Methods of calculating electric current

The electric current, popularly called amperage, is one of the quantities that we have within the study of electricity. As well as the current, there are other fundamental quantities, which are also the basis of electricity, which are: electrical voltage, electrical power, and electrical resistance. Through the combination of these quantities we can find each other. We will show you step by step the methods of how to calculate electric current, in a simple way. Come on, guys!

Before going directly to the calculations, it is necessary to understand some concepts of electricity, how the electrical quantities behave in a series, parallel or mixed circuit and especially the electric current.

We intend to teach how to calculate the electric current using various methods of analysis and formulas, which can be applied in all circuits, but of course, depending on the complexity of the same, several methods and formulas must be used together.

What is electric current:

The electric current is called the ordered movement of electrons. It arises through a potential difference between two points because through those differences the electrons will flow from one point to another through a conductor.

We must know that the electric current behaves differently in the circuits, and in the series circuit the value of it will always be the same at all points since in a parallel circuit the electric current will divide inversely proportional to the value of each resistance or reactance, that is, the higher the resistance the lower the electric current will be. Throughout this article, we will explain other features that will be needed.

Ohm’s law:

Ohm’s law is one of the most important laws of electricity, and it is undeniable that the formula is applied over the electrical calculations. The importance of understanding the use of this law is undoubtedly the basis for almost all other studies and applications of electricity.
This law lists the three main electrical quantities and shows how they are connected, namely electrical voltage, electrical current and electrical resistance. Ohm’s law is very simple, when we have the value of two of these quantities it is possible to find the third variable, for that it is enough to use a formula that is: V = IR

By varying this formula, we can find the other variables, such as: I = V / R or R = V / I. whereas:

V: Electric voltage, given in volts (V).
A: Electrical resistance, given in ohm (Ω).
I: Electric current, given in ampere (A).

Another way to find the electric current is through the electric power (P), which is given in Watt (W), considering that P = VI, that is, we can find the electric current by varying this formula, being: I = P / V or with relation of the electric power formula to the voltage formula, which is I = √ (P / R).

Series circuit:

As we already know, the current in a series circuit is the same in all loads, that is, when calculating the total current in this circuit, it will be the same for all resistances contained in it. Here we will use in the examples the association of resistors in series in parallel to calculate.

Resolution 1:

Circuit with two resistors connected in series.

Let’s go to the first example, there are several ways to find the value of the electric current, one of them is to use the voltage divider method to find the voltage over one of the resistors and then use the ohm law, that way we will have the current in over the resistor, then this will be the same current in all other loads.

Another way is to use the association of resistors in series to find the total resistance (Rt) of the circuit, for that, just add the value of the two resistors, then just apply ohm law, dividing the value of the total voltage (Vt) of the circuit. by the total resistance of the circuit, this way we will have the total current (It) of the circuit and consequently the current in all loads, because it is the same in the whole circuit. This second way is here we will use, as shown in the example below:

Rt = R1 + R2
Rt = 270 + 330
Rt = 600Ω

It = Vt / R
it = 12/600
It = 20 mA or 0,02A

Resolution 2:

We have as an option to find the current of the electrical circuit the electrical power, it is widely used for calculations of dimensioning of conductors in electrical installations, because we do the lifting of the loads of the circuit and the voltage is the same in all loads, because they are all connected in parallel.

In order to perform the above circuit calculations, we need the power value of each of the loads, which is 108mW for R1 and 132mW for R2. As in the previous example, we can solve by voltage divider, finding the potential difference over the load and dividing the power by the voltage and then we find the value of the electric current.

In this case, since we have the power and resistance value in the resistors, we can calculate the electric current in only one of them, so we will have the total current of the circuit, so just use the formula I = √ (P / R).

Calculating the current in resistor 1 (IR1):

IR1 = √ (P / R)
IR1 = √ (108mA / 270)
IR1 = √0.4mA
IR1 = 0.02 or 20mA

Parallel circuit:

As seen before, the electric current in a parallel circuit is divided according to the resistance or impedance of each load, this is what happens in electrical installations, so we have equipment and circuits in the installation with different values ​​of electrical current.

Just like in the series circuit, here we can also calculate the electrical current with the ohm law and with the formulas used previously, to find electrical power, but the current of each charge should be done separately, which may give a little more depending on the situation.

Resolution 1:

Electrical circuit with two resistors connected in parallel.

This way of solving is simpler than we imagined, as the voltage is the same over all loads in a parallel circuit, that is, the same as the source.

To find the value of the current that passes through the resistor, just divide the voltage over the resistance and to find the total current of the circuit, just add the two currents, because according to the kirchhoff law for the currents, the sum of the currents that comes in a knot is the same as the sum of the outgoing currents.

Calculating the current in resistor 1 (IR1):

IR1 = V / R
IR1 = 12v / 270Ω
IR1 = 44.44mA

Calculating the current in resistor 2 (IR2):

IR2 = V / R
IR2 = 12v / 330Ω
IR2 = 36.36mA

Calculating the total circuit current (It):

It = IR1 + IR2
It = 44.44mA + 36.36mA
It = 80.8mA

It is also possible to find the total current of the circuit making the association of resistors in parallel, dividing the circuit voltage by the equivalent resistance and through the power of each resistor, as in the previous example, in the series circuit.

Current divider:

The current divider is only applied to loads that are in parallel. Using this method it is not necessary to have the voltage value at any point in the circuit or the power of each resistor, but it is necessary to have the current entering the Node and the value of both resistors in parallel.

The formula used to find the current that passes through the resistor (IR) is the current that enters the node, which in this case is the total current (It), times the resistance of the resistor contrary to what is trying to find the current value, divided by the sum of both resistors in parallel, as follows: IR1 = I.R2 / (R1 + R2).

We will use the same circuit as the previous example, and as we have the total current value, we will use it, as in the example below:

IR1 = I.R2 / (R1 + R2)
IR1 = 80.8mA.330Ω / (270Ω + 330Ω)
IR1 = 80.8mA.300Ω / 600Ω
IR1 = 44.44mA

Final considerations:

All of these calculations that have been presented should be used according to the availability of information obtained, and are used together in mixed circuits. Below is a video that shows in detail how to perform resistor association calculations in a mixed circuit.

It is important to highlight that there are other methods of analysis of circuits or theorems, in addition to the ones we use, these methods are used in more complex circuits and that vary according to what is needed. The other methods for example are: mesh and nodal analysis, superposition theorems, Thévenin and Norton.