Electrical engineering basics

Electrical engineering basics is a beginner-friendly learning guide to the core ideas behind electric circuits. Electrical engineering is the study and design of systems that use electricity, from tiny circuits inside a phone to the power grid that lights a city. This page explains the key quantities, the most useful formulas, common components, and how to start building simple projects safely. It is written for students, hobbyists, and anyone curious about how electrical devices work.

Charge, voltage, current, and resistance

Four quantities describe almost every circuit.

Charge is a basic property of matter. Electrons carry negative charge, and charge is measured in coulombs (C).
Voltage is the electrical "push" between two points. It is measured in volts (V). A common battery provides 1.5 V, and a USB port provides 5 V.
Current is the rate at which charge flows through a circuit. It is measured in amperes, or amps (A). One amp equals one coulomb of charge passing a point each second.
Resistance is how strongly a material opposes the flow of current. It is measured in ohms. Copper wire has very low resistance, while rubber has very high resistance.

A helpful comparison is water in a pipe. Voltage is like water pressure, current is like the amount of water flowing, and resistance is like a narrow section of pipe that limits the flow.

Ohm's law

Ohm's law connects voltage, current, and resistance:

V = I x R

Here V is voltage in volts, I is current in amps, and R is resistance in ohms. The formula can be rearranged as I = V / R or R = V / I.

Worked example: A 9 V battery is connected across a 450 ohm resistor. The current is I = 9 / 450 = 0.02 A, which is 20 milliamps (mA). If the resistor were replaced with a 900 ohm resistor, the current would drop by half to 0.01 A, or 10 mA.

Power

Power is the rate at which electrical energy is used or converted into heat, light, or motion. It is measured in watts (W):

P = V x I

Worked example: A 12 V lamp draws 2 A of current. Its power is P = 12 x 2 = 24 W.

By combining this with Ohm's law, power can also be written as P = I x I x R or P = V x V / R. For the 9 V battery and 450 ohm resistor above, P = 9 x 0.02 = 0.18 W. A standard small resistor is rated for 0.25 W, so it would handle this load, although it would become slightly warm.

Series and parallel circuits

Components can be connected in two basic ways.

Series circuits

In a series circuit, components are connected end to end, so the same current flows through each one. The total resistance is the sum of the individual resistances:

R total = R1 + R2 + R3 + ...

Example: A 100 ohm resistor and a 220 ohm resistor in series give a total of 100 + 220 = 320 ohms. With a 6.4 V supply, the current is 6.4 / 320 = 0.02 A.

Parallel circuits

In a parallel circuit, components are connected across the same two points, so each one has the same voltage. The current splits between the branches. The total resistance is found with:

1 / R total = 1 / R1 + 1 / R2 + ...

Example: A 100 ohm resistor and a 400 ohm resistor in parallel give 1 / R total = 1/100 + 1/400 = 0.01 + 0.0025 = 0.0125, so R total = 80 ohms. The total resistance of a parallel group is always smaller than its smallest resistor.

Household outlets are wired in parallel, which is why each device receives the full supply voltage and one device can be switched off without affecting the others.

AC and DC

Direct current (DC) flows in one direction only. Batteries, solar cells, and USB chargers supply DC. Most electronic circuits run on DC.
Alternating current (AC) reverses direction many times per second. Mains electricity is AC, at 60 hertz (Hz) in North America and 50 Hz in many other regions. AC is used for power distribution because its voltage can be raised or lowered easily with transformers, which reduces losses over long distances.

Phone chargers and laptop power adapters convert mains AC into low-voltage DC.

Components

These are some of the building blocks found in circuits. Older equipment used vacuum tubes, which were later largely replaced by transistors.

Vacuum tubes: glass tubes that control the flow of electrons through a vacuum, used in early radios, computers, and some audio amplifiers today.
Transistors: semiconductor devices that act as switches or amplifiers, and the foundation of modern computer chips.
Resistors: limit current and divide voltage, marked with colored bands that show their value.
Capacitors: store energy in an electric field and smooth out changes in voltage.
Diodes: allow current to flow in only one direction. A light-emitting diode (LED) also gives off light.
Inductors: coils of wire that store energy in a magnetic field and resist changes in current.
Switches: open or close a circuit to turn current off or on.

Reading a schematic

A schematic is a diagram that shows how components are connected, using standard symbols instead of pictures. A resistor is drawn as a zigzag line or a small rectangle, a battery as a pair of long and short parallel lines, and a ground connection as a set of horizontal lines that get shorter. Straight lines represent wires. A dot where two lines cross means the wires are joined, and lines that cross without a dot are not connected. Each component is usually labeled with a name such as R1 or C2 and a value such as 220 ohms or 10 microfarads. Practicing with simple schematics, such as a battery, resistor, and LED, is a good way to build confidence.

Tools

Multimeter: measures voltage, current, and resistance. It is the most important tool for a beginner and is useful for finding faults.
Breadboard: a reusable board with rows of connected holes that lets you build circuits without soldering.
Bench power supply: provides an adjustable, steady DC voltage with a current limit, which protects both the user and the circuit. A battery pack is a simple alternative for small projects.

Safety

Electricity can cause burns, fires, and serious injury. Beginners should work only with low-voltage projects, such as circuits powered by batteries or USB at 12 V or less. Never work on mains wiring, wall outlets, or household electrical panels unless you are qualified and permitted to do so. Disconnect power before changing a circuit, watch for components that become hot, and be careful with large capacitors, which can hold a charge after power is removed. Lithium batteries should never be short-circuited or punctured. This page is for educational purposes only and is not a substitute for professional training or local electrical codes.

Where to go next

After learning the basics, there are many directions to explore:

Digital logic: how transistors form logic gates that make up computers.
Signals: how audio, radio, and sensor signals are measured, filtered, and processed.
Power systems: how electricity is generated, transmitted, and distributed.
Microcontrollers: small programmable computers such as Arduino boards or the Raspberry Pi Pico, which are a fun way to combine circuits with code to control lights, motors, and sensors.

Glossary

Ampere (amp): The unit of electric current.
Capacitance: The ability of a component to store charge, measured in farads.
Circuit: A closed path through which current can flow.
Ground: A common reference point in a circuit, treated as zero volts.
Ohm: The unit of resistance.
Resistance: Opposition to the flow of current.
Semiconductor: A material, such as silicon, whose ability to conduct can be controlled.
Short circuit: An unintended low-resistance path that lets too much current flow.
Volt: The unit of voltage, or electrical potential difference.
Watt: The unit of power, equal to one volt times one amp.

External links