PHYS2424 – SPRING 2000
EXAM #1 – PART 1
Electric Force, Fields, Potential, Capacitance, and Circuits
NAME ______________________________________________________
I.
Basic Equation and Definitions
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(50 marks)
II.
Multiple Choice
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(40 marks)
III.
Short Problems
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(45 marks)
TOTAL PART 1
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(135 marks)
TOTAL PART 2
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(310 marks)
TOTAL POINTS
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GRADE
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I.
Basic Equations and Definitions – ( 5 marks each)
1.
Write Coulomb’s Law :
2.
Definition of Electric Field
3.
Definition of Electric Flux
4.
Gauss’s Law
5.
Mathematical formula used to find the electric potential difference if one knows
the electric field.
6.
The equation that defines capacitance
7.
The definition of circuit elements in series
8.
The definition of circuit elements in parallel
9.
The definition of an open circuit
10.
The definition of a short circuit
II.
Multiple Choice ( 5 marks each)
1.
Two small spherical charges repulse each other with a force “F” when placed a
distance “d” apart. If the charge on each sphere is doubled, and the distance
between them is doubled, then the new force of repulsion is:
2.
A.
F/2
B.
F
C.
D.
F/4
E.
None of These
2F
The conducting thick spherical shell shown below has a hollow, empty center and
carries a total charge q.
D
A
B
C

The electric field E will be zero at:
A.
Point A only.
B.
Points A and B only.
C.
Points A,B, and C only.
D.
Point D only.
E.
None of These.
3.
4.
Compared to a single light bulb connected to a battery, two identical light bulbs
connected in series with the same battery will use
A.
more current.
B.
less current.
C.
the same current.
D.
Not enough information given.
The cross-section of a charged, solid, metal sphere, existing under electrostatic
conditions, is shown in the figure below. The electrostatic potential difference is
zero between points
D
B
E
A
C
A.
A and B.
B.
A and C.
D.
All of the above.
E.
None of these.
C.
D and E.
5.
Circle the most probable trajectory for a negatively charged fluorine ion fired
horizontally between the two charged capacitor plates shown below:
+ + + + + + + + + + + + + +
B
A
C
E
D
- - - - - - - - - - - - - -
6.
A point charge Q is placed in the center of a cube with sides of length A. The
electric flux through the right face of the cube
A.
D.
Q
ε0A
Q
6 ε0A 2
B.
Q
ε0
E.
None of these.
C.
Q
6 ε0A
7.
8.
Two identical conducting spheres, one with charge +3q and the other with charges
–q, are allowed to touch, and then they are separated. The force between the two
bodies before and after contact will be
A.
repulsive before and attractive after contact.
B.
repulsive before and after contact.
C.
attractive before and after contact.
D.
attractive before and repulsive after contact.
E.
Attractive before contact, but no attraction or repulsive after contact.
What is the equivalent resistance between points A and B of the circuit shown
below?
4
6
A
B
10 
A.
12.4 ohms
B.
1.93 ohms
D.
20.0 ohms
F.
None of these
C.
5.00 ohms
III.
Short Problems ( 15 marks each)
1.
Draw and label the electric field lines and equipotential surfaces for the following
charge distribution.
+2 Q
-Q
2.
Starting from the definition of a parallel capacitance, show that the capacitance of
εA
a parallel plate capacitor is given by the formula: C 
d
3.
Show that the voltage drop across a coaxial cable with an inner conductor of
radius A surrounded by an outer annular conductor of inner radius B is given by
λ
A
VBA 
ln   .
2πε 0  B 