Physics 272: Advanced Honors Physics II
Spring 2018

HW6 due in class Monday, February 26, 2018
Reading: Purcell 4.7-9

Note: the solution for each problem and your name should be written on a separate page (sorry, trees!) to facilitate the grading. Please paperclip the pages together.
I have put the answers (or in some cases partial answers) at the bottom of this page. You can use them to check your own answers -- if they don't agree, you know you need to go back and look for at least one mistake.

1. 4.21(a) Current pulse from an alpha particle: planar electrodes
The current flows in the circuit to transport negative charge from the surface of the left-hand conducting plate to the surface of the right-hand conducting plate as the positively charged particle moves to the right (the charges on the plates can be found from the superposition argument in Exercise 3.37. You just need the answer: the total charge on the surface of the left-hand plate Q1 = -(s-x) Q/s and the total charge on the right-hand plate Q2 = -Q-Q1=-Qx/s, where s is the separation between the plates and x is the perpendicular distance from Q to the left-hand plate. Here's the derivation of the answer if you are interested).
Note that this is NOT a steady current system as the charge distribution is changing with time.

2. 4.38 Two light bulbs *

3. 4.39 Maximum power *
If you are unfamiliar with the concept of internal resistance of a battery, look at Figure 4.21 on p. 211 and corresponding short discussion at the bottom of p. 210 of Purcell. If you need more words, check out the discussion from HRW10.

4. 4.40 Minimum power dissipation **

OPTIONAL **4.21(b)
Current pulse from an alpha particle: cylindrical electrodes
As part of the solution, you have to generalize the superposition argument in Exercise 3.37 to concentric cylinders.


4.21(a) 1.6 x 10^-10 A
4.38. (a) 2 is larger; (b) 2 is brighter
4.39. proof
4.40. proof