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I. Multiple Choice (50 pts total) 4 points each unless noted. Please choose the answer that is most
correct.
1) An electron is stuck to a neutral blob. The blob, in deep, deep space, (ignore gravity!) moves from left
to right and increases speed. The blob has moved from:
a) high to low potential
b) low to high potential
c) no potential difference
d) not enough information
2) From 1), what is the direction of the electric field, if any, that the blob moves through?
a) to the right
b) to the left
c) none
d) not enough information
3) (2pts) In a capacitor with a dielectric in it, why does the electric field not go to zero due to the induced
charges on the dielectric?
a) there aren’t enough charged particles in the dielectric to cancel the free charges on the plate
b) the charges run into obstacles as they move through the dielectric
c) the resonance of the charged dipoles isn’t sustained for a long enough time
d) the polarization of the dielectric isn’t enough to cancel the free charges on the plate
e) none of the above
4) A parallel plate capacitor, insulated from its surroundings, has a potential of Vo with a separation of d.
With all other variables remaining constant, what happens to the potential when d is halved?
a) 4Vo
b) 2Vo
c) Vo
d) Vo/2
e) Vo/4
5) A resistor has a potential drop of 5V when a current of 1.25A flows through it. If all other variables
remain constant, what is the current through the resistor if its length is tripled?
a) 0.104A
b) 0.209A
c) 0.417A
d) 0.833A
e) not enough information
6) Can a charged particle be moved through an electric field and have the field do no work on it?
a) no, it is impossible since electric fields always apply a force to a charged particle
b) no, when you change you position in the field you change your potential which means work was
done by the field
c) yes, by moving along equipotential lines the field does no work.
d) yes, the potential doesn’t necessarily do work if the field has a constant gradient
e) not known
7) Three capacitors are connected in series across a battery of potential 12V. If each capacitor has a
capacitance of 3F, what is the magnitude of the charge on each plate of the capacitors?
a) 3C
b) 6C
c) 9C
d) 12C
e) 24C
Exam 2 Physics 122A Aut05
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II. Multiple Choice Continued
8) A parallel plate capacitor has a charge of 1.2 C and the plates are separated by a distance of 3mm. The
left plate has a potential of 12V and right plate has a potential of 20V. What is the area of the
capacitor in 107m2?
a) 1.1
b) 2.34
c) 2.54
d) 5.08
e) 10.2
9) In a hydrogen atom, the ionization energy is defined as the energy required to move the electron from
the lowest energy (ground) state to infinity. If, in the ground state, the electron is an average distance
of 1.5a0, where ao=0.5x10-10m, from the proton, what is the ionization energy of the hydrogen atom in
10-18 joules? (this is actually slightly off from the actual value, quantum mechanics will give the
correct value)
a) 3.07
b) 4.47
c) 6.67
d) 9.34
10) From 9), with what speed must an electron be started if it is to escape the hydrogen atom, in 10 6m/s?
a) 1.2
b) 2.6
c) 3.6
d) 5.5
e) 9.1
11) A current of 1.4A flows through a cylindrical resistor of radius 3mm, what is the current density in the
wire in 104 C/m2?
a) 2.26
b) 3.56
c) 4.35
d) 4.95
e) 7.75
12) A region of space has a uniform electric field given by E= 13V/mi. Point A is at x=2m, and point B is
at x= -2m. Which point has the highest potential?
a) A
b) B
c) Both same potential
d) Not enough information
13) From 12) calculate the change in potential from A to B, or VAB.
a) 52V
b) 26V
c) 0V
d) -26V
e) -52V
Exam 2 Physics 122A Aut05
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Written Solutions: Show your work for full credit.
III. Cylindrical Conductors (30 pts): A cylindrical conductor with radius a and a cylindrical conducting
shell with inner radius b and outer radius c. The inner conductor has a total charge of –Q and the outer
conductor has a total charge of +Q and both have a length L, which is much greater than the radius c, L>>c.
We will define λ=Q/L All vectors should be in unit vector notation.
a)
Find the electric field, E, between the cylinders using Gauss’ law.
+Q
-Q
b) Find the potential difference, V, between the cylinders.
c) Find the capacitance per unit length, C/l, of the cylinders. l is some arbitrary length along the cylinders.
Exam 2 Physics 122A Aut05
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