## What is an Electric Circuit?

An electric circuit is a path that transmits electric currents. Circuits include devices such as batteries or generators that provide energy to the charged particles that form the current; or devices such as lamps, motors, or computers that consume currents. Interconnection between electrical elements is known as electric circuits.

### Electric Circuit Diagram

A simple electric circuit is shown below:

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## Components in an Electric Circuit

We know different components are needed to build an electrical circuit. To understand the electrical circuit and the flow of Current, one must understand these components and their respective symbols and functions in an electrical network. The essential components of an electric circuit can be listed in the following categories.

### Supply

An electric power supply converts Current from a power source, such as the mains, into voltage and Current needed to power loads, such as motors and electronic devices.

A power supply supplies Current and voltage to a wide variety of loads in a controlled manner and at a precise voltage - typically simultaneously, often without changing the input voltage or affecting the output in other connected devices.

- DC supply

In a Direct current power supply, DC voltage is provided continuously even though electrical power is transmitted, generated, and disturbed by AC voltage.

Examples of DC supply - Batteries and Regulator Circuits

- AC supply

An AC power supply converts alternating Current (AC) power from an electrical source into voltage, Current, and frequency required by the load. The power input may be AC or DC. A DC signal is achieved using a step-down or step-up on voltages and a filter.

Examples of AC supply - Power distribution systems, Solar power systems, UPS or generators

### Current path

The path through which the current flows and drives the load is called an electric current path. There must be a complete path to transfer power between the source and the load. This path offers some resistance to the current flow and is also where different network elements are connected. We will see these elements later in the article.

### Load

Electric load refers to electrical components or paths of circuits that consume electricity (active), such as household appliances and lights.

Examples of Load - Motor, Bulb, heater, appliance

### Indicator

An indicator only serves the purpose of indicating the working condition of the circuit. For example, an ON indication means the circuit is complete, and a path exists for the current to flow through the circuit, while an OFF indication represents a break in the current path. The break can be a disjoint, an open circuit, or a damaged device. This helps in troubleshooting circuit faults.

Examples of Indicators - LEDs, Sirens, or Buzzers

### Circuit Breaker

Circuit breakers are electrical switches that interrupt current flow after monitoring relays detect a fault in an electrical circuit that could cause damage to the circuit. They are used to protect circuits against damage caused by over-current/ overload or short circuits.

Examples of Circuit breakers - are MCB (Miniature circuit breaker), Fuse connections.

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## Elements in an Electric Circuit

An electric Circuit is the interconnection of elements. Joining them via an electric path will give us a complete circuit.

### Classification of Elements

The circuit elements can be classified based on their nature and relational behavior concerning applied voltage and current or applied power.

### Active and Passive Elements

Active Elements are elements that drive an electric circuit. They are the power generators delivering the power to a circuit. The source of the circuit is an Active element. These sources can be independent or dependent.

- Independent - provides a constant voltage to the circuit, irrespective of the Current flowing through the terminals. Examples - Batteries, Power sources
- Dependent - provides Current to the circuit, depending upon its voltage. Examples - Transistors, Operational Amplifiers

For example, voltage source, current source, transistors, etc.

Passive Elements can be defined as elements that can control the flow of electrons through them. They either increase or decrease the voltage. The passive elements in an electric circuit are Resistors, uncharged capacitors, and inductors.

**Note:** From an element's characteristics, the element will be active if the voltage to Current ratio is negative at any point. Otherwise, it will be passive.

### Bidirectional and Unidirectional Elements

When the properties and characteristics in an element are independent of the direction of current flow, then the element is called a bidirectional element.

When the properties and characteristics depend on the direction of current flow, the element is called a unidirectional element.

**All linear elements are bilateral, but the reverse is not true.**

**Note:** From the characteristics of an element, when the graph is similar in opposite quadrants, the element is bidirectional; otherwise, it is unidirectional.

### Linear and Non–linear Elements

When the relationship between excitation and response is linear, then that property of an element is linear.

This property is a combination of the homogeneity property and additivity property.

- The homogeneity property is a property in which when the input is multiplied by a constant. The output should also get multiplied by the same constant.
- The additivity property requires the response to a sum of input is the sum of the responses to each input applied separately.

Some examples of different types of elements can be seen from the V-I characteristics.

## Classification of Electric Circuit

As we have seen that electric circuits constitute interconnected elements. A complete circuit can be classified into several categories -

### Classification of Electric Circuit Based on Connections

**Open Circuit**

A circuit with no return path for Current to flow (i.e., which is not completed) is known as an open circuit. In other words, an open circuit where voltage tends to the EMF (of generating source) and no current flows at all is called an open circuit.

**Closed Circuit**

A circuit with a return path for Current to flow (i.e., a completed circuit) is known as a closed circuit.

A circuit with a return path for Current flows where the value of resistance = zero. (i.e., completed or closed circuit without connected load) is known as a short circuit. In other words, A circuit where voltage tends to zero and Current tends to infinity is called a short circuit.

**Short Circuit**

A short circuit is a closed circuit with a return path for Current without a load attached. In other words, it is a circuit where the voltage tends to zero, and the current tends to infinity.

### Based on Arrangements

**Series Circuit**

As the name implies, these circuits have a single path for electricity travel, e.g., a voltage or current source, inductor, capacitor, resistor, etc., connected in series.

**Parallel Circuit**

The elements of these circuits (voltage and current sources, inductors, capacitors, resistors, etc.) are connected in parallel, i.e., there is more than one path for traveling electricity.

**Series-Parallel Circuit**

The combination of series and parallel circuit elements is called a series-parallel circuit.

**Star-Delta Circuit**

A delta or star connection circuit has electrical elements that are not in series, parallel or series-parallel arrangement. For example, Star delta circuits can be solved by Star to Delta and Delta to Star transformation.

Below we can see different network or circuit connection topologies.

### Classification of Electric Circuit Based on Supply

**AC Circuit**

A circuit containing an AC supply voltage source is known as an AC circuit. The supply sources, for example, are alternators and synchronous generators.

**DC Circuit**

A circuit containing a DC supply voltage source is known as a DC circuit. The DC power supply example is batteries and DC generators.

### Based on the nature of Elements

**Active Circuit**

A circuit containing one or more EMF (Electromotive force) sources is called an Active circuit.

**Passive Circuit**

There is no source of EMF in a passive circuit.

**Linear Circuit**

Linear circuits are electric circuits whose parameter values remain constant, such as resistance, inductance, capacitance, waveform, and frequency. In addition, the linear word circuit refers to a circuit whose parameters don't change concerning Current or voltage.

**Non-Linear Circuit**

Electric circuits with non-linear parameters have varying voltages and currents. In other words, a non-linear circuit is one in which the characteristics of the circuit (resistance, inductance, capacitance, waveform, frequency, etc.) are not constant.

**Unilateral Circuit**

A unilateral circuit allows current to flow only in one direction regardless of the direction of supply voltage or Current. A diode or rectifier is an example of a unilateral circuit because it does not perform the rectification in both directions of supply.

**Bi-lateral Circuit**

When the current flows in both directions, in a bilateral circuit, the property of the circuit is unchanged regardless of the direction of the supply voltage or Current. The transmission line is the best example of a bilateral circuit because the circuit properties remain constant, providing the supply voltage from any direction (starting or finishing).

## Theorems to Solve Electric Circuit

### Thevenin's Theorem

In Thevenin's equivalent circuit, as per Thevenin's theorem, an independent and dependent 2-terminal linear and bilateral network can be represented as a simplified equivalent circuit.

In Thevenin's equivalent circuit, the voltage source, V_{th}, is connected to the resistance, R_{th}, in series. Thus, there is a practical voltage source based on the series combination of voltage source and resistor. So, it would be appropriate to say that Thevenin's equivalent circuit is nothing but a practical voltage source.

### Norton's Theorem

According to Norton's theorem, any 2-terminal linear bilateral network can be represented using a simplified circuit called Norton's equivalent.

As Norton's equivalent circuit, we have Norton's current source I_{N}, parallel with Norton's resistance, R_{N}. This parallel combination provides a practical current source. So, it would be appropriate to say that Norton's equivalent circuit is nothing but a practical current source.

### Superposition Theorem

As per the Superposition theorem, in any linear and bilateral network or circuit having multiple independent sources, the response of an element will be equal to the algebraic sum of the responses of that element by considering one source at a time.

### Millman's Theorem

In Millman's theorem, circuits with parallel voltage sources and internal resistance can replace voltage sources connected in series.

### Tellegen's Theorem

Based on the law of energy conservation, Tellegen's theorem states that various elements in various branches consume the same amount of power as the amount delivered.

^{b}Σ_{b=1}[v_{b}i_{b}] = 0

Where b represents the number of branches, and vb and ib are the branches' current and voltage values.

## Laws of an Electric Circuit

There are various Kirchoff laws that are used in electric circuits to solve complex circuits. Two major laws of Kirchoff are explained below.

### Kirchoff's Current Law

According to Kirchoff's current law, currents entering a closed boundary or a node are equal to zero.

If there are N number of branches connected to a node and it is the Current of the nth branch, then mathematically, KCL states,

^{N}∑_{n=1}i_{n }= 0

**Current leaving=current entering**

### Kirchoff's Voltage Law

Based on Kirchoff's voltage law, a circuit's algebraically summated voltages around a closed path are zero. If there are M number of voltages in a loop and Vm is the mth voltage, then mathematically, KVL can be written as:

^{M}∑_{n=1}v_{m }= 0

**Sum of voltage drop=sum of voltage rise**

## Terms Related to Electric Circuit

We need to know some terminologies to understand the electric circuit better. These terms cover the elements and parameters of such circuits.

**Conductor-**A substance or material that allows electrons, or electrical Current, to flow through it.**Insulato**r-Any material that will not allow electricity to flow through easily.**Current-**The movement or flow of electricity through a conductor.**The capacitance-**The ability of a component to store an electrical charge.**The circuit-**The path followed by a flow of electric Current.**Electricity-**The flow of electrons.**Generator-**A machine which converts mechanical energy into electrical energy.**Ground-**An electrical connection to the earth.**Switch-**An electrical component used for connecting, breaking, or changing the connections in an electrical circuit.**Load-**An electrical device or devices that use(s) electric power.**Resistance-**The resistance to the flow of electricity through a material.**Capacitance-**The property of an electric conductor is measured by the amount of charge that can be stored.**Inductance-**Property of an electric conductor to oppose the change in electric Current flowing through.**Current-**The flow of electrons is referred to as Current.**Voltage-**The potential difference between two terminals**Power-**Energy used to do work measured in watts.**Node-**A point or junction where two or more circuit elements (resistor, capacitor, inductor, etc.) meet is called Node.**A branch-**A part or section of a circuit between two junctions is called a branch. For example, an element with two terminals can be connected to another element through a branch.**Loop-**A closed path in a circuit where more than two meshes can occur is called a loop. There can be many meshes in a loop, but there is only one Mesh in a loop.**Mesh-**A closed loop with no other loop within it or a path that does not contain other paths is called Mesh.

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