Physics: Mechanics, Fields, Electricity and Modern Physics
Physics in Thanawiya Amma tests mechanics, circular and gravitational motion, electric and magnetic fields, electromagnetic induction, alternating current, and modern physics. Numerical answers need correct units and direction.
Core rules
- Newton's second law and circular motion: centripetal force points toward the centre; a = v²/r.
- Electric field E = F/q; magnetic force on a moving charge F = qvB sin θ, direction by the right-hand rule.
- Faraday and Lenz: induced emf depends on rate of change of flux; induced current opposes the change.
- AC: rms values are peak/√2 for sinusoidal quantities.
- Photoelectric effect: K_max = hf - φ; threshold frequency satisfies hf₀ = φ.
Worked example
Light of frequency 1.0 × 10^15 Hz falls on a metal with work function 2.0 eV. Using h = 4.14 × 10^-15 eV·s, the photon energy is hf = 4.14 × 10^-15 × 1.0 × 10^15 = 4.14 eV. Therefore K_max = 4.14 - 2.0 = 2.14 eV. The reasoning is to subtract the work function from the photon energy; if hf < φ, no emission occurs.
Traps
- Mixing joules and electronvolts.
- Using peak values where rms values are required.
- Reversing the direction of induced current by ignoring Lenz's law.
- Forgetting that magnetic force is zero when velocity is parallel to the field.
- Reporting a vector magnitude without its direction.