic is the same; there is simply one more digit encoded.

Key Takeaways

Before moving on to more complex circuits, make sure you understand these core ideas:

Voltage is electrical potential difference. It is the "push" that drives current, measured in volts.
Current is the flow of electrons. It is the rate of charge movement, measured in amps.
Resistance opposes current flow. It is a property of materials, measured in ohms.
Ohm's Law ties them together: $U = I \times R$ . If you know any two, you can find the third.
Power is the rate of energy dissipation: $P = U \times I$ (or $P = I^2 \times R$ or $P = U^2 / R$ ). Power matters for battery life and component ratings.
Series circuits share current and divide voltage. Total resistance is the sum.
Parallel circuits share voltage and divide current. Resistances combine with reciprocals.
Resistor color codes encode resistance value using colored bands. Learning the mnemonic makes reading them automatic.

These fundamentals are not just theory. Every circuit you design will depend on understanding them. The good news is that once these concepts are solid, you have the tools to analyze any circuit made of resistors and power supplies.