# Reciprocity Theorem | What is Reciprocity Theorem?

By Mohit Uniyal|Updated : May 4th, 2022

Reciprocity Theorem: The theorem that we use for solving the given electrical network/circuit is known as Network Theorem / Circuit theorem. Pair of terminals is known as a port. If the network has 2 such ports, then it is known as a 2-port network. The reciprocity theorem is one of the important Network theorems and it plays a vital role in 2 port networks.

In this article, first, you will get to know what the theorem is and the statement of the Reciprocity theorem. Then, you will get to know how to apply the Reciprocity theorem for solving an electric circuit/network problem.

## Reciprocity Theorem Statement

In any linear and bilateral network or circuit having only one independent source if the ratio of response to excitation is constant even though the source is interchanged from the input terminals to output terminals. Since we are having two basic electrical quantities like voltage & current, we will get 4 possible ways regarding excitation and response of the electrical network in terms of voltage & current. Now let’s discuss how to apply the Reciprocity theorem in the following 2 cases.

• Reciprocity Theorem to an electric circuit
• Reciprocity Theorem to a 2-port network

## Reciprocity Theorem to an Electric Circuit

In the following circuit, VS is the input voltage and the current, and I flowing through the resistor R3 is the response or output current.

In the following circuit, we interchanged the position of input voltage and output current. In this circuit, VS is the input voltage and the current, and I flowing through the resistor R1 is the response or output current.

In the first circuit, whatever the amount of current that flows through resistor R3, the same amount of current that will flow through resistor R1 in the second circuit because the magnitude and polarities of voltage, VS are the same in both circuits & the output current direction is same in both circuits.

## Reciprocity Theorem to a 2-Port Network

As the name implies, a 2-port network consists of 2 ports namely port 1 and port 2. In the following 2 port network, the voltage source, VS is connected to port 1. This is the input voltage of the 2-port network. The current, I2 flows through port 2 is the response or output current of the 2-port network.

In the following 2-port network, the voltage source, VS' is connected to port 2. This is the input voltage of the 2-port network. The current, I1' flows through port 1 is the response or output current of the 2-port network.

According to the Reciprocity theorem,

I1'VS'=I2VS

If VS'=VS, then we will get I1'=I2.

## Reciprocity Theorem Questions

Q1. Find the current, I of the circuit shown in fig(b) by using the Reciprocity theorem.

Ans: 1.5 A

Q2. Find the current, I of the 2-port network shown in fig(b) by using the Reciprocity theorem.

Ans: 0.2 A

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## FAQs on Reciprocity Theorem

•  If any electrical network satisfies the Reciprocity theorem, then it is known as a Reciprocal network. All the electrical networks which contain passive elements are always Reciprocal networks because they satisfy the Reciprocity theorem.

•  No, we can apply the Reciprocity theorem directly to the circuits which contain only one independent source. So, if the circuits contain multiple independent sources, then first we can apply the Superposition theorem & then Reciprocity Theorem.

• Yes. We can apply the Reciprocity theorem to the AC circuit just like the way we applied the Reciprocity theorem to the DC circuit. If the AC circuit consists of linear and bilateral elements and only one independent source, then we can apply the Reciprocity theorem.

• We can’t apply the Reciprocity theorem if the network is having non-linear and/or unilateral elements. Similarly, we can’t apply the Reciprocity theorem if the network has a dependent source because the dependent source makes the network active.

• Yes. Antennas will satisfy the Reciprocity theorem. Hence, we can use the same antenna as either a transmitting antenna or receiving antenna based on the requirement. For effective communication, there should be impedance matching between the transmitter and receiver.

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