## 20110104

### Physics final exam problem: relativistic RHEED speeds

Physics 205B Final Exam, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 26.55

[20 points.] Reflection high-energy electron diffraction2 is a technique where electrons with a kinetic energy of up to 100,000 eV probe the atomic structure of crystalline surfaces. How fast do these electrons travel, as measured from a stationary reference frame? Express your answer as a factor of c. Show your work and explain your reasoning.

• p = 20/20:
Correct.
• r = 16/20:
Nearly correct, but includes minor math errors.
• t = 12/20:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
• v = 8/20:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner.
• x = 4/20:
Implementation of ideas, but credit given for effort rather than merit.
• y = 2/20:
Irrelevant discussion/effectively blank.
• z = 0/20:
Blank.

Section 70856
p: 6 students
r: 1 students
t: 0 students
v: 2 students
x: 0 students
y: 0 students
z: 1 student

A sample "p" response (from student 1010), although the final result is expressed as a factor of c, "m/s" units are inserted: