Ohm’s law


Ohm’s law is a very important physical law in the electrical field. This law connects three elements: the value of resistance (in ohms), the current flowing through it (in amps) and the voltage across its terminals (in volts).

The three elements are represented by the letters U, R and I:

  • U: voltage across the resistance, expressed in volt (symbol: V).
  • R: resistance value expressed as ohm (symbol: Ω).
  • I: current flowing through the resistance, expressed in Ampere (symbol A).


The attached image shows a dipole (here a resistor) through which an electric current. The electrical resistance (R) is other than the black rectangle, the current (I) is shown in red and the potential difference (V) is green.


For the above arrangement, the formula of Ohm’s law is:


Note that this last formula can be written according to the current:
formule-loi-d-ohm-02Or you could write a function of the resistance: formule-loi-d-ohm-03


Calculate the voltage

To calculate the voltage across a resistance of 1kΩ which is traversed by a current of 12mA, it is necessary to perform the following calculation:


The voltage across such a resistor will be 12V.

Calculate the current

The calculation necessary to determine the current flowing through a resistor 3kΩ and is Form submitted to a voltage of 5V is the following:


The current calculated for this example is equal to 1.66mA.

Calculate the resistance

It is possible to calculate the value of a resistance by knowing the voltage across and the current through it. In the case of a potential difference of 9V and a current of 2.25mA, the calculation to be performed is as follows:


The resistance value will 4kΩ  for this example.


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Post Author: gyoby

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