Open Circuit: What is it? (And How Does it Differ To a Short Circuit)

what is an open circuit
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Key learnings:
  • Open Circuit Definition: An open circuit is defined as a state in an electrical system where no current flows due to a break in the circuit, maintaining a non-zero voltage across its terminals.
  • Current Flow: In open circuits, the flow of current is zero because the electrical path is interrupted.
  • Voltage Presence: Despite no current flow, open circuits can still exhibit a voltage difference between terminals.
  • Resistance Characteristic: The resistance of an open circuit is infinitely high since no current passes through, resulting in a calculation of voltage divided by zero.
  • Comparison with Short Circuit: Contrasting with open circuits, short circuits have negligible or zero resistance and allow a high amount of current to pass, often leading to dangerous conditions.

What is an Open Circuit?

An open circuit is defined as a condition in an electric circuit where the current does not flow. This occurs when there is no continuous path—referred to as a “closed circuit.” A break in any part of the circuit results in an open circuit, stopping the flow of current.

In an open circuit, the terminals are disconnected, breaking the circuit’s continuity. Although this prevents current from flowing, a voltage drop still exists between two points in the circuit.

Hence in an open circuit, the current flowing through the circuit is zero, and voltage is present (non-zero).

Now Power is equal to V \times I, and the current is equal to zero.

Hence power is also equal to zero, and no power dissipates from an open circuit.

The resistance of an open circuit is explained in detail further below.

Open Circuit Resistance

The behavior of a resistor is given by Ohm’s law. The voltage across the resistor is proportional to the current. Hence, the equation of ohm’s law is,

    \[ V = IR \]

    \[ R = \frac{V}{I} \]

In an open circuit condition, the current is zero (I = 0).

    \[ R = \frac{V}{0} \]

    \[ R = \infty \]

Hence, for any value of voltage, the resistance is infinite in open circuit conditions.

Open Circuit vs Short Circuit

In the basics of electrical engineering, open circuit and short circuit are two special configurations with opposite behavior.

Both concepts represent the connection of two terminals of the circuit. So, the question is what is the difference between an open circuit and a short circuit?

In open-circuit conditions, the current passing through the circuit is zero. While in the case of short circuit conditions, a very high amount (infinite) of current passing through the circuit.

The resistance between two terminals of open circuits is infinite. And the resistance between two terminals of shorts circuits is ideally zero. But practically there is very low resistance.

The voltage across terminals of the open circuit is equal to supply voltage. And in the short circuit, the voltage across short circuit terminals is zero.

When a circuit is running in normal condition and the current passing through the components, this condition is known as a close circuit. The current only flows when a close path is created. In a close path, a current will flow from the negative to the positive polarity of the voltage.

The differences between a closed circuit, an open circuit, and a short circuit are illustrated in the figure below.

close circuit open circuit and short circuit
open circuit, short circuit, and close circuit

Open Circuit Example

In most cases, open circuit causes due to breaking in a conductor. If the circuit is not closed and there is a break in anywhere in the loop, the current cannot flow. It creates an open circuit condition.

For a simple example, consider a bulb connected to a battery via a switch. The circuit diagram is provided in the figure below.

open circuit example
Open Circuit Example

When a switch is open, it will break a path. It means the current cannot flow in this condition. And this is an open circuit condition.

But when you close a switch, it will make a close path. And some amount of current will flow through the load (bulb). This is a normal condition or closed circuit.

Due to any conditions, if positive and negative terminals of a battery are connected, the current has a close path to flow. But there is no load (resistance). Hence, it causes a short circuit and a high amount of current flow.

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