Cambridge IGCSE Physics (0625) Study Notes

Section 1: Motion, Forces & Energy

1.1 Physical Quantities & Measurement Techniques

• Scalar: Magnitude only (e.g., speed, distance, mass, time, energy).
• Vector: Magnitude and direction (e.g., velocity, displacement, force, acceleration, momentum).

1.2 Motion

• Speed vs Velocity: Speed = distance / time | Velocity = Δdisplacement / Δtime
• Acceleration: a = (v - u) / t
• Graphs:
  • Distance–Time Graph: Gradient = speed. Flat line = stationary.
  • Speed–Time Graph: Gradient = acceleration. Area under graph = distance travelled.

1.3 Mass & Weight

• Mass (m): Quantity of matter in a body (kg). Scalar; constant everywhere.
• Weight (W): Gravitational force acting on an object (N). Vector.
• Formula: W = mg (where g ≈ 9.8 m/s² or 10 N/kg).

1.4 Density

• Formula: ρ = m / V
• Determining Density:
  • Regular Solids: Measure dimensions with calipers/ruler to calculate volume V, weigh on a balance for mass m.
  • Irregular Solids: Use a displacement can or measuring cylinder with water (ΔV = V_final - V_initial).

1.5 Forces

• Newton’s Laws:
  1. An object remains at rest or constant velocity unless acted upon by a resultant force.
  2. F_net = ma
  3. Action-reaction pairs are equal in magnitude, opposite in direction, and act on different objects.
• Hooke’s Law: F = kx (Valid up to the limit of proportionality).
• Circular Motion: Constant speed, changing velocity (due to direction change) → resultant centripetal force acts towards the center of the circle.

1.6 Momentum

• Formula: p = mv (Unit: kg·m/s or N·s).
• Impulse: Impulse = F Δt = Δp = mv - mu
• Conservation of Momentum: Total momentum before collision = Total momentum after collision in an isolated system.

1.7 Energy, Work & Power

• Work Done: W = F × d (Joules, J)
• Kinetic Energy: E_k = ½ m v²
• Gravitational Potential Energy: ΔE_p = m g Δh
• Efficiency: Efficiency = (Useful Energy Output / Total Energy Input) × 100%
• Power: P = W / t = ΔE / t (Watts, W)

1.8 Pressure

• General Pressure: p = F / A (Pascal, Pa)
• Liquid Pressure: Δp = ρ g Δh

Section 2: Thermal Physics

2.1 Kinetic Particle Model of Matter

• States of Matter:
  • Solids: Fixed volume and shape; strong intermolecular bonds; vibrate about fixed positions.
  • Liquids: Fixed volume, adapts to container shape; moderate forces; particles slide past each other.
  • Gases: No fixed shape or volume; negligible forces; rapid, random translational motion.
• Gas Laws:
  • p ∝ T (at constant volume)
  • p₁V₁ = p₂V₂ (Boyle's Law: at constant temperature)

2.2 Thermal Properties & Temperature

• Thermal Expansion: Solids expand least, gases expand most for the same temperature rise.
• Specific Heat Capacity: c = ΔE / (m ΔT) (Energy required to raise 1 kg of a substance by 1°C).
• Melting & Boiling: Temperature remains constant during a phase change despite continuous heat input.

2.3 Thermal Energy Transfers

• Conduction: Transfer of thermal energy through atomic vibrations and free electron diffusion (in metals).
• Convection: Fluid circulation driven by density differences (hotter fluid expands, becomes less dense, and rises).
• Radiation: Infrared electromagnetic waves. Can travel through a vacuum.
  • Best emitters/absorbers: Dull, matte black surfaces.
  • Best reflectors/poor emitters: Shiny, silver/white surfaces.

Section 3: Waves

3.1 General Properties of Waves

• Wave Equation: v = f λ
• Frequency: f = 1 / T
• Transverse Waves: Vibration perpendicular to energy transfer (e.g., light, water waves).
• Longitudinal Waves: Vibration parallel to energy transfer (e.g., sound). Consists of compressions and rarefactions.

3.2 Light

• Reflection: Angle of incidence i = Angle of reflection r (measured from the normal).
• Refraction: Snell’s Law: n = sin(i) / sin(r) = c / v
• Total Internal Reflection (TIR): Occurs when light travels from a denser to a less dense medium and i > c (critical angle).
  Formula: sin(c) = 1 / n
• Converging (Convex) Lens:
  • Real Image: Rays intersect; can be projected onto a screen.
  • Virtual Image: Rays diverge; cannot be projected (e.g., magnifying glass when object distance < focal length).

3.3 Electromagnetic Spectrum

• Order (Increasing frequency / Decreasing wavelength): Radio → Microwave → Infrared → Visible → Ultraviolet → X-rays → Gamma rays.
• Speed: All EM waves travel at c = 3.0 × 10⁸ m/s in a vacuum.

3.4 Sound

• Human hearing range: 20 Hz to 20,000 Hz (20 kHz).
• Pitch depends on frequency; loudness depends on amplitude.
• Speed of sound: Air ≈ 330–350 m/s | Water ≈ 1500 m/s | Steel ≈ 5000 m/s.

Section 4: Electricity & Magnetism

4.1 Simple Phenomena of Magnetism

• Magnetic Materials: Iron, steel, nickel, cobalt.
• Soft Iron: Easily magnetized/demagnetized (used in electromagnet cores, transformers).
• Hard Steel: Retains magnetism permanently (used in permanent magnets).

4.2 Electrical Quantities

• Charge: Q = I t (Coulombs, C)
• Current (I): Rate of flow of electric charge (Amperes, A).
• Potential Difference (V): Energy provided per unit charge (V = W / Q).
• Resistance (R): R = V / I (Ohms, Ω)
• Resistivity/Dimensions: R ∝ L / A (Longer wire = higher resistance; thicker wire = lower resistance).
• Power & Energy: P = IV = I²R = V²/R | E = IVt

4.3 Electric Circuits

• Series: I_total = I₁ = I₂ | V_total = V₁ + V₂ | R_total = R₁ + R₂
• Parallel: V_total = V₁ = V₂ | I_total = I₁ + I₂ | 1 / R_total = 1/R₁ + 1/R₂

4.4 Electrical Safety

• Fuses & Circuit Breakers: Protect cables from overheating by breaking the circuit when current exceeds rated capacity.
• Mains Wiring: Live (brown), Neutral (blue), Earth (green/yellow).

4.5 Electromagnetic Effects

• Motor Effect: Current-carrying conductor in a magnetic field experiences a force (Fleming's Left-Hand Rule).
• Electromagnetic Induction: Moving a conductor across magnetic field lines induces an e.m.f. (Faraday's Law / Lenz's Law).
• Transformers: V_p / V_s = N_p / N_s | For 100% efficiency: I_p V_p = I_s V_s

Section 5: Nuclear Physics

5.1 The Nuclear Atom

• Structure: Protons (+1) and Neutrons (0) in the nucleus; Electrons (-1) orbiting in shells.
• Notation: AZX (A = nucleon number, Z = proton number).
• Isotopes: Atoms of the same element with the same proton number Z, but different nucleon number A.

5.2 Radioactivity

• Half-life (T_1/2): Time taken for half the radioactive nuclei in a sample to decay.

Section 6: Space Physics

6.1 Earth & the Solar System

• Orbital Speed: v = 2πr / T
• Solar System: Sun at center, 8 planets (terrestrial inner planets, gas/ice giant outer planets), dwarf planets, asteroids, comets.

6.2 Stars & the Universe

• Stellar Life Cycle:
  1. Nebula → Protostar → Main Sequence Star
  2. Solar-mass star: Red Giant → Planetary Nebula → White Dwarf
  3. Massive star: Red Supergiant → Supernova → Neutron Star OR Black Hole
• Cosmology:
  • Redshift: Increase in observed wavelength of light from distant galaxies moving away from Earth.
  • Hubble's Law: v = H₀ d
  • CMBR (Cosmic Microwave Background Radiation): Radiation coming uniformly from all directions in space; key evidence for the Big Bang Theory.