CPHY 122
Fall 2009 Exam 1
Instructor: H. L. Neal
February 6, 2009
INSTRUCTIONS: (1) Work all of the problems below. (2) Do not put more than one
problem on a single sheet of paper. (3) Write neatly and legibly in dark ink. (4) No cheating
or collaboration of any kind is allowed on exams. All questions must be directed to me by
E-mail or at phone (404-577-9007, anytime after 5 PM and before 1 AM). (5) .The exam is
due promptly at the start of class on Wednesday, February 11. HAPPY LEARNING!
PROBLEMS
1. Protons and neutrons are made from combinations of the two most common quarks,
the d(or down) quark and the u(or up) quark. The dquark has a charge of 1
3eand
the uquark has a charge of +2
3e. How could three of these quarks combine to make
(a) proton and (b) a neutron?
2. Draw an equilateral triangle with sides of length L. Label each of the three vertices with
the numbers 1, 2, and 3, respectively. At the three vertices place three point charges as
follows: qat 1, qat 2, and qat 3. (a) What is the total electric force on the charge at
vertex labeled 1? (b) If two of the charges form an electric dipole moment, what is the
electric dipole moment (vector)? (c) What is the torque on the electric dipole moment
due to the electric …eld of the third charge?
3. What is the electric ‡ux through the hemispherical open surface of radius Rshown
below?
E
α
Created usi ng UNREGISTERED Top Dr aw 3.10 Feb 7, '108 8:58:51 AM
The uniform electric …eld E is at an angle relative to the horizontal.
4. A point charge Qis …xed at the center of a uncharged spherical conducting shell of
inner radius aand outer radius b. What is the induced surface charge densities on the
(a) inner and (b) outer surfaces?
1
