Week 4, clip 2

Timepoint:  09:09.9

1) Which of the following sources of energy could power the Sun?

- Nuclear fission
- Nuclear fusion
- Chemical reactions
- Gravitational heating

Timepoint:  17:49.1

2) Which of the following physical quantities obeys a conservation law?  Check all that apply.

- Heat
- Light
- Electric charge
- Momentum
- Angular momentum
- Velocity
- Mass-energy
- Electron number

Answers:
1) Nuclear fusion

Explanation:  
Chemical potential energy could only provide enough power for the Sun to burn 10^4 years, while gravitational potential energy could only power the Sun for 10^7 years, as we learned in the introduction video.

Nuclear fission can potentially provide enough energy, but our evidence suggests that the Sun is made of light elements like hydrogen and helium, not heavy, fissionable elements like uranium and californium.

Nuclear fusion provides enough energy to power the Sun for 10^11 years and uses light elements as the reactants, which leads us to suggest this as the fuel that powers the Sun.

2) Electric charge, momentum, angular momentum, mass-energy, and electron number (actually lepton number) are all conserved quantities.

Explanation:  
See the slide earlier in this video clip at ~10:00 to identify the correct answers.