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

Contents

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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 a break in an electric circuit so there is no complete loop. Current in that path is zero. A closed circuit is the opposite: a complete path. An open switch, a cut wire or a loose terminal are everyday opens.

The two sides of the break are disconnected, so current cannot cross the gap. If a source is still connected, a voltage can appear across those open terminals. Call that the open-circuit (Thevenin) voltage, not a drop through a conducting load.

Current through the open branch is zero. Voltage across the open is not always non-zero: a dead battery or a fully isolated pair of wires can read 0 V.

Power in a branch is $V \times I$ . With I = 0 in the open branch, that branch dissipates no power.

Other closed branches in the same system can still draw power. An open lamp does not prove the whole installation is at zero watts.

In circuit models an ideal open is infinite resistance. The next section is that model, not a meter reading of true infinity.

Open Circuit Resistance

A linear resistor follows Ohm’s law: voltage proportional to current. The stored lines below are that algebra.

$V = IR$

$R = \frac{V}{I}$

Set I = 0 in that model:

$R = \frac{V}{0}$

$R = \infty$

The stored V/0 step is a teaching limit, not a legal arithmetic operation. The model says R → ∞. A real open is a very large resistance, limited by insulation and air.

Open Circuit vs Short Circuit

Open and short are opposite ideal cases of two terminals.

Open means the terminals are not joined by a conducting path. Short means they are joined by a path of (ideally) zero resistance.

Open branch: I = 0. Ideal short across an ideal voltage source: current is unbounded in the model. A real short is limited by source impedance, wire resistance and the protective device.

Ideal open: R = ∞. Ideal short: R = 0. A real short still has a small resistance.

Ideal open at the source terminals: V equals the source (Thevenin) voltage. Ideal short: V = 0 across the short. Real shorts show a small voltage.

A closed circuit is the normal loop through the load. Conventional current leaves the positive terminal and returns to the negative. (Electron flow is the other way.) The stored wording had that polarity backwards.

The stored figure compares closed, open and short. Captions on that image still say “close circuit.”

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

Open Circuit Example

Common causes are a cut conductor, a blown fuse, a tripped breaker or an open switch. Any break in the loop stops current in that loop.

Lamp, battery and switch:

open circuit example — Open Circuit Example

An open switch breaks the loop, so the lamp draws no current.

A closed switch completes the loop, so lamp current flows.

A different fault: join the battery terminals with no load and you have a short. Current is then limited only by the battery and the shorting path, unlike the open-switch case above.