Summary: Physics 208 Exam 1 Name_______________________________________________
You are graded on your work, with partial credit. See the last pages of the exam for formula sheets.
Please be clear and well-organized, so that we can easily follow each step of your work.
1. A solid conducting sphere carrying charge Q has radius a . It is inside a concentric hollow conducting
sphere with inner radius b and outer radius c . The hollow sphere has no net charge. Use Gauss's law (and
what you know about conductors) to derive expressions for the electric field magnitude E in each of the
You should include sketches which show the appropriate charges and appropriate Gaussian surfaces.
(a) (5) r < a:
(b) (5) a < r < b :
(c) (5) b < r < c :
(d) (5) r > c :
(e) (5) Sketch a graph of E versus r from r = 0 to r = 2c.
2. (25) A small sphere with mass 5.0 grams hangs by a thread between two parallel vertical plates 10.0 cm
apart. The plates are conducting and have uniform surface charge densities +! and !" . The charge on the
sphere is q = 2.0 !10"5
coulomb. The thread makes an angle of 30!
with respect to a vertical line.
(a) (5) Calculate the vertical component Ty of the tension in the thread.
(b) (5) Calculate the horizontal component Tx of the tension in the thread.