CBSE Class 12 Physics Chapter 3 Revision Notes
Chapter 3: Current Electricity Revision Notes

Electric current is the directed flow of electric charge through any crosssection of a conductor.

If ΔQ charges flow in time Δt, current is at any time t.

It’s important to remember that current is a scalar quantity.

I is in the positive charge flow direction and opposite to the negative charge flow direction.

The ampere is the SI unit of current and is represented by A.

The current density at a point in a conductor is equal to the ratio of the current at that point in the conductor to the area of crosssection of the conductor at that point, assuming the area is kept normal to the current flow direction.

The current density is a vector variable.

Electric Charge Flow in Metal Conductors: Metals are the best conductors of electricity among the solids. Free electrons are the source of conductance.

Atoms in a solid conductor (such as Cu, Fe, Ag, and others) are tightly bound to one another. They contain a large number of free electrons.

If it’s a liquid conductor, there are positive and negative charged ions in electrolytic solution that can move when an electric field is applied.

Drift Velocity: Under the influence of an external electric field, it is defined as the average velocity with which free electrons move towards the positive end of a conductor.

Electric current as a function of velocity of drift

Current density at any conductor point,
j = nev_{d}
where j is a vector quantity.

The mobility of electrons is defined as the ratio of their drift velocity to the applied electric field.

Ohm’s Law: The potential difference V across the ends of a given metallic wire (conductor) in an electric circuit is directly proportional to the current flowing through it at constant temperature.
V∝I

The graph depicts the variation of current in relation to the applied potential difference.
V = IR, where R denotes conductor resistance.
V and I have no effect on R because as V rises, I rises as well, but R remains constant.
 Resistance: It is the ratio of the potential difference applied across the conductor’s ends to the current flowing through it in mathematics.
R = V/I
The SI unit for resistance is the ohm.
R = ⍴L/A,
where L denotes the length of the conductor, A denotes the area of crosssection, and ⍴ denotes the resistivity of the material. It is determined by the material’s nature.

Temperature coefficient of resistance

Conductivity: It is defined as the reciprocal of a conductor’s resistivity.

It is written as σ = 1/ρ
SI unit is mho per meter

Superconductivity occurs when the resistivity of a metal or alloy drops to zero when cooled below a certain temperature. Prof. Kamerlingh discovered it in 1911.

Current density (j), electric field (E), and conductivity (σ) have the relationship
j = σE.

If a conductor is stretched or compressed to n times its original length, l’ = nl => R’ = n^{2}R, where R’ represents the new resistance and R represents the original resistance.