Can You Solve This 20 Mark IB Physics Paper 2 Question?

IB Physics HL Paper 2 May 2025 TZ2 Question 7 (20 Marks) fully solved! Nuclear fusion, mass defect, stellar lifetime, Wien's displacement law, Doppler effect, and the Sun's rotation explained step by step. Can you solve an entire 20-mark IB Physics question that moves from nuclear fusion to stellar evolution, thermal radiation, and wave phenomena? In this video, we work through Question 7 from the May 2025 Time Zone 2 Higher Level Physics Paper 2. And according to the official examiner's report, some parts of this question were particularly challenging. Parts (c)(i–iii), involving the Sun's lifetime and mass loss, were identified as the most challenging section of the examination. And towards the end of the question, fewer than half the candidates identified the Doppler effect! Interestingly, the question doesn't require particularly advanced mathematical techniques. The real challenge is understanding how different physics concepts connect and recognizing which principles apply to each part. *WHAT WE COVER* Identifying the conditions required for nuclear fusion in the Sun Why high core temperature and high core density are essential for fusion Understanding hydrostatic equilibrium in stars The proton–proton chain and nuclear fusion reactions Calculating mass defect and energy released during fusion Why the positron's mass must be included in the mass-defect calculation Using the conversion of 1 u = 931.5 MeV/c² Why four hydrogen nuclei are consumed, net, to produce one helium-4 nucleus Calculating the total energy released by nuclear fusion Estimating how long the Sun remains on the main sequence Using luminosity to calculate energy-transfer time Estimating the mass lost by the Sun using E = mc² Applying Wien's displacement law to the Sun's spectrum Recognizing Doppler blueshift in spectral lines Calculating the Sun's rotational speed and period The important lesson here is that a single IB Physics Paper 2 question can bring together concepts from multiple topics. And in a question worth 20 marks, being able to recognize those connections is crucial. --- *VIDEO CHAPTERS* 00:00 Introducing the 20-Mark IB Physics Question 00:30 Part (a)(i): Conditions Required for Nuclear Fusion 01:42 Part (a)(ii): Hydrostatic Equilibrium of the Sun 02:33 Part (b): Nuclear Fusion and Mass Defect 04:33 Converting Atomic Mass Units into MeV 06:07 Part (c)(i): Main Sequence Lifetime of the Sun 06:40 Why Four Hydrogen Nuclei Produce One Helium-4 Nucleus 08:25 Calculating Total Fusion Energy and Stellar Lifetime 10:17 Part (c)(ii): The Assumption of Constant Luminosity 10:35 Part (c)(iii): Estimating the Mass Lost by the Sun 11:55 Part (d): Wien's Displacement Law 12:59 Part (e)(i): Understanding Doppler Blueshift 14:51 Part (e)(ii): Calculating the Sun's Rotation Period 16:59 Final Remarks --- *ORIGINAL IB PHYSICS EXAM QUESTION* *May 2025 | Time Zone 2 | Higher Level | Paper 2 | Question 7 | 20 Marks* The following data are available for the Sun when it entered the main sequence. Mass = 2.0 × 10³⁰ kg Radius = 7.0 × 10⁸ m Surface temperature = 5800 K Core temperature = 1.5 × 10⁷ K Core density = 1.6 × 10⁵ kg m⁻³ *(a)(i)* State and explain which of the above features make nuclear fusion in the Sun possible. *[3]* *(a)(ii)* Outline how the Sun maintains its equilibrium state. *[2]* *(b)* The following sequence of nuclear fusion reactions takes place in the Sun. Stage 1: ¹₁H + ¹₁H → ²₁H + e⁺ + ν Stage 2: ¹₁H + ²₁H → ³₂He + γ Stage 3: ³₂He + ³₂He → ⁴₂He + 2¹₁H The nuclear mass of ¹₁H is 1.007276 u and that of ²₁H is 2.013550 u. Estimate the energy released in stage 1 of these reactions. *[2]* *(c)* The Sun will leave the main sequence when it has converted 10% of its hydrogen mass into helium. The total energy released in the reactions in (b) is 4.3 × 10⁻¹² J. The current luminosity of the Sun is 3.8 × 10²⁶ W. When the Sun entered the main sequence its hydrogen mass was 1.5 × 10³⁰ kg. *(c)(i)* Show that the Sun will stay on the main sequence for about 8 × 10⁹ years. *[3]* *(c)(ii)* State one assumption that was made in getting the answer to (c)(i). *[1]* *(c)(iii)* Estimate the total mass lost by the Sun in this time. *[2]* *(d)* The surface temperature of the Sun is 5800 K. Determine the peak wavelength in the Sun's spectrum. *[2]* *(e)* The Sun rotates about its axis. P is a point on the Sun's equator. A particular spectral line of hydrogen from a laboratory source has wavelength 656.2797 nm. The same spectral line emitted from P has wavelength 656.2753 nm when measured on Earth. *(e)(i)* Explain this observation. *[2]* *(e)(ii)* Calculate the period of revolution of the Sun. *[3]* Refer to the video for the original diagram showing the Sun's rotation and point P.