Ohm’s law

introduction

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).

formulas

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.

resistance-u-i

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

formule-loi-d-ohm-01

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

Examples

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:

math_989-5_a098aa1065f509ff8cbb74f830f83f79

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:

math_977_7f3c8699c6a824e1e24b17b00bd0ea08

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:

math_952_9a5ea520ffbd4a8083490a43a432d20b

The resistance value will 4kΩ  for this example.


 

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