Ohm's Law Definition and Formula


Ohm's Law Definition

The great scientist of Germany, Dr. George Simon Ohm, gave a rule in 1826. This law establishes the relationship between the charged potential difference at the ends of a conductor and the current flowing in that conductor. This law is called Om's law.

Ohm's Symbol

According to Ohm's law "If the physical states of the conductor such as length, area, volume, heat pressure, etc. remain unchanged, then the difference between the current charged at the ends of the conductor and the current flowing in it remains constant.


Om stated in his law that if the physical state is kept constant, the value of the current flowing in the conductor is proportional to the potential at its ends.
So according to Om's rules
V ∝ I
V = R I

Here R is a proportional constant, it is called the resistance of the conductor.
Hence the resistance of the conductor R = V / I
S.I. of resistance The unit is om, it is denoted by Ω.
By drawing from Ohm's law, when we draw a graph between the current and the current flowing in the conductor, this graph is obtained.

Electric current

If two different potential objects are added to a metal wire, the charge will flow from one object to another. This current of charge in a conductor is called electric current. The current flows from low voltage to high potential, but according to the tradition we assume that the current flows in the opposite direction to the flow of electrons.

That is, the direction of flow of positive charge is considered to be the direction of electric current. Despite having both magnitude and direction, the electric current is a scalar, as it does not follow the triangle law of addition. Often in solid conductors, the electric current is through electrons and in liquids both ion and electron. In semiconductors, electric current is through electrons and holes.

If the current in a circuit always flows in the same direction, we call it a direct current, and if the current flows in parallel and back and forth in alternating order, such a current is called a alternating current. To summarize the direction, D.C. And alternating current It is said. The unit of electric current is ampere.

If an electric current of 1 ampere (I) Is flowing in a conductor wire, it means that 6.25 c 1018 electrons per second are entered in that wire and the same number of electrons would exit the other end is.

The electric carrying force is required to obtain a constant current of current in an electric circuit, it is received by an electric cell or generator.


The property of a conductor that opposes the current flowing in it is called resistance. When an electric current is carried in a conductor, the moving electrons in the conductor constantly collide with the electrons, atoms and ions coming in their path, that is why resistance arises. If the voltage between the ends of a conductor is t volt and the current flowing in it is ampere.

Resistance = Potential

The unit of resistance is ohms. The resistance of a conductor depends on the following things-

  • On the nature of the conductor- The resistance of a conductor depends on the nature of its substance.
  • On the temperature of the conductor - The resistance of a conductor depends on its temperature. The resistance of the conductor increases when the temperature increases, but the resistance of the semiconductors decreases when the temperature increases.
  • Length of conductor - Resistance of a driver is directly proportional to its length. That is, the resistance of the driver increases as the length increases and the resistance of the driver decreases by decreasing the length.
  • Transverse cut area of the conductor - The resistance of a conductor is inversely proportional to the area of its transverse cut. That is, the resistance of the driver decreases as the thickness increases.

Relation between drift velocity and potential difference

  • Let PQ be the conductor of length l with a V potential applied to its ends. An electric field E is produced from the positive end Q inside the conductor towards the negative end P. Intensity of this region -
  • E = V / l. . . . . … Equation-1 
  • Each free electron of the conductor is located in this region, so the electric force exerted on each free electron -
  • F = -E.e. . . . . … Equation-2
  • if the mass of the electron is m, the acceleration generated by the electron due to the electric force is -
  • a = F / m = -E.e / m. . . .… Equation-3
  • Since the average velocity of the free electron is zero.
  • Since the initial velocity u = 0
  • Final velocity v = vd = trailing velocity Maximum acceleration achieved by electron -
  • a = -eE / m (from equation 3)
  • Time taken to collide τ Since the first equation of motion -
  • V = u + at
  • Putting the value, Vd = 0 + (-eE / m) τ
  • Vd = -eEτ / m When the value of electric field from equation -1 is - Vd = (-eτ / m) V / l
  • Magnitude of velocity | Vd | = | (-eτ / m) V / l |
  • Hence Vd = eτ / m.v / l
  • This equation shows the relationship between the trajectory velocity and the voltage.

Relation between drift velocity and electric field

Let A be the PQ driver of transverse passage and l length. Applies voltage between its ends. As the voltage is applied, each free electron of the conductor starts moving from the velocity Vd towards the positive end Q. The electron at the Q end will first leave the conductor and then the electrons behind it will continue to leave the Q end. By the time the electron at the P end is crossing the Q end, all the free electrons of the conductor must have crossed the Q end. Time taken in this action t = l / Vd If the number of free electrons in the unit volume of the conductor i.e. electron density is n, then the charge of the conductor is q = number of electrons x charge of electron q = volume x electron density x electron charge or q = Al.ne Therefore Current flowing in the conductor i = q / t = A.l.ne/l/Vd = A.ne.Vd or Vd = i / Ane This is the relation between follow and stream.

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