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 cross-section 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 cross-section 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 cross-section, 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’ = n²R, where R’ represents the new resistance and R represents the original resistance.

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