# Physics 3221 Fall Term 2004 Test 1, October 1, 2004

```Physics 3221
Fall Term 2004
Test 1, October 1, 2004
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This is an open notes test lasting 50 minutes.
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There are two problems, divided into subsections. Problem 1 has 2 parts
and problem 2 has 4 parts. The points for each part are marked. Bold
face letters denote vectors.
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Begin each problem on a fresh sheet of paper. Use only one side of a
sheet of paper.
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Put your name, the problem number, and the page number in the upper
right hand corner of each sheet.
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To receive partial credit you must explain what you are doing. Carefully
labeled figures are important. Randomly scrawled equations aren't helpful.
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Draw a box around important results.
Some constants which may be useful:
Permittivity of free space ε0 = 8.85 &times; 10-12 C2/N.m2
electronic charge e = 1.6 &times; 10-19 C
electronic mass m = 9.1 &times; 10-31 kg
Boltzmann’s constant kB = 1.38 &times; 10-23 J/K
There are 2 pages including this page. Do not forget to look at all parts of the
problems.
Problem 1.
(a) The conductivity (σ) of a material is the inverse of its resistivity (ρ) (resistivity is
defined using the formula R = ρl/A). The σ of a metal depends on the number of
free electrons per unit volume (n), the electronic charge (e), the electronic mass
(m) and a parameter τ which denotes the time between two collisions of an
electron with other objects in the metal such as impurities. Using dimensional
analysis express σ in terms of n, e, m and τ. (6 points)
(b) For the metal Lithium, the value of resistivity (ρ) at 77 K is 1.04 &micro;Ω-cm and the
value at 273 K is 8.55 &micro;Ω-cm. If the τ at 77 K is 7.3 &times; 10-14 seconds, use your
result from (a) to find the value of τ at 273 K. (2 points)
Problem 2.
(a) Let R be the distance from a fixed point A with coordinates (a,b,c) to any point P
with coordinates (x,y,z). Show that ∇R is a unit vector in the direction AP = R. (5
points)
(b) Let P be any point on an ellipse whose foci are at points A and B as shown in the
figure. The sum of the distances from A to any point P on the ellipse and from B
to point P on the ellipse is a constant (AP + BP = constant). Using this property
write down the equation of the ellipse in terms of R1 and R2. (2 points)
(c) Find a normal vector to the ellipse at point P using the equation from part (b). (2
points) [Hint: Use the necessary property of the gradient operator that we
worked out in class, only in this case there are only 2-dimensions. You can