Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Cutnell/Johnson
Physics
Classroom Response System Questions
Chapter 19 Electric Potential Energy and the
Electric Potential
Interactive Lecture Questions
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.1.1. Two electrons are separated by a distance R. If the distance between the
charges is increased to 2R, what happens to the total electric potential energy of
the system?
a) The total electric potential energy of the system would increase to four times its
initial value.
b) The total electric potential energy of the system would increase to two times its
initial value.
c) The total electric potential energy of the system would remain the same.
d) The total electric potential energy of the system would decrease to one half its
initial value.
e) The total electric potential energy of the system would decrease to one fourth its
initial value.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.1.2. The electric potential energy for two positive charges of
magnitude q and separated by a distance r is EPE1. What will the
electric potential energy be if one of the charges is completely
removed and replaced by a negative charge of the same
magnitude?
a) EPE2 = 2  EPE1
b) EPE2 = EPE1
c) EPE2 = EPE1
d) EPE2 = 2  EPE1
e) There is no way to determine this without knowing the value of q.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.2.1. Which one of the following statements best explains why it is
possible to define an electrostatic potential in a region of space that
contains an electrostatic field?
a) The work required to bring two charges together is independent of the
path taken.
b) A positive charge will gain kinetic energy as it approaches a negative
charge.
c) Like charges repel one another and unlike charges attract one another.
d) Work must be done to bring two positive charges closer together.
e) A negative charge will gain kinetic energy as it moves away from another
negative charge.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.2.2. Four point charges are individually brought from infinity and
placed at the corners of a square as shown in the figure. Each charge
has the identical value +Q. The length of the diagonal of the square
is 2a. What is the electric potential at P, the center of the square?
kQ
a)
4a
kQ
b)
a
c) zero volts
2kQ
d)
a
4kQ
e)
a
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.3.1. Consider the four arrangements of three point charges. Rank the values of
the total electric potential at point P in each case in descending order (with the
largest first).
a) VA > VD > VC > VB
b) VC > VB > VD > VA
c) VC > VD > VA > VB
d) VB > VC > VB > VA
e) VD > VB > VA > VC
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.3.2. Two point charges lie along the x axis. One charge, located at
the origin, has a magnitude +2q. The other charge of unknown
magnitude and sign is located at x = 5 units. If the electric
potential at x = 4 units is equal to zero volts, what is the
magnitude and sign of the second point charge?
a) q/2
b) q/4
c) 2q
d) +q/2
e) +2q
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.4.1. A proton is moved from point B to point A in an electric field
as shown. As a result of its movement, its potential increases to V.
If three protons are moved from point B to A, how much will the
electric potential of the protons increase?
a) V/9
b) V/3
c) V
d) 3V
e) 9V
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.4.2. Which one of the following statements concerning
electrostatic situations is false?
a) No work is required to move a charge along an equipotential
surface.
b) If the electric potential with a region of space is zero volts, the
electric field within that region must also be zero V/m.
c) The electric field is never perpendicular to equipotential surfaces.
d) The electric field is zero V/m everywhere inside a conductor.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.4.3. The drawing shows three point charges of equal magnitude, but
one is positive (shown in blue) and two are negative (shown in
yellow). Some of the equipotential lines surrounding these charges
are shown and five are labeled using letters A, B, C, D, and E. At
which of the labeled points will an electron have the greatest electric
potential energy?
a) A
b) B
c) C
d) D
e) E
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.4.4. The drawing shows three point charges of equal magnitude, but one is positive (shown
in blue) and two are negative (shown in yellow). Some of the equipotential lines
surrounding these charges are shown and five are labeled using letters A, B, C, D, and E.
What is the direction of the electric field at the location of the letter “D?”
a) perpendicular to the equipotential
line marked “D” and directed toward
the negative charge closest to it
b) parallel to the equipotential line
marked “D” and directed toward the
location of the letter “C”
c) perpendicular to the equipotential
line marked “D” and directed toward
the location of the letter “A”
d) toward the negative charge in the lower part of the drawing
e) toward the positive charge
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.5.1. An electrical outlet has two vertical slots and a hole into
which a three prong plug may be inserted. The maximum
potential difference between the two vertical slots is 120 volts.
The hole is connected to earth ground. Estimate the maximum
electric field that exists between the two vertical slots.
a) 240 V/m
b) 480 V/m
c) 6000 V/m
d) 120 V/m
e) 63000 V/m
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.5.2. The plates of an isolated parallel plate capacitor with a
capacitance C carry a charge Q. What is the capacitance of the
capacitor if the charge is increased to 4Q?
a) C/2
b) C/4
c) 4C
d) 2C
e) C
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.5.3. A parallel plate capacitor with plates of area A and plate
separation d is charged so that the potential difference between its
plates is V. If the capacitor is then isolated and its plate
separation is increased to 2d, what is the potential difference
between the plates?
a) 4V
b) 2V
c) V
d) 0.5V
e) 0.25V
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.5.4. The plates of an isolated parallel plate capacitor with a
capacitance C carry a charge Q. The plate separation is d.
Initially, the space between the plates contains only air. Then, an
isolated metal sheet of thickness 0.5d is inserted between, but not
touching, the plates. How does the potential difference between
the plates change as a result of inserting the metal sheet?
a) The potential difference will decrease.
b) The potential difference will not be affected.
c) The potential difference will increase.
d) The potential difference will be zero volts.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.5.5. The plates of an isolated parallel plate capacitor with a
capacitance C carry a charge Q. The plate separation is d. Initially,
the space between the plates contains only air. Then, a Teflon ( =
2.1) sheet of thickness 0.5d is inserted between, but not touching, the
plates. How does the electric field between the plates change as a
result of inserting the Teflon sheet?
a) The electric field will decrease to approximately one-half its initial
value.
b) The electric field will not be affected.
c) The electric field will increase to approximately twice its initial value.
d) The electric field will be zero volts per meter.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.5.6. The plates of an isolated parallel plate capacitor are separated
by a distance d and carry charge of magnitude q. The distance
between the plates is then reduced to d/2. How is the energy
stored in the capacitor affected by this change?
a) The energy increases to twice its initial value.
b) The energy increases to four times its initial value.
c) The energy is not affected by this change.
d) The energy decreases to one fourth of its initial value.
e) The energy decreases to one half of its initial value.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
19.5.7. A parallel plate capacitor is connected to a battery that
maintains a constant potential difference across the plates.
Initially, the space between the plates contains only air. Then, a
Teflon ( = 2.1) sheet is inserted between, but not touching, the
plates. How does the stored energy of the capacitor change as a
result of inserting the Teflon sheet?
a) The energy will decrease.
b) The energy will not be affected.
c) The energy will increase.
d) The energy will be zero joules.