Physics Lecture: Current Electricity

Introduction to the Chapter

• Focus on Chapter 3: Current Electricity.
• Importance of understanding every line/example from NCERT for board exams and NEET.
• Attending all NCERT series important.
  • NDA questions are based on NCERT.

Current Electricity Overview

• Starts with basic current parameters:
  • Density, Drift Speed, Relaxation Time, Resistivity, Resistance, EMF.
• Progresses to circuit problems:
  • Batteries, Resistors in combination, Wheatstone Bridge, Power.
• Current electricity is everywhere - appliances like fans, ACs, lights, torches.
• Natural phenomena like lightning involve current as flow of charges from clouds to Earth.

Electric Current

• Defined by charge flow per unit time: ( I = \frac{Q}{T} ).
• Positive current means charge flows in the assumed direction.
• Negative current means actual flow is opposite to assumed direction.

Example Scenarios

• Charges: Positive charge flow, negative charge flow, combined scenarios.
• Direction interpretation based on charge movement.

Electric Current in Conductors

• Electrons in metals are free to move when an electric field is applied, contributing to current.
• In non-conductors, electrons are bound and do not contribute to current.
• Conductors allow electron flow due to less restrictive atomic structures.

Ohm's Law

• ( V = IR ): Voltage is proportional to current through a constant resistance ( R ).
• Resistance unit: Ohm (Ω).
• Only applicable to ohmic materials (mostly conductors).

Resistance Dependence

• Proportional to length, inversely proportional to cross-sectional area:
  • ( R = \rho \frac{L}{A} )
• ( \rho ): Resistivity, material-specific property.
• Conductivity ( \sigma = \frac{1}{\rho} ).

Drift Velocity

• Electrons move with a constant average velocity due to collisions.
• Drift velocity ( v_d = \frac{eE\tau}{m} ).
  • ( \tau ): Relaxation time, average time between collisions.

Current Density

• ( J = \frac{I}{A} ): Current per unit area.
• ( J ) and Electric Field ( E ) relation: ( E = \rho J ).
• Vector forms represent directions of vectors.

Temperature Dependence of Resistivity

• Conductors: Resistivity increases with temperature due to increased collisions.
• Semiconductors: Resistivity decreases with temperature.
• Formula: ( \rho_T = \rho_0(1 + \alpha \Delta T) ).
  • ( \alpha ): Temperature coefficient of resistivity.

Practical Examples

• Solving for drift speed, resistivity, and temperature effects using given formulas and relationships.
• Understanding the instantaneous setup of electric fields versus slow drift speeds of electrons.