How to find amperage through power, resistance and voltage

One of the main characteristics of an electrical circuit is its current strength. It is measured in amperes and determines the load on conductive wires, buses, or board tracks. This value reflects the amount of electricity that has flowed in the conductor per unit of time. There are several ways to define it, depending on the data you know. Accordingly, students and novice electricians because of this often face problems when solving educational tasks or practical situations. In this article, we will tell you how to find the amperage through power and voltage or resistance.

If the power and voltage are known

Let's say you need to find the current in a circuit, while you only know the voltage and power consumption. Then, to determine it without resistance, use the formula:

P = UI

After some simple ones, we get the formula for the calculations

I = P / U

It should be noted that this expression is valid for DC circuits. But when calculating, for example, for an electric motor, its full power or cosine Phi is taken into account. Then, for a three-phase motor, it can be calculated as follows:

We find P taking into account the efficiency, usually it lies in the range of 0.75-0.88:

Р1 = Р2 / η

Here P2 is the active net power on the shaft, η - efficiency, both of these parameters are usually indicated on the nameplate.

We find the total power taking into account cosF (it is also indicated on the nameplate):

S = P1 / cosφ

Determine the consumed current by the formula:

Inom = S / (1.73 U)

Here 1.73 is the root of 3 (used for calculating a three-phase circuit), U is the voltage, depending on the inclusion of the motor (triangle or star) and the number of volts in the network (220, 380, 660, etc.). Although in our country, 380V is most often found.

If voltage or power and resistance are known

But there are problems when you know the voltage on the section of the circuit and the magnitude of the load, then to find the current strength without power, use Ohm's law, with its help we calculate the current strength through resistance and voltage.

I = U / R

But sometimes it happens that you need to determine the current strength without voltage, that is, when you only know the power of the circuit and its resistance. In this case:

P = UI

Moreover, according to the same Ohm's law:

U = IR

That:

 P = I²* R

So the calculation is carried out according to the formula:

I²= P / R

Or take the expression on the right side of the expression under the root:

I = (P / R)^1/2

If EMF is known, internal resistance and load

Trick student problems include cases when you are given the EMF value and the internal resistance of the power source. In this case, you can determine the current in the circuit according to Ohm's law for the complete circuit:

I = E / (R + r)

Here E is the EMF, r is the internal resistance of the power source, R is the load.

Joule-Lenz law

Another task that can throw even a more or less experienced student into a stupor is to determine the current strength, if the time, resistance and the amount of heat generated by the conductor are known. To do this, remember Joule-Lenz law.

Its formula looks like this:

Q = I²Rt

Then carry out the calculation like this:

I²= QRt

Or root the right side of the equation:

I = (Q / Rt)^1/2

A few examples

As a conclusion, we propose to consolidate the information received on several examples of tasks in which you need to find the current strength.

1 task: Calculate I in a circuit of two resistors in series connection and in parallel connection. R resistors 1 and 2 Ohm, 12 volt power supply.

It is clear from the condition that you need to give two answer options for each of the connection options. Then, to find the current in series connection, first add the resistances of the circuit to get the total.

R₁+ R₂= 1 + 2 = 3 ohms

Then you can calculate the current strength according to Ohm's law:

I = U / R = 12/3 = 4 Amperes

When two elements are connected in parallel, Rtotal can be calculated as follows:

Rtot = (R1 * R2) / (R1 + R2) = 1 * 2/3 = 2/3 = 0.67

Then further calculations can be carried out as follows:

I = 12 * 0.67 = 18A

Task 2: calculate the current with a mixed connection of the elements. At the output of the power supply is 24V, and the resistors are: R1 = 1 Ohm, R2 = 3 Ohm, R3 = 3 Ohm.

First of all, you need to find the R total of R2 and R3 connected in parallel, using the same formula that we used above.

R priv = (R2 * R3) / (R2 + R3) = (3 * 3) | (3 + 3) = 9/6 = 3/2 = 1.5 Ohm

Now the diagram will look like this:

Next, we find the current according to the same Ohm's law:

I = U / (R1 + R priv) = 24 / (1 + 1.5) = 24 / 2.5 = 9.6 Amperes

Now you know how to find the amperage by knowing the wattage, resistance, and voltage. We hope the provided formulas and calculation examples have helped you master the material!

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