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UNIVERSITY OF SOUTH AUSTRALIA School of Electrical & Information Engineering (Applied Physics) ENGINEERING PHYSICS QUIZ 3 [ week 13 ] Time: 20 minutes

Marks: 20 Name .....................................................................................

QUESTIONS ( 8 MARKS )

ANSWERS

1. If the net external torque on a rigid body about some origin is zero, it means that A. B. C. D.

the angular momentum of the rigid body about the origin is constant, the angular acceleration of the rigid body about the origin is zero, the angular speed of the rigid body about the origin is constant all of the above are true.

2. Which of the following is incorrect? A. B. C. D.

An elastic collision is when the total kinetic energy of the system is conserved. A perfectly elastic collision is one in which the colliding bodies stick together after collision. Momentum is always conserved. An inelastic collision is one in which the total kinetic energy of the system is not conserved.

3. Which of the following is not a requirement for laser emission? A. Spontaneous emission B. Stimulated emission

C. A population inversion D. A metastable excited state

4. A DVD, an LP (vinyl record) and a CD are rotated in their respective instruments. Which has the greater moment of inertia? (assume they all have the same mass) A. The DVD C. The CD B. The LP D. They all have the same moment of inertia

5. What does a scanning tunneling microscope measure? A. B. C. D. E.

electrons electron density of states electron probability all of the above none of the above

6. Isotopes of an element contain A. B. C. D. E.

the same number of neutrons but a different number of electrons the same number of electrons but a different number of protons the same number of neutrons but a different number of protons the same number of nucleons none of the above

7. An external observer sees two lightning bolts, A + B, hit a train simultaneously, at the front and back of a carriage A. B. C. D.

respectively. A person on the moving train would observe: A striking the front and B the back of the carriage simultaneously A striking the front before B striking the back of the carriage A striking the front after B striking the back of the carriage Nothing

8. Which of the following statements is true? A. B. C. D.

Tbe combined mass of the final nuclei produced by fission is less than the mass of the original nucleus. Tbe mass of the final nucleus produced by fission is more than the combined mass of the original nuclei. Tbe combined mass of the final nuclei produced by fusion is less than the mass of the original nucleus. Tbe mass of the final nucleus produced by fusion is more than the combined mass of the original nuclei.

9. The emission of radiation from an atom or molecule arises from A. B. C. D. E.

the vibration of the molecular bonds the transition of an electron between energy levels the rotation of a molecule all of the above none of the above

QUESTIONS & PROBLEMS ( 14 MARKS ) Question 1 (4 marks)

Explain the difference between alpha, beta and gamma particles (3marks) and how one could distinguish them apart experimentally (1mark). Discuss properties such as charge, mass and relative penetrating power. ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… Problem 1 (5 marks) A disk 8.00 cm in radius and with a moment of inertia of 1.6 x 10–3 kg.m2 rotates at a constant rate of 1200 revolutions per minute about its central axis. Determine (a) its angular speed in SI units (1 mark), (b) the linear speed of a point on the rim of the disk (1 mark), (c) the magnitude and direction of the linear acceleration of a point on the rim (1 mark), (d) the angular acceleration of the disk (1mark), and (e) the rotational kinetic energy of the disk (1 mark). ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… Problem 2 (4 marks) The half-life of carbon-14 is 5730 years and its original activity (when the plant was alive) was 0.255Bq per gram of carbon. A 200mg piece of charcoal found in an aboriginal burial site was measured to have an activity of 140 decays /hour. Find the decay constant for C-14. How old is the charcoal (in years)? ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..……………………………………………………………………………