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Physics Core Concepts

Jun 16, 2025

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Overview

This lecture covers the core concepts, equations, and principles needed for OCR A Level Physics, including mechanics, electricity, waves, quantum phenomena, fields, nuclear physics, and practical skills.

Forces & Motion

  • Forces are vectors (arrows) showing magnitude and direction.
  • Resultant force is the vector sum of all forces; use negatives for opposite directions.
  • Use Pythagoras/trig for perpendicular vectors.
  • Scalars have magnitude only; vectors have magnitude and direction.
  • Displacement and velocity are vector forms of distance and speed respectively.
  • Weight = mass × gravitational field strength (g = 9.81 N/kg).
  • Work done = force × distance moved parallel to force.
  • Gravitational potential energy (GPE) = mgh.
  • Power = work done ÷ time or power = force × velocity.
  • Newton’s Laws: 1st (constant motion if no resultant force), 2nd (F = ma), 3rd (every action has equal and opposite reaction).

Dynamics, Energy & Momentum

  • On a slope: perpendicular force = mg cosθ; down slope = mg sinθ.
  • Conservation of energy: mgh (top) = ½mv² (bottom) if no friction.
  • Momentum = mass × velocity (kg m/s); conserved in all collisions.
  • Impulse = change in momentum = force × time.
  • In collisions, total momentum before = total momentum after.
  • Elastic collision: kinetic energy conserved; inelastic: some energy lost.

Circular Motion & SHM

  • Centripetal acceleration a = v²/r; centripetal force F = mv²/r.
  • Angular velocity ω = 2π/T, v = ωr.
  • SHM: acceleration = –ω²x, force restores to equilibrium.
  • Displacement in SHM: x = A sin(ωt) or A cos(ωt).
  • Damping reduces amplitude; resonance increases amplitude at natural frequency.

Materials & Deformation

  • Hooke’s Law: F = kx (elastic limit not exceeded).
  • Stress = force/area (Pa); strain = extension/original length.
  • Young modulus = stress/strain; gradient of stress–strain graph.
  • Energy stored in a spring = ½kx² (area under force–extension graph).

Thermal Physics & Gases

  • Specific heat capacity: energy = mcΔT.
  • Latent heat: energy for state change = mL.
  • Ideal gas law: PV = nRT; P₁V₁/T₁ = P₂V₂/T₂.
  • Kinetic theory: average kinetic energy per molecule = 1.5kT.

Electricity

  • Current (I) = Q/t; charge (Q) in coulombs.
  • Potential difference (V) = energy (E) / charge (Q).
  • Resistance (R) = V/I (Ohm’s law); resistors in series and parallel.
  • Power (P) = VI; EMF = terminal pd + Ir (internal resistance).
  • Capacitance: Q = CV; E = ½CV² = ½QV = ½Q²/C.

Waves & Optics

  • Wave speed v = fλ; frequency f = 1/T.
  • Intensity ∝ amplitude².
  • Snell’s Law: n₁sinθ₁ = n₂sinθ₂; critical angle for total internal reflection.
  • Young’s double slit: fringe spacing w = λD/s.
  • Diffraction grating: nλ = d sinθ.

Fields (Gravitational & Electric)

  • Field strength: g = F/m (gravity), E = F/Q (electric).
  • Gravitational force: F = GMm/r²; Electric force: F = kQ₁Q₂/r².
  • Potential energy: GPE = –GMm/r, EPE = kQ₁Q₂/r.
  • Orbital speed: v = √(GM/r); Kepler’s third law: T² ∝ r³.

Nuclear Physics & Quantum

  • Activity A = λN; half-life t½ = ln2/λ.
  • Binding energy = Δmc²; 1 u = 931.5 MeV.
  • Alpha (α), Beta (β), Gamma (γ) radiations: different emissions and properties.
  • Photoelectric effect: E_k = hf – ϕ; threshold frequency for emission.

Practical Skills & Data

  • Use suitable measuring tools, avoid parallax, consider uncertainty.
  • Absolute uncertainty: equals instrument resolution.
  • Combine uncertainties in sums/multiplications accordingly.
  • Use log graphs to determine relationships; gradient gives proportionality.

Key Terms & Definitions

  • Scalar — quantity with magnitude only.
  • Vector — quantity with magnitude and direction.
  • Impulse — change in momentum.
  • SHM — simple harmonic motion, periodic oscillation.
  • Latent heat — energy per kg for state change at constant temp.
  • Capacitance — charge stored per volt, unit: Farad (F).
  • Resonance — maximum amplitude when driving frequency matches natural frequency.
  • Critical angle — minimum angle for total internal reflection.
  • Binding energy — energy to separate a nucleus into nucleons.
  • Half-life — time for activity/nuclei to halve.
  • Young modulus — stiffness, stress/strain.

Action Items / Next Steps

  • Review formula sheet and memorize key equations.
  • Practice problems on vectors, momentum, energy, and circuits.
  • Complete assigned textbook readings and past paper questions where relevant.

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