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PHYS 102 Midterm Examination #1 (version B)
June 19, 2015
Time: 50 minutes
Last Name :
____________________________
First Name : ____________________________
Student No. : ___________________________
Computing ID : __________________________
Tutorial Section (TA): _____________________
score
Maximum
Multiple Choice
10
Written # 11
5
Written # 12
5
Written # 13
5
Total
25
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Part I (Multiple choice questions. 1 mark each.)
1.
You are sitting a certain distance from a point charge, and you measure an
electric field of E0. If the charge is doubled and your distance from the charge is also
doubled, what is the electric field strength now?
A) 2E0 ;
B) 4E0 ;
C) E0 ;
D) 1/4E0 ;
E) 1/2E0.
2. Which of the arrows best represents the direction of the net
force on charge +Q due to the other two charges?
A) 1
B) 2
C) 3
D) 4
E) 5
3. Does a water molecule carry an electric dipole moment?
A) No, because the molecule is electrically neutral.
B) Yes, because of the uneven distribution of electron
density and the asymmetrical structure.
C) No, because the molecule is symmetrical and the net
dipole moment is zero.
D) Can’t tell from the structure of the molecule.
E) It depends on the environment such as the existence
of an external field.
4. A parallel-plate capacitor carries charge Q and is then disconnected from a battery.
The two plates are initially separated by a distance d. Suppose the plates are pulled
apart until the separation is 2d. What happens to the stored energy and charge?
A) Neither the stored energy nor the stored charge changes.
B) The stored energy is decreased but the charge remains the same.
C) The stored energy is increased but the charge remains the same.
D) The stored energy does not change but the stored charge is increased.
E) The stored energy does not change but the stored charge is decreased.
5. The lightbulbs in the circuits below
are identical with the same
resistance R. Which circuit
consumes more power?
A) circuit I
B) circuit II
C) both the same
D) it depends on R
E) it depends on V
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6. The electric field lines (solid) and the equipotential lines
(dashed) in a certain region of space are as shown.
Compare the work done moving an electron from A to D
and from B to C. Which one requires more work?
A) More work is required to move an electron from A
to D than from B to C.
B) More work is required to move an electron from B
to C than from A to D.
C) The same amount of work is required to move an
electron from A to D as to move it from B to C.
D) Cannot determine without performing the
calculation.
A
B
C
7. An electron enters a uniform magnetic field that is perpendicular to the electron’s
velocity. What happens to the kinetic energy of the electron?
A) It increases.
B) It stays the same.
C) It decreases.
D) It depends on the velocity direction.
E) It depends on the B field direction.
8. A positive charge +q and an electric
current I are both in the y-z plane. The
charge moves in the z-direction and the
current flows in the opposite direction, as
shown in the figure. What is the direction
of the force acting on the charge?
A) +x (out of page)
B) -x (into page)
C) +y
D) -y
E) +z
D
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9. In a typical animal cell, the cell membrane separates the fluids inside and outside the
cell, which can be modeled by a parallel-plate capacitor. Assuming that the diameter of
a cell is about 10-5 m, the thickness of the cell membrane is about 6 nm, and the
dielectric constant of the membrane is 7, estimate the capacitance.
A) 10-3 F
B) 10-6 F
C) 10-9 F
D) 10-12 F
E) 10-15 F
10. The membrane potential of a living cell described in the previous question is
approximately
A) 120 V
B) 12 V
C) 1.5 V
D) 70 kV
E) 70 mV
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Part II (Full solution questions, 5 marks each. SHOW ALL WORK FOR FULL MARKS!)
10.
A sphere of radius R carries a volume charge density ρ which is constant and
positive. A spherical cavity of radius R/2 is then scooped out and left empty, as shown.
Points A and C are at the centers of the respective spheres.
(a) What is the magnitude and direction of the electric field at point A?
(b) What is the direction and magnitude of the electric field at point B?
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11. The circuit shown consists of four resistors and three (ideal) batteries. The values of all
circuit elements are given in the figure.
What is the magnitude and direction of the current through resistor R3?
R1
R1 = 1 
R2 = 2 
R3 = 3 
R4 = 1 
E1
R3
R2
R4
E3
E2
E1 = E2 = E3 =3 V
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The diagram below depicts an RC circuit where C=5.0μF, R1=R2=40Ω, and
V0=6.0V.
(a) What is the current through R1 immediately after
the switch S is closed?
(b) What is the current through R1 after the switch S
has been closed for a very long time? (Assume that
the battery does not go dead.)
(c) Calculate the time constant of charging the
capacitor.
12.
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Physics 102 Formula Sheet (for Midterm #1)
Q1Q2
1 Q1Q2

(magnitude )  0  8.85  10 12 C 2 / N  m 2
2
2
4 0 r
r
 
Q
Gauss's law:  E   E  A   F A  encl
0
closed
closed
Coulomb's law: F  k
surface
surface
Electric potential due to a point charge: V 
1
Q
4 0 r
(assuming V  0 at r  )
Capacitance: Q  CV
A
d
Ceq  C1  C2  C3
Parallel-plate capacitor: C  K  0
Capacitors in parallel:
Capacitors in series:
1
1
1
1



Ceq C1 C 2 C3
Energy stored in a capacitor: U 
I
Ohm's law:
Resistivity : R  
V
,
R
1
1
1 Q2
QV  CV 2 
2
2
2 C
V  IR
l
A
Electric power: P  IV  I 2 R 
V2
R
Root-mean-square (rms) value: I rms 
Resistors in series:
I0
2
Vrms 
,
V0
2
Req  R1  R2  R3
1
1
1
1



Req R1 R2 R3
Time constant of RC circuits:   RC
Resistors in parallel:


Charging and discharging: VC   1  e t / RC ;
VC  V0 e t / RC .
Magnetic force on an electric current: F  IlB sin 

 
Magnetic force on a moving charge: F  qv  B