PHYS 308 Homework
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   Homework #2: Do not turn this in since we have our first test next Friday. I will post my solutions so you can check your work. I always like to give plenty of time to take tests. I will bring a sheet to class next week so people can sign up for a 2-hour block of time in which to take the test (in Reed 119) next Friday.

Note: When you check your answers with my solutions, be sure to have worked the problems yourself first as if you had to turn in this homework. It's tempting to just look at my solutions and agree with what I have done. That does not help you in understanding the material. Avoid that trap.

Note: While there is only one type of, say, the first problem below, you should be able to work each of these problems in general. For example, what if I would ask you for the energy required to bring perhaps 3 charges together in a line, or a triangle with sides of unequal length? You should always work all homework problems with the thought in your mind, "How might this be reworded to make a problem with the same concept, but a different physical situation?"

  1. Find the energy required to bring 4 identical charges together in a line, with each one separated by 2.0A ("A"="angstrom"=10-10m). Each charge has a magnitude of (+)2e (e=1.602x10-19C).
  2. Find the energy required to bring three charges together, whose location and charge are this: +3e at (x, y) = (0,0), -2e at (2A, 0), and -1e at (0,1A). Here, A means "angstroms" and has a value A=1.0x10-10m.
  3. The example Mathematica worksheet in the class web page will be helpful with this. Plot both the electric field vectors and the electric field unit vectors due to a (-)45μC charge located at the point (x,y)=(0.90m, 1.30m). You are to plot this in the range x=-4.0m to 4.0m and y=4.0m to 4.0m.
  4. An electric field line points to the left (negative x-direction, or "west"). (a) Assume for this part that it is increasing in strength. Draw the magnetic field lines associated with this electric field E. (b) Assume for this part that it is decreasing in strength. Draw the magnetic field lines associated with this electric field E.
  5. A solid sphere of radius 75cm has a uniform change density given by ρ=+3.5μC/m3. Find the magnitude of the electric field due to this charge at a radius of (a) 35cm, and (b) 90cm.
  6. A solid sphere of radius 75cm has a change density that depends on r and is given by ρ(r)=+2.0μC/m6r3. Find the magnitude of the electric field due to this charge at a radius of (a) 35cm, and (b) 90cm.
   That's all for homework #2.