Describe the life cycle of a star: (a) a star is formed from interstellar clouds of gas and dust that contain hydrogen (b) a protostar is an interstellar cloud collapsing and increasing in temperature as a result of its internal gravitational attract

IGCSE Physics (0625) – Core Topics & Key Concepts (2026‑2028)

This set of notes follows the Cambridge IGCSE Physics syllabus (2026‑2028). It is organized by the six syllabus units, includes concise explanations, essential equations, worked‑example style questions and a short extension on astronomy. Practical/experimental advice (AO3) is provided at the end.

Unit 1 – Motion, Forces & Energy

1.1 Kinematics (speed, velocity, acceleration)

• Speed = distance ÷ time \(v = \dfrac{s}{t}\) (scalar)
• Velocity = displacement ÷ time \(\vec v = \dfrac{\vec s}{t}\) (vector, includes direction)
• Acceleration = change in velocity ÷ time \(a = \dfrac{\Delta v}{\Delta t}\)

Example: A car travels 150 m in 12 s. Speed = \(150/12 = 12.5\ \text{m s}^{-1}\).

1.2 Graphs of Motion

• Distance‑time graph – gradient = speed (if straight line) or instantaneous speed (tangent).
• Speed‑time graph – gradient = acceleration; area under the curve = distance travelled.
• Velocity‑time graph – area = displacement; gradient = acceleration; horizontal line = constant velocity.

1.3 Forces & Newton’s Laws

• Resultant force – vector sum of all forces acting on an object.
• Newton’s 1st law (inertia) – an object remains at rest or in uniform motion unless acted on by a net external force.
• Newton’s 2nd law – \(\displaystyle \vec F_{\text{net}} = m\vec a\).
• Newton’s 3rd law – for every action there is an equal and opposite reaction.

1.4 Density & Pressure

• Density \(\displaystyle \rho = \frac{m}{V}\) (kg m⁻³).
• Pressure \(\displaystyle p = \frac{F}{A}\) (Pa = N m⁻²).
• Hydrostatic pressure in a fluid: \(\displaystyle \Delta p = \rho g \Delta h\).

1.5 Momentum, Impulse & Conservation

• Momentum \(\displaystyle \vec p = m\vec v\).
• Impulse \(\displaystyle \vec J = \vec F\Delta t = \Delta \vec p\).
• In a closed system, total momentum is conserved.

1.6 Work, Energy & Power

• Work \(W = F s \cos\theta\) (J = N·m).
• Kinetic energy \(E_k = \tfrac12 mv^2\).
• Gravitational potential energy \(E_p = mgh\).
• Work‑energy theorem Net work = change in kinetic energy.
• Power \(P = \dfrac{W}{t}\) (W = J s⁻¹).

Example (work‑energy): A 2 kg stone is lifted 5 m vertically.

\(E_p = mgh = 2 \times 9.8 \times 5 = 98\ \text{J}\). The work done against gravity is 98 J.

1.7 Elastic Behaviour, Springs & Limits

• Hooke’s law \(F = kx\) (\(k\) = spring constant, \(x\) = extension).
• Limit of proportionality – end of the linear region of the force‑extension graph.
• Elastic limit – maximum extension for which the material returns to its original shape.
• Plastic deformation – permanent change after the elastic limit.

1.8 Centre of Gravity & Moments

• Centre of gravity (CG) – point where the total weight of a body may be considered to act.
• Moment of a force \(\displaystyle \tau = Fr\) (where \(r\) is the perpendicular distance from the line of action to the pivot).
• For equilibrium: \(\displaystyle \sum \tau = 0\).

Unit 2 – Thermal Physics

2.1 Temperature, Heat & Specific Heat Capacity

• Temperature measured in °C or K; \(T(\text{K}) = \theta(^{\circ}\text{C}) + 273\).
• Heat is energy transferred because of a temperature difference.
• Specific heat capacity \(c = \dfrac{Q}{m\Delta T}\) (\(Q\) in J, \(m\) in kg, \(\Delta T\) in K).

2.2 Kinetic Particle Model & States of Matter

• All matter consists of particles in constant motion.
• Solid – particles vibrate about fixed positions; fixed shape & volume.
• Liquid – particles move past one another; fixed volume, no fixed shape.
• Gas – particles move freely; no fixed shape or volume; pressure arises from collisions with container walls.

2.3 Methods of Heat Transfer

• Conduction – transfer through a material; rate \(\displaystyle \dot Q = \frac{kA}{d}\,\Delta T\) (\(k\) = thermal conductivity).
• In metals, free electrons carry the majority of the heat.
• Convection – bulk movement of fluid; driven by density differences due to temperature gradients.
• Radiation – emission of EM waves; power \(P = \varepsilon\sigma AT^4\) (Stefan‑Boltzmann law, \(\sigma = 5.67\times10^{-8}\ \text{W m}^{-2}\text{K}^{-4}\)).

2.4 Thermal Expansion

• Solids (linear) \(\displaystyle \Delta L = \alpha L_0 \Delta T\) (\(\alpha\) = linear expansion coefficient).
• Area expansion \(\displaystyle \Delta A = 2\alpha A_0 \Delta T\).
• Volume expansion (solids) \(\displaystyle \Delta V = 3\alpha V_0 \Delta T\).
• Liquids \(\displaystyle \Delta V = \beta V_0 \Delta T\) (\(\beta\) ≈ \(2.1\times10^{-4}\ \text{K}^{-1}\) for water).
• Gases (ideal) \(\displaystyle \frac{\Delta V}{V} = \alpha{\text{gas}} \Delta T\) with \(\alpha{\text{gas}} \approx 1/273\ \text{K}^{-1}\).

2.5 Example Calculation (Calorimetry)

0.5 kg of water (\(c = 4180\ \text{J kg}^{-1}\text{K}^{-1}\)) is heated from 20 °C to 80 °C.

\(Q = mc\Delta T = 0.5 \times 4180 \times (80-20) = 1.25\times10^{5}\ \text{J}\).

Unit 3 – Waves

3.1 Wave Terminology

• Wavelength \(\lambda\), frequency \(f\), speed \(v\) \(v = f\lambda\).
• Amplitude – maximum displacement from equilibrium.
• Period \(T = 1/f\).
• Wavefront – line (2‑D) or surface (3‑D) of points in phase.

3.2 Light Waves

• Visible spectrum ≈ 400–700 nm.
• Reflection – angle of incidence = angle of reflection.
• Refraction – Snell’s law \(n1\sin\theta1 = n2\sin\theta2\).
• Dispersion – different wavelengths refract by different amounts (prism).
• Monochromatic light – light of a single wavelength (e.g., laser).
• Diffraction – bending of waves around an obstacle or through a narrow slit; the amount of spreading increases as the slit width approaches \(\lambda\).
• Applications: lenses, mirrors, fibre optics.

3.3 Sound Waves

• Longitudinal wave; requires a material medium.
• Speed in air ≈ 340 m s⁻¹ at 20 °C; varies with temperature and medium.
• Pitch ∝ frequency; loudness ∝ amplitude.
• Doppler effect \(f' = f\frac{v\pm vo}{v\pm vs}\) (signs chosen according to motion).

3.4 Wave Phenomena (Diffraction & Interference)

• When two coherent sources produce overlapping wavefronts, constructive interference occurs where path difference = \(n\lambda\); destructive where = \((n+\tfrac12)\lambda\).
• Diffraction patterns (single‑slit, double‑slit) illustrate the wave nature of light and sound.

Unit 4 – Electricity & Magnetism

4.1 Charge, Electric Field & Potential

• Charge \(Q\) measured in coulombs (C); 1 C ≈ \(6.24\times10^{18}\) e⁻.
• Electric field \(E = \dfrac{F}{q}\) (N C⁻¹); direction is the force on a positive test charge.
• Potential difference \(V = \dfrac{W}{Q}\) (V = J C⁻¹).

4.2 Current, Resistance & Ohm’s Law

• Current \(I = \dfrac{Q}{t}\) (A = C s⁻¹).
• Resistance \(R = \dfrac{V}{I}\) (Ω).
• Resistivity \(R = \rho\frac{L}{A}\) (\(\rho\) material property).
• Power \(P = VI = I^{2}R = \dfrac{V^{2}}{R}\).

4.3 Series & Parallel Circuits

4.4 Magnetism

• Magnetic field lines exit the north pole and enter the south pole.
• Right‑hand rule (straight conductor) – thumb = current direction, fingers = magnetic‑field direction.
• Solenoid – field inside is uniform and parallel to the axis; strength \(B = \mu_0 n I\) (where \(n\) = turns per unit length).
• Field‑line diagrams help visualise direction and relative strength.

4.5 Electromagnetic Induction

• Faraday’s law (quantitative) \(\displaystyle \mathcal{E} = -\frac{d\Phi}{dt}\) (EMF induced equals the negative rate of change of magnetic flux).
• Lenz’s law – the induced EMF produces a current whose magnetic field opposes the change that produced it.
• Applications: generators, transformers, induction cookers.

4.6 Common Electrical Components

• Diode – allows current to flow in only one direction (forward bias).
• LED – light‑emitting diode; forward voltage typically 2–3 V.
• Variable potential divider (rheostat/potentiometer) – provides a controllable voltage output.

4.7 Electrical Safety (AO3)

• Hazards: electric shock, burns, fire.
• Protective devices: earthing/grounding, fuses, circuit breakers, residual‑current devices (RCDs).
• Safe working practices – use insulated tools, never work on live circuits, switch off at the supply.

Unit 5 – Nuclear Physics

5.1 Atomic Structure & Isotopes

• Atom = nucleus (protons + neutrons) + electrons.
• Atomic number \(Z\) = number of protons; mass number \(A\) = protons + neutrons.
• Isotopes have the same \(Z\) but different \(A\). Example: \(\,^{12}\!C\) and \(\,^{14}\!C\).
• Relative atomic mass \(A_r\) is a weighted average of isotopic masses.

5.2 Radioactivity

• Three main types of decay:

  • Alpha (\(\alpha\)) – emission of a \(\,^{4}\!He\) nucleus; reduces \(A\) by 4, \(Z\) by 2.
  • Beta (\(\beta\)) – conversion of a neutron to a proton + electron; \(A\) unchanged, \(Z\) + 1.
  • Gamma (\(\gamma\)) – high‑energy photon; no change in \(A\) or \(Z\).

• Half‑life \(t{1/2}\) – time for half the nuclei in a sample to decay. Activity \(A = \lambda N\) where \(\lambda = \ln 2 / t{1/2}\).
• Safety: shielding (lead for \(\gamma\), plastic for \(\beta\)), distance, time, and use of dosimeters.

5.3 Applications

• Medical imaging (X‑rays, PET), sterilisation, carbon dating, power generation in nuclear reactors.

Unit 6 – Space Physics & Astronomy (Extension)

6.1 Earth‑Sun‑Moon System

• Orbital motion – Earth orbits the Sun in ~365 days; Moon orbits Earth in ~27.3 days.
• Apparent daily motion of the Sun is due to Earth’s rotation.
• Seasons – result from the tilt of Earth’s axis (≈ 23.5°) and its orbital position.
• Tides – caused by the differential gravitational pull of the Moon (and Sun) on Earth’s oceans.

6.2 Overview of the Solar System

• Inner planets (Mercury, Venus, Earth, Mars) – rocky, relatively small.
• Outer planets (Jupiter, Saturn, Uranus, Neptune) – gaseous or icy giants.
• Minor bodies – asteroids, comets, Kuiper‑belt objects.

6.3 Life Cycle of a Star

Understanding stellar evolution illustrates gravity, energy conversion, and nuclear physics.

Formation – Nebula → Protostar

• Nebula: Giant cloud of gas (≈ 90 % hydrogen) and dust.
• Gravitational collapse of a dense region → protostar. Gravitational potential energy → thermal energy; core temperature rises.

Main‑Sequence – Hydrostatic Equilibrium

• When core temperature reaches ≈ \(10^{7}\) K, hydrogen fusion begins: \(4\,^{1}\!H \rightarrow\,^{4}\!He + 2\gamma + 26.7\ \text{MeV}\).
• Radiation pressure balances gravity → stable main‑sequence phase.

Post‑Main‑Sequence Evolution

• Hydrogen exhaustion – core contracts, outer layers expand.
• Red giant (low‑mass) / Red supergiant (high‑mass) – helium fusion (and heavier elements for massive stars) begins in the core; envelope becomes cool and large.

End States

• Low‑mass stars (\(<8\,M_\odot\)):

  • Outer layers expelled → planetary nebula.
  • Core becomes a white dwarf (electron‑degeneracy pressure supports it).

• High‑mass stars (\(\ge 8\,M_\odot\)):

  • Fusion proceeds up to iron; iron fusion is endothermic.
  • Core collapses → supernova explosion.
  • Remnant:

    • Neutron star if core mass \(<3\,M_\odot\) (neutron‑degeneracy pressure).
    • Black hole if core mass \(\ge 3\,M\odot\) (Schwarzschild radius \(Rs = 2GM/c^{2}\)).

Recycling of Material

• Supernova remnants enrich the interstellar medium with heavy elements.
• Enriched gas can later form new nebulae, restarting the stellar cycle.

Summary Table – Stellar Life Cycle

Key Equations (Astronomy Extension)

• Hydrogen fusion energy: \(4\,^{1}\!H \rightarrow\,^{4}\!He + 2\gamma + 26.7\ \text{MeV}\).
• Hydrostatic equilibrium: \(\displaystyle \frac{dP}{dr} = -\frac{G M(r)\rho(r)}{r^{2}}\).
• Schwarzschild radius: \(\displaystyle R_s = \frac{2GM}{c^{2}}\).

Practical & Experimental Skills (AO3)

• Planning experiments – define hypothesis, identify variables, choose appropriate apparatus.
• Safety – wear goggles, use fume hoods for gases, follow electrical safety rules (earthing, isolation).
• Data handling – record raw data in tables, calculate uncertainties, use appropriate significant figures.
• Error analysis – distinguish random vs systematic errors; calculate percentage uncertainty; suggest improvements.
• Graphical analysis – plot with correct scales, label axes, draw best‑fit line, determine gradient/intercept and their uncertainties.
• Use of instruments – calibrate timers, voltmeters, ammeters, thermometers; understand zero‑error and parallax.

Quick Revision Checklist

• Write down the three kinematic equations and know when to use each.
• Apply Newton’s three laws to free‑body diagrams, including moments and centre of gravity.
• Memorise \(Ek = \tfrac12 mv^2\), \(Ep = mgh\) and the work‑energy relationship.
• Convert between °C and K; use \(Q = mc\Delta T\) and the appropriate expansion coefficient.
• Recall \(v = f\lambda\), Snell’s law, and the conditions for diffraction and interference.
• Use Ohm’s law, series/parallel resistance formulas, and power equations confidently.
• Draw magnetic‑field lines for straight conductors, solenoids and bar magnets; apply right‑hand rules.
• Apply Faraday’s law and Lenz’s law to generator and transformer problems.
• Identify isotopes, write decay equations, and calculate half‑life problems.
• Explain the main stages of stellar evolution and the associated nuclear processes.
• Practice experimental design, error analysis and data presentation for AO3 marks.

Use these notes to practise past‑paper questions, create flashcards, and test yourself with the example problems provided. Good luck with your IGCSE Physics preparation!