Open circuits are as necessary as closed circuits, and the most common example on hand is the light bulb that illuminates a room, which is activated by an electrical switch or switch.
When the switch is closed, it allows the flow of electrons from the source of electricity to pass through the bulb, so that it emits light. Subsequently, the current carriers return to the source, completing a closed path.
On the other hand, when the switch is opened, the flow of electronic current ceases and consequently the bulb turns off. In short, the switch or electrical switch is a device that allows you to choose between having an open circuit or a closed one.
Characteristics of an open circuit
An open electrical circuit, by its very definition, is one that prevents a closed path for electrical current from being established. As a result of this, the open circuit has the following characteristics:
1.- There is no circulation of electric current
In a common electrical circuit, the carriers of electrical charge are the free electrons of the metal, which interconnects its electrical parts or components.
For the carriers to have a non-zero net movement, two things must happen:
- The existence of a source of electromotive force and
- A closed conductive path that begins at the source and ends at the source without interruption.
As in an open circuit the condition of an uninterrupted closed path does not exist, even if the source exists (either a battery or a generator), there is no circulation of free electrons. Therefore, the electric current is zero.
It should be remembered that in physics electric current is defined as:
Amount of charge (in coulombs) per unit time (in seconds) that passes through a cross section to the conductor that interconnects the elements of the circuit.
When 1 coulomb of charge circulates in a (closed) circuit every second, then you have 1 ampere (A) of current. In an open circuit the electric current I is equal to 0 A.
2.- Finite open circuit voltage
In an open circuit that has a source, the voltage, potential difference or voltage between the ends of the opening, is numerically equal to the voltage of the source.
This is due to the fact that since current flow in an open circuit is zero, then there is no voltage drop or loss across the elements that make up the circuit.
For this reason, the voltage between the ends of the open switch matches that of the source.
3.- Infinite open circuit resistance
As previously said, there is a voltage at the terminals of the opening of an open circuit, but since the current flowing is zero, then the resistance of the opening is infinite.
The reason is in Ohm’s law, which states that the electrical resistance of an electrical component is the quotient between the voltage between its terminals divided by the electrical current that circulates through said element:
R = V / I
But, since the current is zero amps, dividing a certain voltage value at the opening by zero circulating amps, the mathematical result tends to a very large number: infinity.
Differences between open circuit and closed circuit
- In a closed circuit, the current starts from one of the terminals of the source (battery or generator) and returns to the other terminal of the same source. On the other hand, in an open circuit this is not possible because the return path is interrupted.
- Through the elements of the closed circuit, which can be of the passive type or of the active type, the carriers of the electric current circulate. These exchange their energy with these components or elements.
The opposite occurs in an open electrical circuit, since the net movement of charge carriers is zero, there is no exchange or transfer of energy from the charge carriers to the circuit component in question. This can be a resistor, capacitor, inductor, transistor, or some other component.
3. In a closed circuit there is a voltage drop between each of the elements of the circuit, therefore each element has an electrical resistance or impedance of a finite modulus.
In particular, when the switch is closed, the resistance tends to zero. But, in an open circuit the resistance or impedance at the terminals of the open switch is undefined, or tends to infinity.
Examples of open circuit
Open circuits are very useful, as can be seen in the following examples:
Terminals of a USB charger
The terminals of a USB charger for a mobile phone constitute an open circuit as long as it is disconnected. At the moment of being connected, current begins to circulate through the mobile and then it becomes a closed circuit.
Homemade electrical outlets
Similarly, household outlets are open circuits through which no current flows unless some device is connected, for example a lamp.
Fuses are elements that are used to protect the elements of a closed circuit from excess current.
When more current flows through a fuse than it can withstand, it melts and immediately opens the circuit. In this way the integrity of the other elements of the circuit is protected, which were not designed for high values of electric current.
A battery powered flashlight while it is off is an open circuit, but as soon as it is turned on it is a closed electrical circuit.