Current Electricity Fundamentals and Principles

May 19, 2024

Current Electricity (Class 12 Physics Chapter 3)

Introduction

  • Objective: Comprehensive coverage of current electricity - numericals, derivations, concepts, graphs
  • Feature: Aid in understanding, solve previous year questions
  • New Series: NCERT Reading Series
  • Information: Difficulty in reading NCERT, line-by-line coverage

Basics of Current Electricity

  • Previous Chapter: Electrostatics
  • Current Electricity: Moving charge
  • Electric Current: Flow of charge, current = stream
  • Current Intensity: Strength of current, I = Q/t, Unit: Ampere

Charge Carriers in Different Things

  • Metals: Free electrons
  • Electrolytes: Ions (positive, negative)
  • Gases: Free electrons and ions

Quantity of Current

  • Current: Scalar quantity (only magnitude, no impact of direction)
  • Direction: From negative to positive (real direction), from positive to negative (conventional direction)

Current Numericals

  • Turbulent format, calculation important

Current Density

  • Definition: Current per unit area
  • Formula: J = I/A
  • Unit: A/m┬▓
  • Vector quantity

Drift Velocity

  • Random motion does not lead to current flow
  • Electron drifts when potential difference is applied
  • Charge Carrier: Free electrons
  • Formula: vd = I / neA (given velocity)

Relaxation Time

  • Time between the first and second collision
  • Relaxation time decreases with increasing temperature

Relationship Between Electric Current Intensity and Drift Velocity

  • I = neAvd
  • Current Density J = n e vd

Discovery Velocity

  • Drift Velocity vs Thermal Velocity
  • th = ~10-4 m/s, thermal = 10^5 m/s

Ohm's Law

  • Electric current is directly proportional to the potential across + resistance (V = IR)
  • Resistance: Obstruction to charge flow
  • Resistivity: Resistance per unit volume (╧Б = R * A / L)
  • Resistance Unit: Ohm, Conductance: Siemens (S)

Dependence on Calibration

  • Increasing length compromises conductance (L тИЭ R)
  • Cutting the cross-sectional area increases conductance and density (A тИЭ 1/R)
  • Resistance and resistivity: Material and temperature dependent
  • R = ╧Б(L/A)

Equalizing Electrodes

  • Main: Error set by angle A is assessed
  • Positive resistance: Increasing slope
  • Negative resistance: Decreasing slope

Wheatstone Bridge Principle

  • Measurement of unknown resistance
  • P/Q = R/S
  • No current in galvanometer when current flows: Balance condition

Electric Power and Energy

  • Power: Rate of doing work (P = VI)
  • Energy: E = Pt
  • Power Unit = Watt, Energy Unit = Joule

Power Rating

  • Rating provided by the manufacturer: Rated power
  • Power Unit: Kilowatt-hour (kWh)
  • Importance of rating, description of each unit

Kirchhoff's Laws

  • Two laws: KirchhoffтАЩs current law (junction rule), KirchhoffтАЩs voltage law (loop rule)
  • Junction rule: The current that comes into a junction is equal to the current that leaves (current reaching = current leaving)
  • Voltage rule: The sum of potential differences in a closed loop is always zero

Numericals

  • Practice on examples of power, energy

Electromotive Force (EMF)

  • Not a force, but energy
  • Source of a battery or cell
  • EMF = V + Ir