Download Exam 1

Name___________________________________________ Student Number_______________________
Last
First
MI
I.
Multiple Choice (4pts each) Fill in the most correct answer on the bubble sheet, and circle the correct
answer on this page for future reference.
1) (5pts)If gravitation in the atom is 1039 times weaker than the electrostatic force, why is it that gravity dominates
over the large scale of the universe?
a) the electrostatic force is a short range force
b) gravitation acts over a larger distance than electrostatics
c) electrostatics doesn’t act on many different particles
d) most matter in bulk is neutral
e) no explanation is known
2) Of the four fundamental forces, the one responsible for the normal force is:
a) Strong
b) Electromagnetic
c) Weak
d) Gravitation
e) Nonsense-the normal force is its own force
3) When using Gauss’ law to do calculations, what aspect of the system will allow a solution to be found?
a) symmetry
b) Coulomb’s law
c) invertibility of the E-field
d) test charges known
e) no aspects are important
4) You rub a glass rod with silk and then touch it to a conducting ball insulated from the ground, then remove the
glass rod. You then connect a wire from the ground to the ball and then disconnect it. You bring the glass rod
close to the ball again, what happens?
a) it is attracted to the rod
b) it is repelled by the rod
c) nothing happens—the ball is now neutral
d) not enough information, you must know the sign of the charge on the glass rod
5) An electron accelerates at a constant rate starting from rest to the right through a distance of 0.65m in a time of
0.015s. What is the magnitude of the electric field through which the electron moves, in 10 -8 N/C?
a)1.22
b)2.29
c)5.79
d)3.89
e)3.29
6) From 5), what is the direction of the electric field?
a) to the right
b) to the left
c) up
d) down
e) not enough information
7) A proton starts from rest near the surface of a sheet of charge. It experiences a force of 14.0 microNewtons
towards the sheet, what is the magnitude of the surface charge density of the sheet in 102 C/m2?
a)3.42
b)5.56
c)8.55
d)11.4
e)15.5
Phys 122 exam 1 sum06
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Name___________________________________________ Student Number_______________________
Last
First
MI
8) Consider a point on the axis of, and some distance from, a disk of charge. If the total charge Q is fixed on the
disk, how would we decrease the electric field at the point?
P
a)
b)
c)
d)
decrease distance between disk and point
decrease the diameter of the disk
increase the radius of the disk
not enough information
9) A square sheet has a charge density of =5xy C/m4 and sides of length 1.4m. What is the total charge on the
square?
a)2.40
b)3.40
c)4.40
d)5.40
e)6.40
10) A cube has sides of 1.2m and a charge of 4C in the center. What is the electric flux through the cube, in 10 11
Nm2/C?
a) 4.52
b) 6.52
c) 7.52
d) 9.52
e) 15.52
11) (5pts)Given two charges Q=-2C at x=0, and q=10C at x=5cm, in which region is there a point at which the
electric field is zero?
a) x<0
b) 0<x<5cm
c) x>5cm
d) It will never be zero
12) From 12), if you were to move a distance away, x>>5cm, what would the electric field look like?
a) E=infinity
b) E=k 8C/x2 in +x direction
c) E=k 8C/x2 in –x direction
d) E=k 12C/x2 in +x direction
e) E=k 12C/x2 in -x direction
Phys 122 exam 1 sum06
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Name___________________________________________ Student Number_______________________
Last
First
MI
II.
Written Solution: 30 Points. Be sure to show all of your work for full credit.
An infinite cylindrical shell of radius R carries a charge density of . Assume the shell is very thin.
II
I
R
A) (2pts)On the two diagrams above, draw the Gaussian surface you would use
to calculated the electric field for the region inside the shell, I.
R
B) (2pts)On the two diagrams above, draw the Gaussian surface you would use
to calculate the electric field for the region outside of the shell, II.
C) (9pts)What is the electric field in region I, 0<r<R. Use Gauss’ law.
D) (9pts)What is the electric field in region II, R<r<. Use Gauss’ law.
E) (8pts)If a line charge, centered at r=0, with a charge density - was inserted, in which region(s) would the
field change? Explain your reasoning using Gauss’ law.
Phys 122 exam 1 sum06
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