Download PH203 exam 1 2014

PH203 2014
Exam #1
Name ____________________________________________
Part I: True/False
_____1) Two nuclei each contain two protons and exert a force 4 N on each other. If we
transfer a proton from one of these nuclei to the other, the force on each nucleus is now
reduced to 3 N.
_____2) Two neutral objects cannot attract each other electrically because they have no
excess charge.
_____3) Two infinite parallel plastic sheets contain equal negative charge densities
uniformly spread over their surfaces. The electric field is zero everywhere between these
sheets, not just midway between them.
_____4) It is impossible for any object to have a charge of 7.5 e.
_____5) Two protons slightly separated from one another form an electric dipole.
_____6) Two equal positive charges are separated by a fixed distance. If you put a third
positive charge midway between these two charges, its electrical potential energy
(relative to infinity) is zero because the electrical forces due to the two fixed charges just
balance each other.
_____7) You want to put an electron somewhere between the plates of a parallel plate
capacitor so that it will have the maximum electrical potential energy relative to one of
the plates. The best place to put it is at the inner surface of the negative plate.
_____8) Suppose you have two point charges, +Q and -q. As you pull them farther and
farther apart, you increase the potential energy of this system relative to infinity.
_____9) If the electrical potential in a region is constant, the electric field must be zero
everywhere in that region.
_____10) Four identical positive point charges lie at the corners of a square. The
electrical potential at the center of this square is zero because the electric fields all cancel
at that point.
Part II: Multiple Choice
_____11) If two macroscopic objects are electrically attracted to each other,
A) both objects must be negatively charged.
B) both objects must be positively charged.
C) one object must be negatively charged and the other object must be positively
charged.
D) None of the above statements are absolutely true.
_____12) A plastic rod is charged up by rubbing a wool cloth, and brought to an initially
neutral metallic sphere. It is allowed to touch the sphere for a few seconds, and then is
separated from the sphere by a small distance. After the rod is separated, the rod
A) is repelled by the sphere.
B) is attracted to the sphere.
C) feels no force due to the sphere.
_____13) A positive test charge q is released near a positive fixed charge Q.
As q moves away from Q, it will move with
A) constant velocity.
B) constant acceleration.
C) increasing acceleration.
D) decreasing acceleration.
_____14) X and Y are two uncharged metal spheres on insulating stands, and are in
contact with each other. A positively charged rod R is brought close to X as shown in
figure 1.
Sphere Y is now moved away from X (figure 2).
The final charge states of X and Y, respectively, are
A) neutral and neutral.
B) positive and neutral.
C) neutral and positive.
D) negative and positive.
_____15) If the voltage at a point in space is zero, then the electric field must be
A) negative.
B) zero.
C) uniform.
D) positive.
E) Impossible to determine based on the information given.
_____16) One charged plastic ball is vertically above another in a test tube as shown. The
balls are in equilibrium a distance d apart.
If the charge on each ball is doubled, the distance between the balls in the test tube would
become
A) d.
B) 2d.
C) 4d.
D) 8d.
_____17) Two identically charged balls are a certain distance apart. The vectors in the
diagram below show the magnitude and direction of the electrostatic force on each ball.
Suppose the charge on the left ball is now doubled (represented by two plus signs).
Which diagram below best represents the forces that now act on the two balls?
A)
B)
C)
D)
_____18) A metal sphere is insulated electrically and is given a charge. If 25 electrons
are added to the sphere in giving a charge, how many Coulombs are added to the sphere?
A) - 4.0 × 10-20 C
B) - 4.0 × 10-18 C
C) - 25 C
D) - 40 C
_____19) A proton is located at x = 1.0 nm, y = 0.0 nm and an electron is located at x =
0.0 nm, y = 4.0 nm. Find the attractive Coulombic force between them. (The value of k
is 9.0 × 109 N·m2/C2.)
A) 5.3 × 10-18 N
B) 5.9 × 10-15 N
C) 1.4 × 10-11 N
D) 5.3 × 108 N
_____20) Two electrons are passing 26.0 mm apart. What is the electric repulsive force
that they exert on each other? (The value of k is 9.0 × 109 N·m2/C2.)
A) 3.4 × 10-27 N
B) 3.4 × 10-25 N
C) 1.3 N
D) 1.3 × 1010 N
_____21) Two equally charged spheres of mass 1.0 g are placed 2.0 cm apart. When
released, they begin to accelerate at 414 m/s2. What is the magnitude of the charge on
each sphere?
A) 75 nC
B) 95 nC
C) 120 nC
D) 140 nC
_____22) What is the magnitude of an electric field that balances the weight of a plastic
sphere of mass 6.4 g that has been charged to -3.0 nC?
A) 2.4 × 106 N/C
B) 4.5 × 106 N/C
C) 6.4 × 106 N/C
D) 2.1 × 107 N/C
_____23) In the figure Q = 5.8 nC and all other quantities are exact. What is the
magnitude of the force on the charge Q?
A) 9.0 × 10-4 N
B) 1.0 × 10-3 N
C) 1.2 × 10-3 N
D) 1.8 × 10-3 N
_____24) The charge in the bottom right corner of the figure is Q =- 45 nC. What is the
magnitude of the force on Q?
A) 1.4 × 10-2 N
B) 1.9 × 10-2 N
C) 2.6 × 10-2 N
D) 3.5 × 10-2 N
_____25) A point charge Q = -12 μC, and two other charges, q1 and q2, are placed as
shown. The electric force components on charge Q are Fx = +0.005 N and Fy = -0.003 N.
In the figure, the number of excess electrons in charge Q is closest to:
A) 5.5 × 1013
B) 6.5 × 1013
C) 7.5 × 1013
D) 8.5 × 1013
E) 9.5 × 1013
_____26) The figure shows two 5.0 g spheres suspended from 1.0-m-long threads. The
spheres repel each other after being charged to +91 nC. What is the angle θ?
A) 4.1 degrees
B) 8.2 degrees
C) 12 degrees
D) 16 degrees
_____27) The diagram shows electric field lines arising from two small charged particles
P and Q.
Consider the following two statements:
i.
The charge on P is smaller than the charge on Q.
ii.
The electrostatic force on P is smaller than that on Q.
Which of the above statements are true?
A) Only i
B) Only ii
C) Both i and ii
D) Neither i nor ii
_____28) The diagram shows two unequal charges +q and -Q, of opposite sign. Charge
Q has greater magnitude than charge q. Point X is midway between the charges.
In what section of the line will there be a point where the resultant electric field is zero?
A) VW
B) WX
C) XY
D) YZ
_____29) The capacitance of a capacitor depends on
A) the charge on it.
B) the potential difference across it.
C) the energy stored in it.
D) More than one of these
E) None of these
_____30) A proton is accelerated from rest through a potential difference Vo and gains a
speed vo. If it were accelerated instead through a potential difference of 2Vo, it would
gain a speed:
A) 8 vo
B) 4 vo
C) 2 vo
D) vo 2
_____31) Four charged particles (two having a charge +Q and two having a charge -Q)
are distributed on the xy-plane, as shown below. Each charge is equidistant from the
origin. The voltage is zero at infinity. The amount of work required to move a positively
charged particle from point P to point O (which is on the z-axis, below the origin) is
A) zero.
B) positive.
C) negative.
D) dependent on the path in which the charged is moved.
_____32) Two parallel plates are separated by 1.0 mm. If the potential difference
between them is
3.0 V, what is the magnitude of their surface charge densities?
A) 27 nC/m2
B) 53 nC/m2
C) 0.16 mC/m2
D) 0.33 mC/m2
_____33) As an electron moves from a high potential to a low potential, its electrical
potential energy
A) decreases.
B) remains constant.
C) increases.
_____34) Suppose a region of space has a uniform electric field, directed toward the
right, as shown in the figure. Which statement is true?
A) The voltage at all three locations is the same.
B) The voltage at points A and B are equal, and the voltage at point C is higher than the
voltage at point A.
C) The voltage at points A and B are equal, and the voltage at point C is lower than the
voltage at point A.
D) The voltage at point A is the highest, the voltage at point B is the second highest, and
the voltage at point C is the lowest.
E) None of the above
_____35) A hydrogen atom consists of a proton and an electron. If the orbital radius of
the electron increases, the potential energy of the electron
A) increases.
B) decreases.
C) remains the same.
D) depends on the zero point of the potential.
_____36) A 1.0 μF capacitor has a potential difference of 6.0 V applied across its plates.
If the potential difference across its plates is increased to 8.0 V, how much additional
energy does the capacitor store?
A) 2.0 μJ
B) 4.0 μJ
C) 14 μJ
D) 28 μJ
_____37) Two conductors are joined by a long copper wire. Thus
A) each conductor carries the same free charge.
B) each conductor must be at the same potential.
C) the electric field at the surface of each conductor is the same.
D) no free charge can be present on either conductor.
E) the potential on the wire is the average of the potential of each conductor.
_____38) A 3.0 μC point charge and a 9.0 μC point charge are initially infinitely far
apart. How much work does it take to bring the 3.0 μC point charge to x = 3.0 mm, y =
0.0 mm, and the 9.0 μC point charge to x = -3.0 mm, y = 0.0 mm? (The value of k is 9.0
 109 N·m2/C2.)
A) 41 J
B) 81 J
C) 27 J
D) 6.8 J
_____39) A dipole with ±6.0 μC charges is positioned so that the positive charge is 1.0
mm to the right of the origin and the negative charge is at the origin. How much work
does it take to bring a
10.0 μC charge from infinity to a position x = 3.0 mm, y = 0.0 mm? (The value of k is 9.0
 109 N·m2/C2.)
A) 75 J
B) 90 J
C) 200 J
D) 450 J
_____40) A proton with a speed of 2.0 x 105 m/s falls through a potential difference V
and thereby increases its speed to 4.0 × 105 m/s. Through what potential difference did
the proton fall?
A) 100 V
B) 210 V
C) 630 V
D) 840 V
E) 1000 V