Physics (PGT) — Study Notes for HTET Level 3
Overview
Physics at the PGT level for HTET covers the entire Class XI-XII NCERT syllabus, testing both conceptual depth and problem-solving ability. The four major domains—Mechanics, Electromagnetism, Optics, and Modern Physics—carry roughly equal weightage, with Mechanics and Electromagnetism typically forming the bulk of numerical questions.
For HTET PGT, you must demonstrate mastery equivalent to teaching senior secondary students. Questions test not just formula recall but also the ability to apply concepts to unfamiliar situations. Dimensional analysis, unit conversions, and sign conventions are frequently tested traps. A strong grasp of vector operations, calculus-based derivations, and circuit analysis is essential.
Success requires balancing breadth (covering all chapters) with depth (solving numericals from each unit). Focus on NCERT examples and back-exercises—most HTET questions are adapted from these or follow similar patterns.
Key Concepts
Mechanics
- Newton's laws form the foundation—identify forces correctly using free-body diagrams before applying F = ma.
- Work-energy theorem and conservation laws (energy, momentum, angular momentum) solve problems faster than kinematics when constraints exist.
- Rotational motion parallels translational motion: torque ↔ force, moment of inertia ↔ mass, angular momentum ↔ linear momentum.
- Simple harmonic motion is characterized by restoring force proportional to displacement; all SHM quantities derive from ω = √(k/m).
Electromagnetism
- Electric field and potential are related: E = −dV/dx; field lines are perpendicular to equipotential surfaces.
- Kirchhoff's laws (junction and loop rules) solve any resistor network; assign current directions consistently.
- Magnetic force on a moving charge is always perpendicular to velocity—it changes direction, not speed.
- Faraday's law (induced EMF = −dΦ/dt) and Lenz's law govern all electromagnetic induction phenomena.
Optics
- Sign convention (New Cartesian) must be applied consistently—distances measured from pole/optical centre, real is positive for mirrors (opposite for lenses).
- Wave optics: interference requires coherent sources; diffraction occurs at edges/slits; path difference determines fringe position.
Modern Physics
- Photoelectric effect proves particle nature of light; de Broglie wavelength proves wave nature of matter.
- Nuclear binding energy per nucleon peaks at iron—fusion releases energy for lighter nuclei, fission for heavier.
Formulas / Key Facts
Mechanics
| Concept | Formula |
|---|---|
| Equations of motion | v = u + at; s = ut + ½at²; v² = u² + 2as |
| Projectile (range) | R = u²sin2θ / g |
| Centripetal acceleration | a = v²/r = ω²r |
| Work-energy theorem | W_net = ΔKE = ½mv² − ½mu² |
| Moment of inertia (rod, centre) | I = ML²/12 |
| Moment of inertia (disc, centre) | I = MR²/2 |
| SHM time period (spring) | T = 2π√(m/k) |
| SHM time period (simple pendulum) | T = 2π√(l/g) |
| Gravitational potential energy | U = −GMm/r |
| Escape velocity | v_e = √(2gR) = √(2GM/R) |
Electromagnetism
| Concept | Formula |
|---|---|
| Coulomb's law | F = kq₁q₂/r² (k = 9 × 10⁹ Nm²/C²) |
| Electric field (point charge) | E = kq/r² |
| Capacitance (parallel plate) | C = ε₀A/d |
| Energy in capacitor | U = ½CV² = ½QV = Q²/2C |
| Ohm's law | V = IR |
| Power | P = VI = I²R = V²/R |
| Resistors in series/parallel | R_s = R₁+R₂; 1/R_p = 1/R₁ + 1/R₂ |
| Magnetic force on current | F = BIL sinθ |
| Biot-Savart (long wire) | B = μ₀I / 2πr |
| Faraday's law | EMF = −dΦ/dt |
| Transformer | V_s/V_p = N_s/N_p (ideal) |
Optics
| Concept | Formula |
|---|---|
| Mirror/lens formula | 1/v + 1/u = 1/f (mirror); 1/v − 1/u = 1/f (lens) |
| Magnification | m = −v/u (mirror); m = v/u (lens) |
| Lens maker's formula | 1/f = (n−1)(1/R₁ − 1/R₂) |
| Young's double slit fringe width | β = λD/d |
| Single slit first minimum | a sinθ = λ |
Modern Physics
| Concept | Formula |
|---|---|
| Photoelectric equation | KE_max = hν − φ (φ = work function) |
| de Broglie wavelength | λ = h/p = h/mv |
| Bohr's radius | r_n = 0.53 n² / Z Å |
| Energy levels (H-atom) | E_n = −13.6 Z²/n² eV |
| Radioactive decay | N = N₀ e^(−λt); T_½ = 0.693/λ |
| Mass-energy equivalence | E = mc² |
Worked Examples
Example 1 (Mechanics): A block of mass 2 kg on a frictionless surface is attached to a spring (k = 200 N/m). Find the time period of oscillation.
Solution: T = 2π√(m/k) = 2π√(2/200) = 2π√(0.01) = 2π × 0.1 = 0.628 s ≈ 0.63 s
Example 2 (Electromagnetism): Two resistors 6 Ω and 12 Ω are connected in parallel across a 12 V battery. Find total current drawn.
Solution: Equivalent resistance: 1/R_p = 1/6 + 1/12 = 2/12 + 1/12 = 3/12 → R_p = 4 Ω Current: I = V/R_p = 12/4 = 3 A
Example 3 (Modern Physics): Light of wavelength 400 nm falls on a metal with work function 2.0 eV. Find maximum KE of photoelectrons.
Solution: Energy of photon: E = hc/λ = (6.63 × 10⁻³⁴ × 3 × 10⁸) / (400 × 10⁻⁹) E = 4.97 × 10⁻¹⁹ J = 4.97 × 10⁻¹⁹ / 1.6 × 10⁻¹⁹ eV ≈ 3.1 eV KE_max = 3.1 − 2.0 = 1.1 eV
Common Mistakes
- Ignoring sign convention in optics → Always mark the principal axis, assign signs before substitution. Real image for concave mirror has v negative when object is beyond F.
- Confusing series and parallel combinations → In series, same current flows; in parallel, same voltage appears. Students often add resistances when they should take reciprocals.
- Using wrong formula for moment of inertia → A disc about its centre has I = MR²/2, not MR². Ring has I = MR². Check axis of rotation.
- Forgetting to convert units → Wavelength often given in nm (10⁻⁹ m) or Å (10⁻¹⁰ m). Mass in grams must become kg for SI calculations.
- Applying Lenz's law incorrectly → Induced current opposes the change in flux, not the flux itself. If flux increases, induced field opposes the original field.
- Mixing up threshold frequency and work function → Work function φ = hν₀. If incident frequency < ν₀, no photoelectrons regardless of intensity.
Quick Reference
- Escape velocity from Earth ≈ 11.2 km/s; orbital velocity ≈ 7.9 km/s.
- Speed of light c = 3 × 10⁸ m/s; Planck's constant h = 6.63 × 10⁻³⁴ Js.
- 1 eV = 1.6 × 10⁻¹⁹ J — memorise for all modern physics conversions.
- Magnetic field inside a solenoid: B = μ₀nI (uniform, independent of position).
- Power of lens in dioptres: P = 1/f (f in metres).
- Half-life and decay constant: T_½ × λ = 0.693 (always).