Open Circuit vs Short Circuit – Full Explanation

Why do we need to differentiate open circuit vs short circuit? Both open circuit and short circuit are the extreme conditions of resistance in the circuit.

What is an Open Circuit?

What does open circuit mean?

Just like an “open” means, there is a “hole”, “not-closed”, or in the electrical term, “disconnected”. An open circuit is an electrical circuit when there is a disconnected terminal or point.

When we learned about electrical current, one important thing to make an electrical charge flow in the circuit is that the circuit must be closed from head-to-tail.

To simply put, an open circuit is when the circuit has a broken electrical path.

The open circuit has zero current flowing in the circuit but it still has a voltage drop between two disconnected points.

Assume that we have a circuit with a two-points terminal.

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An open circuit is a pair of terminals that are externally disconnected from each other. Because they are not connected then no current flows through them.

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Open Circuit Resistance

An open circuit voltage is the voltage measured across a pair of terminals when no current flows through it (no current drawn or supplied).

Let’s rollback to the Ohm’s law theory where we use it to calculate the resistance in the circuit.

From Ohm’s law:

    \begin{align*}R=\frac{V}{I}\end{align*}

We can leave the voltage V alone and we put I = 0 since there is no current flowing in the circuit.

Thus,

    \begin{align*}R=\frac{V}{0}=\infty\end{align*}

We can do another approach using Ohm’s law to prove that there is no current flowing in an electrical circuit with an open path.

    \begin{align*}I=\frac{V}{R}\end{align*}

Where R will be infinity as stated in the equation before, then

    \begin{align*}I=\frac{V}{\infty}=0\end{align*}

What can an open circuit cause?

We can safely say that an open circuit doesn’t cause serious damage to the circuit or to its surroundings. But be safe not to accidentally connect the open terminal with anything, especially your body parts.

One thing to be wary of is that very high voltage on the open terminal can produce arcs through vacuum gaps.

Open Series Circuit

When a series circuit has an open terminal then the current is unable to flow through the circuit. An example is a flashlight with its switch opened. It is not different from turning off your flashlight.

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Open Parallel Circuit

When a parallel circuit has an open terminal on one of its branches then the current will flow to another branch. Of course this makes the current in the closed branch have higher value than if all the branches are closed.

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What is a Short Circuit

The opposite of an open circuit is a closed circuit. A closed circuit is a circuit where there is no disconnected point, disconnected terminal, or broken current path.

What does a closed circuit look like?

A closed circuit is an electrical circuit without any broken electric current’s path. The simplest example of this circuit is a flashlight with its switch closed.

open circuit vs short circuit 5

So what is a short circuit?

Short circuit is a closed circuit where there is no load in that circuit.

When we are talking about no load, it means there is only a source with a conductor wire (for example) connecting its positive polarity to its negative polarity.

Assume that we have a circuit with a two-points terminal.

open circuit vs short circuit 1

 

A short circuit is a pair of terminals that are externally connected to each other. Because they are connected directly only with a conductor wire then there is zero voltage across them.

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Above is the short circuit diagram.

Short Circuit Resistance

A short circuit current is a current flow through a pair of terminals with zero voltage difference.

In an ideal illustration, a short circuit has zero resistance since there is no load installed in the circuit.

But in the real circuit, it still has very low resistance from the conductor wire.

Assume that R = 0, then

    \begin{align*}I=\frac{V}{R}=\frac{V}{0}=\infty\end{align*}

What can a short circuit cause?

Short circuits cause high current flowing in the circuit and may damage it. Not only that, if it has a high voltage source then it will produce a large amount of heat or even a fire. Your electrical devices can be broken and any conductor wires can be melted.

Keep in mind that in the practical field, a conductor wire has non-zero resistance. It can be very low resistance but not absolutely zero.

Short Series Circuit

A short series circuit is not different with a closed switch. It will act as a conductor wire to carry electrical currents.

open circuit vs short circuit 7

 

Short Parallel Circuit

Opposite from the short series circuit, a short parallel circuit means we put a conductor wire in a parallel connection in the circuit.

Remember that current will likely choose a path with the least resistance as its flowing path. The lower the resistance in a branch, the more current will flow through that branch.

If we use an ideal short circuit then the resistance on that branch will be zero and the current will be infinite.

open circuit vs short circuit 8

 

Using parallel resistance formula:

    \begin{align*}R_{eq}&=\frac{R_1\times R_2}{R_1+R_2}\\&=\frac{0\times R}{0+R}\\&=0\end{align*}

Open Circuit vs Short Circuit

Both open circuit and short circuit can be considered as circuit failures. But two of them have different characteristics.

Both open circuit and short circuit represent a terminal with a “failure”.

In an open circuit, the current flowing in the circuit is zero. On the other hand, in a closed circuit, the current flowing in the circuit is very high (infinite).

The resistance in an open circuit is very high (infinite), while a short circuit has very low resistance (zero).

The voltage drop across the open circuit terminal is equal to the voltage source. Across the short circuit terminal, the voltage drop is zero since the resistance is zero.

The summary between open circuit vs short circuit can be observed from the figures below.

 

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Open Circuit and Short Circuit Applications

Even though these two are considered harmful, we can make use of these extreme conditions to make something.

An ideal voltmeter is an open circuit which has very large resistance (not infinite).

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An ideal ammeter is a closed circuit which has very low resistance (not zero).

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