Know that energy is released by nuclear fusion in the Sun
1.7.3 Energy Resources – Nuclear Fusion in the Sun 🌞
Objective
Understand that the Sun releases energy through nuclear fusion of hydrogen atoms into helium.
What is Nuclear Fusion?
Imagine two Lego bricks that snap together and release a burst of light. In the Sun, tiny hydrogen atoms (protons) collide at high temperatures and pressures, overcoming their natural repulsion and fusing to form a helium nucleus. This process releases a huge amount of energy because the mass of the resulting helium nucleus is slightly less than the sum of the original protons. The missing mass is converted into energy by Einstein’s equation $E=mc^2$.
Fusion in the Sun 🌞
The Sun’s core is a gigantic fusion reactor. Here’s a simplified view of the main reaction chain (the proton–proton chain):
| Step | Reaction | Energy Released |
|---|---|---|
| 1 | $^1\!H + ^1\!H \rightarrow ^2\!H + e^+ + u_e$ | 0.42 MeV |
| 2 | $^2\!H + ^1\!H \rightarrow ^3\!He + \gamma$ | 5.49 MeV |
| 3 | $^3\!He + ^3\!He \rightarrow ^4\!He + 2\,^1\!H$ | 12.86 MeV |
| Total per 4 H → He | 26.37 MeV | |
The tiny mass difference (≈0.7 %) between the four hydrogen nuclei and the resulting helium nucleus is the source of the Sun’s energy. This energy travels outwards as light and heat, sustaining life on Earth.
Exam Tips 📚
- Show the fusion reaction: Write the balanced nuclear equation clearly.
- Calculate mass defect: Subtract the mass of the products from the mass of the reactants.
- Use $E=mc^2$: Convert the mass defect to energy (Joules or MeV). Remember 1 MeV = 1.602 × 10⁻¹³ J.
- Explain the Sun’s core conditions: Mention high temperature (~15 million K) and pressure that enable fusion.
- Link to energy resources: Discuss how fusion could be a clean energy source for Earth in the future.
Analogy & Example
Think of fusion like a team game where two players (hydrogen atoms) join forces to create a stronger player (helium). The team’s combined strength (mass) is slightly less than the sum of the individual strengths, and the “extra” strength is released as energy, just like a burst of light when a goal is scored in a soccer match! ⚽️✨
Revision
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