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Electronics Circuits Basics: Voltage, Current, Resistance and the Laws That Tie Them Together

Every electronic device, from a pocket calculator to a substation protection relay, is built from circuits, and every circuit obeys the same handful of rules. Learn those rules properly once, and schematics stop looking like spaghetti and start reading like sentences. This guide covers the foundations: what voltage, current and resistance really mean, Ohm's law, Kirchhoff's two laws, the three basic passive components, and series and parallel circuits, with worked examples you can check with a calculator.

One note before we start: this guide stays at low, safe voltages for learning. The examples use batteries and bench supplies of a few volts to a few tens of volts. Working on mains-powered equipment is a different subject with its own safety rules, and nothing here is a substitute for those rules or for your project specifications. If you want to go from theory to building, our Arduino projects for beginners guide applies exactly these ideas at safe low voltages, and our schematics guide shows how the same circuits look on paper and in simulation.

Voltage, current and resistance: the three quantities

Voltage, measured in volts (V), is the electrical pressure that pushes charge around a circuit. Think of it as the height of a water tank: the higher the tank, the more pressure at the tap. A battery or power supply creates that pressure difference between its two terminals. Current, measured in amperes (A), is the flow of charge itself, like the water actually flowing through the pipe. Resistance, measured in ohms (Ω), is whatever opposes that flow: a resistor, a lamp filament, a length of wire, or the internal resistance of a component. The three are linked, and the link is the most useful equation in electronics.

Power, measured in watts (W), is the rate at which energy is delivered or converted, and it matters because it decides whether a component survives. A resistor that has to dissipate more power than its rating allows will overheat and fail, even if the voltage and current look reasonable on their own. Whenever you design anything, check power as well as voltage and current. The AI tools for engineers guide shows how engineers use software to double-check exactly these calculations before committing to hardware.

Ohm's law: V = I × R

Ohm's law states that the voltage across a resistor equals the current through it multiplied by its resistance: V = I × R. Rearranged, it gives the other two forms you will use constantly: I = V / R and R = V / I. If a 12 V battery drives 0.02 A (20 mA) through a resistor, the resistance must be 12 / 0.02 = 600 Ω. If you know any two of the three quantities, you can find the third, and that is most of circuit analysis for simple circuits.

Power follows from Ohm's law too: P = V × I, which can be rewritten as P = I² × R or P = V² / R. A resistor carrying 0.02 A at 12 V dissipates 0.24 W, so a standard quarter-watt resistor would be running at its limit and a half-watt part would be the sensible choice. That habit, checking the power rating after every current calculation, is what separates a circuit that works on the bench from one that works for years.

Kirchhoff's laws: what goes in must come out

Kirchhoff's current law (KCL) says that at any junction in a circuit, the total current entering equals the total current leaving. Charge is not created or destroyed at a junction, so if 100 mA arrives at a junction and splits into two branches, the two branch currents must add up to 100 mA. This is the rule you use to work out how current divides in parallel circuits.

Kirchhoff's voltage law (KVL) says that around any closed loop, the sum of all voltage rises and drops equals zero. Walk around a loop adding up battery voltages as rises and resistor voltage drops as falls, and you must end back at zero. In practice this means the supply voltage is shared out among the components in a loop: in a series circuit, the resistor voltages add up to the supply voltage, which is exactly how you analyse voltage dividers and LED circuits. Together, Ohm's law and Kirchhoff's laws can solve any circuit made of resistors and supplies, however complicated it looks at first glance.

Resistors: the workhorse component

A resistor limits current and drops voltage, and it does so predictably: double the voltage across it and the current doubles. Resistors in series add up directly, so 100 Ω + 200 Ω + 300 Ω in series behaves as a single 600 Ω resistor, and the total supply voltage divides across them in proportion to their values. Resistors in parallel combine differently: the reciprocal of the total equals the sum of the reciprocals, so two 100 Ω resistors in parallel give 50 Ω, and the current splits between them in inverse proportion to their values.

Practical points matter as much as the maths. Resistor values follow standard series, so you will buy 150 Ω or 220 Ω rather than 173 Ω, and you round your calculated value to the nearest standard value and re-check the current. Tolerance tells you how far the real value may sit from the marked value, and temperature coefficient tells you how much it drifts as it warms up. For precision circuits those two specifications matter; for an LED or a pull-up resistor they rarely do. Always check the power rating, and leave margin: a resistor run at its rated power runs hot, and hot components age fast.

Capacitors: storing charge, blocking DC

A capacitor stores charge on two plates separated by an insulator. Its capacitance, measured in farads (F), with microfarads (µF) and picofarads (pF) the practical units, tells you how much charge it stores per volt. A capacitor blocks direct current once charged, because no charge can cross the insulator, but it passes changing signals, which makes it essential for coupling, filtering and timing. In a power supply, a large capacitor smooths the ripple after rectification. In a timing circuit, a capacitor charging through a resistor creates a predictable delay.

The time constant of a resistor-capacitor pair, τ = R × C, is one of the most useful numbers in electronics. After one time constant, a charging capacitor reaches about 63% of the supply voltage, and after about five time constants it is effectively fully charged. That single idea explains filter cut-off frequencies, debounce timing, and why a capacitor across a supply rail is called a decoupling capacitor: it supplies short bursts of current locally, keeping the rail steady when a chip switches. Capacitor voltage rating is a hard limit, never exceed it, and electrolytic capacitors are polarised: connect them backwards and they can fail violently.

Inductors: the dual of the capacitor

An inductor stores energy in a magnetic field around a coil of wire. Its inductance, measured in henries (H), with millihenries the common practical unit, tells you how strongly it opposes changes in current. Where a capacitor opposes changes in voltage, an inductor opposes changes in current, and that symmetry makes inductors the other half of filters, power supplies and tuned circuits. Switch-mode power supplies, radio circuits and motor drives all depend on inductors.

The behaviour that catches beginners out is back electromotive force: when current through an inductor is interrupted suddenly, the collapsing magnetic field generates a voltage spike that can be many times the supply voltage. That is why a diode is placed across a relay coil or a motor, to give that spike a safe path, and why switching inductive loads needs care in design. Inductors also have resistance in their windings, so a real inductor is an ideal inductor in series with a small resistor, and that resistance matters in power circuits where it wastes energy as heat.

Series and parallel: the two ways to connect things

In a series circuit there is only one path for current, so the same current flows through every component and the voltages divide. In a parallel circuit each branch connects across the same two points, so every branch sees the same voltage and the currents divide. Most real circuits mix both, and the standard technique is to simplify step by step: combine series groups and parallel groups until the circuit reduces to a single loop, solve it, then work back out to find each branch voltage and current.

A voltage divider is the most common series circuit of all: two resistors across a supply, with the output taken from their junction. The output voltage equals the supply voltage multiplied by the lower resistor divided by the total. Dividers are perfect for sensing and for setting reference voltages, but poor for powering loads, because anything connected to the output changes the division. That distinction, sensing versus powering, is a recurring theme: a circuit that works beautifully unloaded can collapse the moment a load is attached, and recognising why is a key engineering instinct.

Worked examples: check these yourself

Example 1, series circuit: a 12 V battery in series with 100 Ω, 200 Ω and 300 Ω. Total resistance is 600 Ω, so current is 12 / 600 = 0.02 A, or 20 mA. Voltage drops are 2 V, 4 V and 6 V, which add up to 12 V, exactly as Kirchhoff's voltage law requires. Power in the 300 Ω resistor is 0.02² × 300 = 0.12 W, comfortably within a quarter-watt rating.

Example 2, parallel circuit: a 12 V battery across two branches, 100 Ω and 200 Ω. Each branch sees the full 12 V, so currents are 120 mA and 60 mA, totalling 180 mA from the battery. Combined resistance is 1 / (1/100 + 1/200) = 66.7 Ω, and 12 / 66.7 = 180 mA, confirming the result by both routes. Always check a parallel calculation both ways when learning; the two routes agreeing is a strong sign the answer is right.

Example 3, the LED circuit every beginner builds: a 5 V supply, an LED with a forward voltage of 2 V, and a target current of 20 mA. The resistor must drop the remaining 3 V at 20 mA, so R = 3 / 0.02 = 150 Ω. Power in the resistor is 3 × 0.02 = 0.06 W, so a quarter-watt resistor is fine. If the LED is too dim, the current is too low; if it is hot or fails, the current was too high, and the resistor value is the first thing to check. This one calculation is the gateway to almost every practical project in our Arduino beginner projects, and the same thinking scales up to the PCB design stage, where every trace and component placement starts from these same numbers.

Common mistakes and how to avoid them

The most common beginner mistake is confusing series and parallel, especially with capacitors, where the rules reverse: capacitors in parallel add directly, while capacitors in series combine by reciprocals. The second is forgetting that a meter affects the circuit: a voltmeter in parallel draws a little current, and an ammeter in series adds a little resistance, so measurements on high-resistance circuits can mislead. The third is unit errors, mixing milliamps with amps or kilohms with ohms, which throw answers out by a factor of a thousand. Writing units at every step of a calculation, not just at the end, prevents nearly all of these errors.

The final habit to build is verification. Simulate before you build, measure after you build, and compare the two. Our schematics and simulation guide walks through exactly that loop with a free simulator, and it is the single most effective way to turn textbook laws into working hardware. Electronics rewards the person who checks twice and solders once.

Frequently asked questions

Next steps: build something with these laws in our Arduino projects for beginners, learn to draw and check your circuits in reading electrical schematics, and see how the same fundamentals scale to board layout in PCB design with AI. For checking calculations with software, see AI tools for engineers.