Current and ohm’s law

1. Drift velocity of electrons

Electrons move in the conductor randomly when no voltage applied to it and so average velocity is zero.

When a voltage applied to the conductor, the free electrons move in the certain direction and its speed is very slow ..just 0.58 cm/minute ..this speed is known as drift velocity or the electrons.

Finding out drift velocity:

Let there is a copper conductor and electric field applied across the conductor

Assumptions :

Number of free electrons : n per cubic meter of conductor.

time under consideration: dt.

Drift velocity = v.

Distance travelled by free electrons = vdt

Area of the conductor = A

Than number of free electrons = vAdt

Than charge will be dq=nAevdt

If e is the charge of each electron

So , I= dq/dt = nAevdt/dt = nAev

Current density J= I/A = nAev/A= nev

Assume J= 1.55×10^6 A/sqm

n= 10A^29 for copper conductor

e= 1.6×10^(-19) Coulomb

Putting all values in above equation

v= 0.58 cm/min

2. Resistance

Resistance is the property of a material due to which it opposes the flow of electric current (flow of electrons ) through it.

You have read above that in conductor , the free electrons are in pelnty available and these electrons move randomly but when electric voltage or field is applied across the conductor, than electrons move at drift speed of 0.58 cm/minute .

These electrons while moving collide with other atoms and electrons ..and produces heat ..

Some material offer less opposition , called metals and some offer great hindrance known as insulators ..

Around the world , conductors are used for transmission and distribution of electric energy .. Aluminium and copper are two most used metals for this. Unit of resistance is ohm. Resistance is expressed by letter ‘R’.

3. Resistance formula :

Resistance of a material depends on :

  • Proportional to length of material means resistance increses or decreases with increses or decreases in length of material respectively.
  • Inversly proptional to cross section of the material means a thicker wire has less resistance than thin wire of same material.
  • Nature of material
  • Temperature of the material

So R= K l/A

Where R is resistance

K is specific resistivity

L is length of material

A is area of material

4. Ohm’s law:

According to ohm’s law, current flowing through the conductor is proportional to the potential difference between across the conductor , provided temperature of the conductor remains constant.

I proportional V

Or I= V/R

Where R is constant , known as resistance of the conductor.

Ohms law only applicable to metals.

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  1. Sonu


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