Short Circuit

A short circuit occurs when a current flows through an electric circuit without resistance. If electrons flow without being impeded in some way, excess heat will be caused by the friction of these electrons moving in the conductive material.

Too much heat will eventually damage the circuit, perhaps through fire or worse. Short circuits occur often when current flows through an unintended path, perhaps due to a flaw in the design, with low electrical impedance.

The Principles

When current flows through a circuit it must encounter some resistance — electrical resistance that impede the flow of electrons so the current doesn’t get out of hand — or else it will short circuit.

In a simple lamp circuit, current provides electric power to the light-bulb in the lamp, resulting in a voltage drop along the circuit by using some of the power. If the bulb is turned off, the switch that turns the lamp off opens the circuit, breaking it completely and leaving no pathway for electrons to flow. So there is no electric current.

After the electrons leave the lamp they have lost a certain amount of energy, kinetic energy. They are moving more slowly. Thus there is also a slight voltage drop after the lamp in the circuit. This resistance is enough to keep current in check.

If we omit the lamp from the circuit, so that in our case there is no load at all with the terminals of the battery being connected to one another by copper wires, electrical energy flows directly through the circuit without being hindered significantly at all. As electrons move from negative to positive terminal of the battery with no voltage drop at all, excess heat is produced in the wire due to the friction of a torrent of electrons pushing their way through the atomic structure of the wire.

If our power source is a battery this excess heat at the terminals of the battery can damage the battery and effect the chemicals in the battery, perhaps causing an explosion, or else damage the wires in the circuit.

A More Technical Perspective

In more complex circuits, a short circuit occurs when electricity flows along an unintentional path. It is one thing to attach a wire directly from one end of the battery to another with nothing between. Only so much thinking goes into such an operation, though it allows us to visualize the principles.

However, circuits in modern technology are far more complex and sensitive. Thus short circuits can occur in more subtle ways when electricity moves in a way that it wasn’t designed to. If a large current flows into an area or loop of a circuit that was not designed for current of that magnitude, it can have unintended consequences, and damage the components in that loop.

From a more technical perspective, a short circuit is the creation of a new path of least resistance for current to flow by the unintended connection of two points — nodes — giving them the same voltage. Thereby omitting the path with more electrical resistance that we intended for the electricity to flow, resulting in the device not functioning properly, or at all. Like shoving a knife in the toaster, the electricity intended for the toaster goes up the knife and into the person, because there is no resistance for that path.

A short circuit is a connection between two nodes of an electrical circuit that were intended to be at different voltages. This forces them to be at the same voltage, and a current flows between them larger than anticipated (limited only by the Thevenin equivalent resistance of the rest of the network) which can cause overheating, damage, fire.

Short Circuit and Ohm’s Law

Ohm’s Law must be considered when designing circuits. Ohm’s law, $V=IR$ gives the voltage as current times resistance. If our circuit has too much resistance built in then we will diminish the current (because greater resistance will lower the current more significantly since current is voltage over resistance, $I=V/R$) which may not deliver enough electric power for the loads to function as intended.

On the other hand, if you don’t provide enough resistance, you might overload a circuit. If the resistance is reduced relative to an equivalent voltage from the power source, the current $I$ rises.

All circuits have a level of current and power they are designed to handle. Overload a circuit and you might produce excessive heat due to friction in the material caused by an elevated quantity of electrons flowing, which might be enough heat to cause explosions or fire.

Resources

1. What is a Short Circuit? MIT. <https://engineering.mit.edu/engage/ask-an-engineer/what-is-a-short-circuit/>.
2. Short Circuit. Wikipedia. <https://en.wikipedia.org/wiki/Short_circuit/>.