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Chapter 20
Electric Potential and Electric Potential Energy
Answers to Even-numbered Conceptual Questions
2.
The electric potential energy of the system decreases.
In fact, it is converted into the
kinetic energy gained by the electron.
4.
The electric potential energy of the system decreases.
In fact, the kinetic energy gained by
the proton is equal to the decrease in the electric potential energy.
6.
The two like charges, if released, will move away from one another to infinite separation,
converting the positive electric potential energy into kinetic energy.
The two unlike
charges, however, attract one another – if their separation is to be increased, a positive
work must be done.
In fact, the minimum amount of work that must be done to create an
infinite separation between the charges is equal to the magnitude of the original negative
electric potential energy.
8.
The electric potential energy of the proton-electron system is negative, as required to
produce a bound atom.
(a)
If the electron is replaced with a proton, the two protons will
repel one another and move off to infinite separation.
This means that the initial electric
potential energy of this system is positive, and that this positive potential energy is
converted to kinetic energy.
Therefore, the electric potential energy of the system
increased by changing the electron to a proton.
(b)
If the proton is replaced with an
electron the electric potential energy of the system increases, for exactly the same reason
given in part (a).
10.
Point 2 is closer to
than to
by a factor of the square root of 2.
Therefore, if the
electric potential is to be zero at point 2, it is necessary that
be negative, and have a
magnitude that is less than the magnitude of
by a factor of
q
2
q
1
q
2
q
1
2
.
We conclude, then,
that
q
2
=
–
Q
/2
.
Notice that point 1 is closer to the positive charge +Q than to the
negative charge
–
Q
, and that the negative charge has a smaller magnitude. It
follows that the electric potential at point 1 is positive.
12.
Not necessarily.
The electric field is related to the rate of change of electric potential, not
to its value.
Therefore, if the electric field is zero in some region of space, it follows that
the electric potential is constant in that region.
The constant value of the electric potential
may be zero, but it may also be positive or negative.
31

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*Sign up*Chapter 20:
Electric Potential and Electric Potential Energy
Physics: An Introduction
14.
An equipotential surface must always cross an electric field line at right angles.
Therefore, the equipotential surfaces in this system must have the shapes indicated here:
1
2
3
E
field lines
equipotential surfaces
16.
We know that the value of the electric potential must decrease as we move in the direction
of the electric field (see, for example, Figure 20-3.)
It follows that the electric potential
decreases in value as we move from point 1 to point 2 to point 3.

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