Aspirant Academy

Science and Technology

Electricity and Magnetism

Physics: Motion, Work/Power/Energy, Gravitation, Light, Heat, Electricity, Magnetism, Sound, EM Waves, Medical Diagnostics, Nuclear Fission/Fusion, Radiation Safety

Paper II · Unit 2 Section 6 of 13 0 PYQs 31 min
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Electricity and Magnetism

5.1 Electric Circuits

Ohm's Law: V = IR (Voltage = Current × Resistance).

Resistance (R) in ohms (Ω) depends on material (resistivity ρ), length (l), and cross-sectional area (A): R = ρl/A.

  • Metals: resistance increases with temperature (positive temperature coefficient).
  • Semiconductors: resistance decreases with temperature.

Series and Parallel Circuits:

  • Series: Same current; R_total = R₁ + R₂ + R₃; V divides.
  • Parallel: Same voltage; 1/R_total = 1/R₁ + 1/R₂; I divides.

Household wiring uses parallel connections — so each appliance gets full mains voltage and can be switched independently.

Joule's Heating Effect: H = I²Rt (heat produced = square of current × resistance × time).

Applied in: electric heaters, electric irons, incandescent bulbs, fuses (fuse wire melts when I² exceeds threshold — circuit protection).

Power in circuits: P = VI = I²R = V²/R (watts).

5.2 Magnetism and Electromagnetic Induction

Magnetic Force on current-carrying conductor: F = BIL sinθ (B = field, I = current, L = length). Basis of electric motors — current in magnetic field → force → rotation.

Electromagnetic Induction (Faraday 1831): Changing magnetic flux through a closed loop induces an EMF. The faster the change, the greater the EMF.

Faraday's Laws:

  1. EMF is induced when magnetic flux through a loop changes.
  2. Magnitude of EMF = rate of change of magnetic flux.

Lenz's Law: The induced current opposes the change causing it (energy conservation in electromagnetic systems).

Applications of EM Induction:

  • AC Generator (Dynamo): Rotating coil in magnetic field → changing flux → AC voltage. Power stations, cycle dynamos.
  • Transformer: Changes AC voltage using mutual induction. Step-up: increases voltage (for long-distance transmission); step-down: decreases voltage (for home use). V₁/V₂ = N₁/N₂ (turns ratio).
  • Induction Motor: Most industrial motors — no brushes, low maintenance, high reliability.

5.3 Semiconductors and Band Theory (PYQ 2016)

Energy band theory:

  • Valence band: Filled with valence electrons.
  • Conduction band: Above valence band; electrons here are free.
  • Band gap (Eg): Energy difference between bands.
Material Band Gap Conductivity
Conductors (metals) 0 (bands overlap) High
Semiconductors (Si, Ge) ~1 eV Intermediate; increases with temperature
Insulators (glass, rubber) > 3 eV Very low

Doping of semiconductors:

  • n-type: Adding pentavalent impurity (phosphorus, arsenic) → extra electron (majority carrier: electrons).
  • p-type: Adding trivalent impurity (boron, aluminium) → hole created (majority carrier: holes).

p-n Junction (diode): One-way current valve.

  • Forward biased → current flows.
  • Reverse biased → current blocked.
  • Applications: rectifiers (AC → DC), transistors (amplifiers, switches), LEDs, solar cells.

Solar cell (Photovoltaic): Light photons absorbed in p-n junction eject electrons → current flows.

  • India's installed solar capacity = ~85 GW (February 2025).
  • Target: 500 GW renewable by 2030.