PH2400 Spring 2001 - Final answers

1. b      2. c      3. c      4. d      5. c      6. e      7. c      8. e     

9. replace "x" with (x+vt) in the two places it appears.
10. both observers see the same space-time interval, so can compute with Earth coordinates.
s = square root of [ (x₂ - x₁)² - c² (t₂ - t₁)² ]
11. As the temperature is raise, more electrons get excited across the gap (leaving more holes behind) and hence there are more charges to carry the current.
12. E_f is the energy between the highest occupied electron level and the lowest unoccupied electron level in a many electron system (e.g. a solid) at T = 0 K. Typically it is a few eV.
13. The baryon number is not conserved.

14. A = 0.33 m, lambda = 3.7 m, speed = 18.8 m/s, f = 5.09 Hz, traveling toward +x
15. 9.87 mm
16. 3.49 x 10^-8 s
17. let z = x/b (for notational simplicity) then for -b < x < b
U(x) = 6 h-bar/(m b²) * z²/(1-z⁴)
and U(x) is infinite for x < -b and x > b.
18. 1.43 mm
19. 0.293 MeV
20. approx. 0.2 eV