Physics 272: Advanced Honors Physics II
Spring 2018

HW5 due in class Monday, February 19, 2018
Reading: Purcell 4.1-3

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 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. **4.20 Combining the current densities
It's fine to leave the answer in unit vector notation with i-hat pointing east and j-hat pointing north. Compute to two sig figs (i.e. assume given info is precise to two sig figs, e.g. n+ = 5.0 x 1016 m3).

2. *4.25 Drift velocity in seawater
Find the average drift SPEED, not average drift velocity as stated in the problem.

3. *4.32 Tapered rod
In class, our approach to finding the resistance was first to find the E field for a given voltage difference between the two "ends," find the current density from j=E/rho, and then do a surface integral to get I. But what if we are not able to find the fields easily for our system, as is the case here? Then you can use the approximate method given in the hint: you know how to find the resistance of a thin disk, and then add the resistances of the disks together. Take a look at Problem 4.6 to understand this better -- you will also find there a related shape (conical with spherical caps) for which we CAN find the E field easily and get the exact resistance as done in the class.

4. 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 method of images as described in Exercise 3.37 which we will discuss in class).
Note that this is NOT a steady current system as the charge distribution is changing with time.

OPTIONAL **4.21(b)
Current pulse from an alpha particle: planar electrodes

1. x component (east-west direction) is -1.3 x 104 A/m2
2. 2.5 x 10-7 m/s