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/m²
• 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 (ρ = R * A / L)
• Resistance Unit: Ohm, Conductance: Siemens (S)

Dependence on Calibration

• Increasing length compromises conductance (L ∝ R)
• Cutting the cross-sectional area increases conductance and density (A ∝ 1/R)
• Resistance and resistivity: Material and temperature dependent
• R = ρ(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: Kirchhoff’s current law (junction rule), Kirchhoff’s 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