# What Is Circuit Theory?

Circuit theory is a set of techniques used describe the flow of energy around an electrical loop. The theory is comprised of a number of different laws, ideas, and definitions. These include Ohm’s law and Kirchhoff’s lawa, which describe the relationship between current, voltage, and resistance. In some cases, the techniques may also refer to hydraulic or pneumatic circuits, which involve fluid and gas respectively. An electrical circuit is formed using a number of components, such as batteries, wires, capacitors, resistors and switches.

Electrical current, at a microscopic level, is the flow of electrons around a circuit. Electrons are negatively charged particles. This is the fundamental reason why "work," i.e., the name for energy given to or performed on an object in physics, can be transmitted by an electric circuit. Circuit theory is a collection of laws and techniques that allow people to devise circuits for specific goals and purposes.

Any circuit is built using two types of components. These are called the “active” components, which supply the energy, and the “passive components” which use the energy. In the case of electronic circuit theory, the active components are usually batteries or generators.

There are a number of passive electric components which are important in circuit theory. A basic “test” circuit is usually made up of a battery, a passive component such as a lamp, and a variable resistor. The variable resistor can be adjusted at any time, which allows for the current through the component to be varied. Other components that are commonly used in electric circuits include diodes, which allow electricity to flow in only one direction, and capacitors, which can store an electrical charge and discharge it when required.

A cornerstone of circuit theory is Ohm’s law, which is a simple formula for working out the relationship between current, voltage, and resistance in a circuit. The formula is written as "current equals voltage divided by resistance." In other words, the resistance of a conductor is independent of the current. This formula is often used in the techniques to determine how much resistance might be required to achieve a certain level of current.

Kirchhoff’s laws are also an important part of circuit theory. There are several laws, each describing conservation of energy and charge in a circuit. Conservation laws are among the most important in physic as a whole, and Kirchhoff’s laws apply these general principles to electronic circuits.

## Discussion Comments

I would love to know more about the theory of electronics. I have spent twenty years in the field but do not feel I know much on the theory part, but in the practical, I am good, because I can follow instructions and get the aim of a project.

In theory I would not mind starting from the elementary level, so I would like someone to help me.

@MrMoody - Everyone who has ever dimmed the lights in a room knows a little something about electronic devices and circuit theory, whether they realize it or not.

That light dimmer is actually a variable resistor. It varies the amount of current going to the lamps and that’s what causes the lamps to dim on or off.

Most people also understand the concept of a charging a battery, too. So there you have it – batteries, charges, resistance and current, the basic concepts behind circuit theory.

@everetra - Most of the guys at my workplace understand circuit theory very well. I don’t think it’s that hard, but some of the guys do get into calculus for some of the applications. That’s where I get lost.

I work for a software company that caters to the energy industry. We do stuff with protective relays and so forth so we need to track how well circuits “ramp” up and down in strength for example, and I think that’s where some of the higher math comes in. But as I said essential circuit theory is easy to understand.

@nony - Circuit design is a science in itself, I would agree. Today we have more useful tools however for those who want to study the theory.

For example, I’ve found circuit simulators online. These applications let you “draw” your circuit and then test it, in a virtual environment. Since there is a direct relationship between current, resistance and voltage, the applicaton correctly calculates how your circuit is working.

You can see the numbers on the screen and tweak your circuit as needed until you get the desired results. Then when you’re ready you can build the circuit by hand and it will work perfectly as designed.

When I got started in electronics years ago I learned some of the concepts behind the components but not the theory unfortunately. I was just interested in throwing circuits together and building stuff.

I knew about Ohm's law, because one of the devices I needed was an Ohm meter. I think they call it something else nowadays. But mainly I just knew how to wire components together by following a circuit diagram.

I was able to build a lot of neat things before I really understood a lot of the theory, but in hindsight I think that theory is very important. It takes you beyond circuit construction to circuit design.

If you can master theory then you can come up with your own circuits rather than just following someone else’s example. Ultimately of course you can build your own electronic inventions, which is the ultimate payoff.

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