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We can say that in an electric circuit if the potential difference applied between its two points is equal to 1 volt, and the partial resistance of the section between these two points is 1 Ohm in this stretch circulates the current of 1 ampere.

Ohm's law states very simply the relationships between the three following electrical quantities: voltage (V), current (I) and resistance (R)

This law was enunciated by the famous German physicist George Simon Ohm, and is certainly the most important of those relating to electricity.

The statement sounds exactly like this:

*"The intensity of current in a circuit is directly proportional to the voltage applied to it and inversely proportional to the resistance of the circuit itself."*

Its mathematical expression is:

**I = V / R**

which allows to calculate the current knowing the voltage and resistance. Derived from this formula:

**V = I * R**

**R = V / I**

which allow to determine the voltage or resistance when the other two are known quantities. If the circuit is applied to a single f.e.m. (Electromotive force) value of E, we see that the formula of Ohm's law becomes the following:

**I = E / (R + r)**

where "r" is the internal resistance of the generator. If we consider the circuit with a single resistor, and assuming that the potential difference between terminals A and B has the value V, the current flowing in the resistance R will be:

**I = V / R**** **

Whereas the other hand the circuit with two resistors fed by a generator emf E and the internal resistance r, if R1, and R2, are the external resistors or load connected in series, we will have:

**I = E / (R1 + R2 + r)**

which yields

**E = I (R1 + R2 + r) = I R1 + I R2 + I r.**

The products I R1, I R2, and I r (-current resistors) respectively expressing the potential differences existing between the points (AC) and (CB), as well as the internal voltage drop of the generator.

We can see that the f.e.m. And applied to the circuit is equal to the sum of the differences of potential partial, that are also called "voltage drops".

The IR1 and IR2 voltage drops, occur in the external circuit, and can produce a useful effect. The voltage drop Ir is the case inside the generator, and has no utility.

Suppose now that the switch is open: no current in the circuit and since I = 0, the internal voltage drop will be null and ddp between the two terminals A and B of the generator will be equal to the emf of the generator itself: VAB = E.

If instead the circuit is closed and circulates a current I, between A and B will have a potential difference (ddp)

**VAB = E - I * r**

Another case in which condition occurs VAB = E is when the internal resistance of the generator is zero (r = 0).

Even though most of us know and properly use the "Ohm's Law," we must not forget that there are people starting out that despite knowing the existence of this law do not know use it in practice so as to derive much advantage as possible.

We refer to the simulator for any examples and applications.