Physics 272: Advanced Honors Physics II
due in class Monday, February 19, 2018
Reading: Purcell 4.1-3
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
2. *4.25 Drift velocity in seawater
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.
4.21(a) Current pulse from an alpha particle: planar
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
x component (east-west direction) is -1.3 x 104
2. 2.5 x 10-7 m/s