Current Electricity Lecture Notes

Chapter Overview

Three units are covered in this chapter:

• Unit A: Fundamental concepts
• Unit B: Circuit elements and combinations
• Unit C: Electrical energy, power, and household consumption

Unit A: Basic Concepts

Charge, Current, Potential, and Resistance

• Charges: Transference of charges when two non-conductors are rubbed together
• Example: Glass rod (loses electrons) and silk cloth (gains electrons)
• SI Units: Coulomb (C). Smaller units include millicoulomb (mC), microcoulomb (µC), nanocoulomb (nC)
• Charge Calculation: Total charge Q = n * e (n = number of electrons, e = charge of one electron)*

Electric Current

• Definition: Flow of electric charge over time, I = Q/t
• SI Unit: Ampere (A). Smaller units: milliampere (mA), microampere (µA), nanoampere (nA)
• Measurement Device: Ammeter (connected in series)
• Flow Direction: High potential to low potential

Ohm's Law and Resistance

• Ohm's Law: V = IR (Voltage = Current x Resistance)
• Resistance: Abstraction given to flow of charges, depends on material nature, temperature, length, and area of cross-section
• SI Unit: Ohm (Ω)
• Factors Affecting Resistance: Nature of material, temperature, length, and area of cross-section

Conductance and Resistivity

• Conductance (G): Reciprocal of resistance
• SI Units: Siemens (S), Ω^{-1}
• Resistivity (ρ): R = ρ(L/A) where ρ depends on material nature and temperature
• SI Unit: Ωm
• Conductivity (σ): Reciprocal of resistivity
• SI Unit: S/m

Choice of Materials

• Applications:
  • Electrical connections: Low resistance, high melting point (e.g., copper, aluminum)
  • Standard resistors: High resistance, temperature-independent (e.g., manganin, constantan)
  • Fuses: High resistance, low melting point (e.g., lead, tin)
  • Filament wires: High resistance, high melting point (e.g., tungsten)
  • Heating elements: High resistance, high melting point (e.g., nichrome)

Superconductivity

• Definition: Zero resistance at very low temperatures
• Example: Mercury below 4.2K

Unit B: Circuits and EMF

Electromotive Force (EMF) and Terminal Voltage

• EMF: Work done to move charge around complete circuit (open condition), W/Q
• Terminal Voltage: Potential difference in a closed circuit
• Relation: EMF = V_terminal + V_drop
• Internal Resistance: Resistance within the electrolyte

Combination of Resistors

• Series Combination: Rs = R1 + R2 + R3 + ... , Current is the same, voltage divides
• Parallel Combination: 1/Rp = 1/R1 + 1/R2 + 1/R3 + ..., Voltage is the same, current divides

Unit C: Electrical Energy and Power

Electrical Energy

• Calculation: W = VQ, W = VIt, W = I²Rt, W = V²t/R
• SI Unit: Joule (J)
• Commercial Unit: Kilowatt-hour (kWh)

Electrical Power

• Definition: Rate of electrical energy consumption, P = W/t
• Formulas: P = VI, P = I²R, P = V²/R
• SI Unit: Watt (W)

Calculation of Cost of Electrical Energy

• Cost: Total energy consumed (in kWh) x Cost per unit

Heating Effect of Current

• Joule's Law of Heating: H = I²Rt
• Factors Affecting Heat Production: Current, resistance, and time

Study Tips

• Regularly review textbooks and take notes for each section
• Practice example problems and exercise questions