What Is a Circuit?
Introduction
Take a moment to think about any electronic device or piece of equipment containing electronics. There’s the coffee pot you use every morning, a smartphone you carry around in your pocket, the computer you use for work, a child’s battery powered toy, an electric car zooming down the interstate, robotic armatures welding together car parts in an automobile factor, a satellite orbiting earth, a robotic vehicle exploring Mars — each of these and other common electronic devices contain circuits within them.
In its simplest form, a circuit is a closed loop through which electric current flows.
Circuits are ubiquitous in our modern world and perform important tasks for the majority of us on the planet every single day, but what exactly is a circuit?
In the proceeding, we will take an unorthodox approach to learn how a circuit works and behaves — we’ll go over varying circuit situations, and observe how different circuit components react within a circuit. We’ll begin our discussion by imagining that circuits can be thought of as the racetrack analogy.
Circuits Explained Using the Racetrack Analogy
Before we begin our discussion on what a circuit is, let’s come up with an analogy we’ll use to help us understand the fundamentals and key-concepts of a circuit more easily.
Most textbooks and teachers in electronics use the analogy of water for electricity and circuits, which is fine, but I’m going to use an analogy of a multi-lane racetrack — because I’m stubborn that way 😁.
Our racetrack has no openings and no forks to go anywhere else, except around a loop. The racetrack’s start and finish are not separated, but are connected or the same position on the track — it is a completely connected loop. The idea of a completely connected loop will be an important distinction for our next discussion on the differences between an open circuit, short circuit, and a closed circuit.
Differences Between an Open Circuit, a Short Circuit and a Closed Circuit
Open Circuit
An open circuit was a closed loop that has been open in some manner — whether it be by removing or cutting a wire or conductor, removing a component from the circuit, or opening a switch (turning a switch off).
Using our analogy of the racetrack, an open circuit would be equivalent to having a construction road crew come tear out a section of the road of the racetrack. If that were to happen, the race cars could not race — the race cars would not be able to move past the gap in the road.
Short Circuit
A short circuit is when electric current is allowed to flow through a part of a circuit that has less resistance than through the components the current is intended to flow through. The current is allowed to bypass the entire rest of the circuit at the short — allowing the current to go straight to ground.
Using our analogy of the racetrack, you can think of a short circuit as being a shortcut through the racetrack, allowing cheaters in the race to drive straight to the finish-line — bypassing the rest of the track.
Closed Circuit
A closed circuit is closed loop through which electric current flows. A closed circuit means that there is no break whatsoever in the circuit — allowing current to flow from the positive terminal (+) to the negative terminal (-) of the circuit’s power supply.
A circuit is typically made up of a conductor (such as wire) that starts the loop connected to a power source (such as a terminal to a battery) that then connects to some components within the circuit (such as resistors, capacitors, inductors, and switches). Wire is then connected from the components to each other in some fashion, then on to some kind of load (such as an electric motor, light, or other component that consumes electric power). Eventually, the conductor makes its way back connecting to the opposite terminal of the battery, creating a closed circuit.
Using our analogy of a racetrack, we can think of the closed circuit as the track itself — our conductor and everything else that makes up the completed circuit. This track could contain race cars, flags, lanes, barriers or other items that we’ll think of as being analogies for circuit components.
Basic Components of Circuits
Components are devices connected within the circuit that perform specific functions to manipulate voltage and current — for the most part. To comprehend circuits fully, it’s crucial to acquaint ourselves with some of the basic components that may make up a circuit.
Resistors
A resistor is an electronic component that “resists” current flow. Resistors control the amount of current flow through a section of the circuit it is connected to.
Using the racetrack analogy, we can think of a large number of race cars on the track as being the electrons flowing through the circuit — each of them driving around at high speeds on our multi-lane racetrack. We can think of a resistor as being like having the racetrack go from several lanes down to one lane.
The race cars drive at a high speed, then come up toward the abrupt reduction in lanes where they see the flag guy waving a yellow caution flag — meaning they have to slow down to proceed through that section of narrow racetrack. Fewer cars are driving through the one lane section of racetrack at a given time than there were when the racetrack was a wide open multi-lane track. The narrowing of the track has slowed the flow of traffic.
Similarly, a multitude of electrons in a circuit come zipping along at high speed. When they encounter a resistor along their path, that path becomes resistant to their flow — fewer electrons now go through this section of the circuit at a given time (usually because some have found another path of least resistance).
Capacitors
Capacitors store electrical energy in an electric field. They are commonly used to smooth out voltage fluctuations or store energy for short bursts.
Using the analogy of the racetrack, we can think of the capacitor as being a concrete wall (barrier) that takes up all lanes of the track. The race cars can drive up to the wall, but cannot go through it, around it, or over it. The cars need to race, but now they can’t because the wall is in their way.
Let’s say that the drivers decide to reconfigure the rules and agree that to be able to race they’ll have to race in teams.
- They’ll paint half the cars red and the other half blue, to make up a red team and a blue team.
- The red team will get set at one side of the wall, while the blue team will get set at the other side of the wall.
- Their plan is to drive away from the wall in opposite directions at the same time.
- The new rules of the game states that each team will have their own lanes.
- The red team will take up lanes 1, 2, and 3 — while the blue team will use lanes 4, 5, and 6.
The object of this new race is that:
- Each team will take-off at the same time when a green flag is waved — racing in opposite directions trying to have each car from each team reach the opposite ends of the wall before the other team.
- Once each car arrives at its opposite side they must slow down and a checkered flag is waved.
- Each car must then stop and wait for every one of his teammates to reach the wall.
- Once everyone has reached their opposite sides of the wall a red flag is waived — each team must turn around and each car must reset again.
- While the red flag is waiving, no one can take-off towards the opposite side of the wall.
- Once everyone has reset, everyone must wait to take-off until the green flag is waved again.
The new race rules and concrete wall barrier prevents any sudden changes in the amount of cars leaving the wall at each start of the race.
The teams change their direction on the closed racetrack, racing from one side of the wall, then back to the other side — alternating their direction each time. This analogy is meant to visualize how electrons might behave when they encounter a capacitor in a circuit.
A capacitor is made up of two plates, separated by a dielectric. The dielectric is either air or some other material that is impervious to electric current flow, to an extent.
The concrete wall (barrier) on the race track acts as the capacitor and dielectric in the circuit. Either side of the wall on the track acts as a plate, whereas the concrete within can be thought of as the dielectric preventing the cars from driving through it.
In AC circuits (alternating current), electrons in a circuit flow toward a plate on one side of a capacitor in a cycle of the current flow — creating a negative charge on one plate of the capacitor and a positive charge on the opposite plate of the capacitor. Then in the next cycle, turn around and go toward the opposite plate of the capacitor — essentially reversing the polarities of the plates of the capacitor. This alternation continues over and over — up to 60 cycles-per-second for the Unites States power supply coming from a typical wall outlet.
A capacitor resists any sudden changes in voltage potential across its plates. This is due to the build up of charges on opposing plates — these charges essentially being equal to one another, yet opposite in charge during any given resting state.
What this means is the voltage across a capacitor, for the most part stays constant. This is what is meant by capacitors smoothing out voltage fluctuations.
For AC current flow, the electrons flow toward a plate on one side of the capacitor for one cycle, then back toward the opposite plate of the capacitor on the next cycle — alternating back-and-forth each cycle.
Inductors
Inductors (also known as coils or chokes) store energy in a magnetic field. They are vital in applications involving magnetic fields, such as transformers and motors.
Using the analogy of the racetrack, we can think of an inductor as being a large oil slick on the racetrack. When the car’s tires come in contact with the oil slick — no matter how much more acceleration the driver tries to give, the tires keep on spinning and the car doesn’t pick up any more speed. If the driver tries to stop on the oil slick, the car just keeps on going — its speed doesn’t change or slow.
The speed of the car as it drove on the road is the same as it drives over the oil slick. The car can neither accelerate or stop, because the oil keeps the car sliding uncontrollably at its original speed and direction (considering the condition to be a friction-less surface).
The oil slick resists any sudden changes in acceleration to the car as it drives over it. Similarly, an inductor resists any sudden changes in current. In other words, the inductor (due to its build up of a magnetic field) does not allow the current to change its rate of flow.
The oil slick in our racetrack analogy acts as the inductor in the circuit. The oil slick does not allow a race car to accelerate or slow down because of it being so slippery to the tires of the car. The inductor does not allow the flow of current to deviate because of its magnetic field.
Switches
Switches are the gatekeepers of a circuit, allowing the flow of current through a circuit or blocking the flow of current in a circuit. Switches come in various forms, from simple mechanical switches to advanced semiconductor devices.
Using our racetrack analogy, you can think of a switch as being the guy who waves the red and green racing flags. The cars cannot go unless the flag guy is waving the green flag and the drivers know when to stop when he’s waving the red flag.
Similarly, the electrons can flow through the circuit when the switch of the circuit is closed (turned ON). The movement of the electrons in the circuit cease when the switch of the circuit is opened (turned OFF).
Circuit Classifications
Circuits can be classified into different types called series or parallel, or a combination of the two — each serving a specific purpose. Let’s explore each of these briefly:
- Series Circuits: Series circuits are when components within a circuit are connected one after the other, forming a single pathway for current flow. The current remains constant throughout the circuit, but the voltage may vary across each component.
- Parallel Circuits: Parallel circuits feature multiple pathways for current to flow — with each component connected separately. Unlike series circuits, the voltage remains constant across each component, but the current may vary.
- Combination Circuits: Combination circuits combine elements of both series and parallel circuits. They are often employed in complex electronic systems to achieve specific functionalities.
Furthering Your Understanding
I would be hard pressed to let you know that if you’re serious about furthering your understanding of electronic circuits, then exploring the basic principles that govern their behavior — including Ohm’s Law, Kirchhoff’s Laws, and the concept of power sources would be beneficial for you.
Getting your hands on a good electronics book for beginners, like Electronics Fundamentals: Circuits, Devices & Applications by Thomas Floyd, or another book of his titled Electronics Fundamentals: A Systems Approach, would provide you a more in depth understanding of circuits than this article offers. I own and have used both books, and they provide great explanations, graphics and charts on the fundamentals of circuits, and both take you deeper into the basic theory and applications of circuits.
Where to go from here?
If this short article on the basics of a circuit has increased you appetite for knowledge of electronics, then I suggest that you explore the basic principles that govern circuit behavior which includes Ohm’s Law, Kirchhoff’s Laws, and the concept of electrical power sources.
Get yourself a good electronics book for beginners, like the ones mentioned above by Thomas Floyd. If you’re interested in some other electronics books based on age and skill set, then check out our post here where I recommend books not only on electronics, but on robotics and programming as well — many of which I own myself and have used!
Final Thoughts
Circuits are everywhere around us. They quietly power the devices we use every day, from the simplest gadgets to the most advanced machines. In this post, we used a simple racetrack analogy to make circuits easier to understand, explored the components that make them work, looked at different types of circuits, and talked about practical ways you can continue learning.
Now that you have this foundation, you may start to see the world a little differently. The next time you pick up a device, flip a switch, or hear a motor start, you’ll know there’s a circuit at work behind the scenes — guiding electricity exactly where it needs to go.
And this is only the beginning. Every circuit you build, every component you test, and every project you try brings you one step closer to truly understanding electronics. So stay curious, keep experimenting, and remember — keep at it and stay motivated.
