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Ch 25 HW
Due: 8:00am on Friday, January 8, 2016
You will receive no credit for items you complete after the assignment is due. Grading Policy
Conceptual Question 25.1
Description: (a) Can an insulator be charged? (b) If so, how would you charge an insulator? If not, why not?
Part A
Can an insulator be charged?
ANSWER:
Yes
No
Part B
If so, how would you charge an insulator? If not, why not?
ANSWER:
3693 Character(s) remaining
An insulator can be charged. Plastic is an insulator. A plastic rod can be charged by rubbing it with wool.
Conceptual Question 25.2
Description: (a) Can a conductor be charged? (b) If so, how would you charge a conductor? If not, why not?
Part A
Can a conductor be charged?
ANSWER:
Yes
No
Part B
If so, how would you charge a conductor? If not, why not?
ANSWER:
3702 Character(s) remaining
A conductor can be charged. A conductor can be charged by touching it with another charged object.
Conceptual Question 25.3
Description: Four lightweight balls A, B, C, and D are suspended by threads. Ball A has been touched by a
plastic rod that was rubbed with wool. When the balls are brought close together, without touching, the
following observations are made: Balls B, C, and D...
Four lightweight balls A, B, C, and D are suspended by threads. Ball A has been touched by a plastic rod that was
rubbed with wool. When the balls are brought close together, without touching, the following observations are made:
Balls B, C, and D are attracted to ball A. Balls B and D have no effect on each other. Ball B is attracted to ball C.
Part A
What are the charge states (glass, plastic, or neutral) of balls A, B, C, and D?
ANSWER:
glass
Ball A is: neutral
plastic
Part B
ANSWER:
glass
Ball B is: neutral
plastic
Part C
ANSWER:
neutral
Ball C is: glass
plastic
Part D
ANSWER:
glass
Ball D is: neutral
plastic
Conceptual Question 25.03
Description: (a) When two point charges are a distance d part, the electric force that each one feels from the
other has magnitude F. In order to make this force twice as strong, the distance would have to be changed
to...
Part A
When two point charges are a distance d part, the electric force that each one feels from the other has
magnitude F . In order to make this force twice as strong, the distance would have to be changed to
ANSWER:
d
/4.
d
/2.
√2 d
/
.
d √2
2d .
Conceptual Question 25.04
Description: (a) A point charge Q is located a short distance from a point charge 3Q, and no other charges are
present. If the electrical force on Q is F, what is the electrical force on 3Q?
Part A
A point charge Q is located a short distance from a point charge 3Q , and no other charges are present. If the
electrical force on Q is F , what is the electrical force on 3Q ?
ANSWER:
F
F/3
3F
/
F √3
√3 F
Conceptual Question 25.8
Description: The two oppositely charged metal spheres in the figure have equal quantities of charge. They are
brought into contact with a neutral metal rod. (a) What is the final charge state of each sphere and of the rod?
The two oppositely charged metal spheres in the figure have equal quantities of charge. They are brought into
contact with a neutral metal rod.
Part A
What is the final charge state of each sphere and of the rod?
ANSWER:
Both the spheres and the rod are charged.
Both the spheres and the rod are neutral.
The rod is neutral, but the spheres have opposite charges.
Both spheres are neutral, but the rod is charged.
Conceptual Question 25.10
Description: Metal spheres A and B in the figure are initially neutral and are touching. A positively charged rod
is brought near A, but not touching. (a) Is A now positive, negative, or neutral?
Metal spheres A and B in the figure are initially neutral and are touching. A positively charged rod is brought near A,
but not touching.
Part A
Is A now positive, negative, or neutral?
ANSWER:
A is positive.
A is negative.
A is neutral.
Conceptual Question 25.14
Description: The electric field strength at one point near a point charge is E. (a) What is the field strength if
the distance from the point charge is doubled? (b) What is the field strength if the distance from the point
charge is halved?
The electric field strength at one point near a point charge is 3000 N/C .
Part A
What is the field strength if the distance from the point charge is doubled?
Express your answer with the appropriate units.
ANSWER:
E
= = 750
Part B
What is the field strength if the distance from the point charge is halved?
Express your answer with the appropriate units.
ANSWER:
E
= = 1.20×104
Problem 25.1
Description: A plastic rod is charged to ­12 nC by rubbing. (a) Have electrons been added to the rod or protons
removed? (b) How many electrons have been added or protons removed?
A plastic rod is charged to ­12 nC by rubbing.
Part A
Have electrons been added to the rod or protons removed?
ANSWER:
electrons have been added
protons have been removed
Part B
How many electrons have been added or protons removed?
Express your answer using two significant figures.
ANSWER:
N
= 7.5×1010 particles Problem 25.3
Description: A glass rod that has been charged to + q_1 touches a metal sphere. Afterward, the rod's charge
is + q_2. (a) What kind of charged particle was transferred between the rod and the sphere, and in which
direction? That is, did it move from the rod ...
A glass rod that has been charged to + 16.0 nC touches a metal sphere. Afterward, the rod's charge is + 9.0 nC
.
Part A
What kind of charged particle was transferred between the rod and the sphere, and in which direction? That is,
did it move from the rod to the sphere or from the sphere to the rod?
ANSWER:
electrons, from the rod to the sphere
protons, from the sphere to the rod
protons, from the rod to the sphere
electrons, from the sphere to the rod
Part B
How many charged particles were transferred?
Express your answer using two significant figures.
ANSWER:
N
= = 4.4×1010 particles Problem 25.4
Description: A plastic rod that has been charged to ­ q_1 touches a metal sphere. Afterward, the rod's charge
is ­ q_2. (a) What kind of charged particle was transferred between the rod and the sphere, and in which
direction? That is, did it move from the...
A plastic rod that has been charged to − 16 nC touches a metal sphere. Afterward, the rod's charge is − 1.0 nC .
Part A
What kind of charged particle was transferred between the rod and the sphere, and in which direction? That is,
did it move from the rod to the sphere or from the sphere to the rod?
ANSWER:
electrons, from the rod to the sphere
electrons, from the sphere to the rod
protons, from the rod to the sphere
protons, from the sphere to the rod
Part B
How many charged particles were transferred?
Express your answer using two significant figures.
ANSWER:
N
= = 9.4×1010 particles Problem 25.9
Description: Two neutral metal spheres on wood stands are touching. A negatively charged rod is held directly
above the top of the left sphere, not quite touching it. While the rod is there, the right sphere is moved so that
the spheres no longer touch. Then the...
Two neutral metal spheres on wood stands are touching. A negatively charged rod is held directly above the top of
the left sphere, not quite touching it. While the rod is there, the right sphere is moved so that the spheres no longer
touch. Then the rod is withdrawn.
Part A
Afterward, what is the charge state of each sphere?
ANSWER:
Both the spheres are neutral.
The left sphere is negatively charged, the right sphere is charged positively.
The right sphere is negatively charged, the left sphere is charged positively.
Both the spheres are charged positively.
Both the spheres are charged negatively.
Problem 25.12
Description: Two m masses are d apart (center to center) on a frictionless table. Each has + q of charge. (a)
What is the magnitude of the electric force on one of the masses? (b) What is the initial acceleration of the
mass if it is released and allowed to...
Two 1.2 kg masses are 1.0 m apart (center to center) on a frictionless table. Each has + 9.0 μC of charge.
Part A
What is the magnitude of the electric force on one of the masses?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
F
= = 0.73
Part B
What is the initial acceleration of the mass if it is released and allowed to move?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
a
= = 0.61
Problem 25.17
Description: (a) What is the magnitude of the net electric force on charge B in the figure? Assume a=## cm
and b=## cm. (b) What is the direction of the net electric force on charge B in the figure?
Part A
What is the magnitude of the net electric force on charge B in the figure? Assume a = 1.7 cm and b = 1.2 cm
.
Express your answer to two significant figures
and include the appropriate units.
ANSWER:
F
= 3.1×10−4
= Part B
What is the direction of the net electric force on charge B in the figure?
ANSWER:
The force is directed down.
The force is directed up.
Problem 25.20
Description: (a) What is the strength of the electric field E_p r from a proton? (b) What is the direction of the
electric field? (c) What is the strength of the electric field E_e r from an electron? (d) What is the direction of
the electric field?
Part A
What is the strength of the electric field Ep 0.60 mm from a proton?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
Ep
= = 4.0×10−3
Part B
What is the direction of the electric field?
ANSWER:
toward proton
away from proton
Part C
What is the strength of the electric field Ee 0.60 mm from an electron?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
Ee
= = 4.0×10−3
Part D
What is the direction of the electric field?
ANSWER:
toward electron
away from electron
Problem 25.22
Description: (a) What magnitude charge creates a E electric field at a point r away?
Part A
What magnitude charge creates a 8.30 N/C electric field at a point 3.40 m away?
Express your answer with the appropriate units.
ANSWER:
= 10.7
Problem 25.25
Description: (a) What is the strength of an electric field that will balance the weight of a m plastic sphere that
has been charged to ­q? (b) What is the direction of an electric field that will balance the weight of a m plastic
sphere that has been charged to ...
Part A
What is the strength of an electric field that will balance the weight of a 4.6 g plastic sphere that has been
charged to ­6.4 nC ?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
E
= = 7.0×106
Part B
What is the direction of an electric field that will balance the weight of a 4.6 g plastic sphere that has been
charged to ­6.4 nC ?
ANSWER:
downward
upward
Problem 25.30
Description: The nucleus of a ^125(Xe) atom (an isotope of the element xenon with mass 125 u) is 6.0 fm in
diameter. It has 54 protons and charge q = + 54e. (a) What is the electric force on a proton l from the surface
of the nucleus? Hint: Treat the spherical ...
The nucleus of a 125 Xe atom (an isotope of the element xenon with mass 125 u) is 6.0 f m in diameter. It has 54
protons and charge q = +54e .
Part A
What is the electric force on a proton 2.9 f m from the surface of the nucleus? Hint: Treat the spherical
nucleus as a point charge.
Express your answer to two significant figures and include the appropriate units.
ANSWER:
F
= = 360
Part B
What is the proton's acceleration?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
a
= = 2.1×1029
Problem 25.33
Description: (a) What is the magnitude of the force F_vec on the 1.0 nC charge in the figure ? (b) What is the
direction of the force F_vec on the 1.0 nC charge in the figure?
Part A
What is the magnitude of the force F ⃗ on the 1.0 nC charge in the figure ?
Express your answer to two significant figures
and include the appropriate units.
ANSWER:
F
−4
= 3.1×10
Part B
What is the direction of the force F ⃗ on the 1.0 nC charge in the figure?
ANSWER:
upward
downward
another direction
Problem 25.40
Description: (a) What is the force F_vec on the 1 nC charge at the bottom?
Part A
What is the force F ⃗ on the 1 nC charge at the bottom?
Write your answer as two vector components, separated by a
comma. Express each component numerically, in newtons, to two
significant figures.
ANSWER:
Fx , Fy
= 0, 1.1×10−5 N Also accepted: 0, 1.14×10−5
Problem 25.51
Description: In a simple model of the hydrogen atom, the electron moves in a circular orbit of radius 0.053 nm
around a stationary proton. (a) How many revolutions per second does the electron make?
In a simple model of the hydrogen atom, the electron moves in a circular orbit of radius 0.053 nm around a
stationary proton.
Part A
How many revolutions per second does the electron make?
Express your answer using two significant figures.
ANSWER:
6.6×1015
Problem 25.57
Description: You sometimes create a spark when you touch a doorknob after shuffling your feet on a carpet.
Why? The air always has a few free electrons that have been kicked out of atoms by cosmic rays. If an
electric field is present, a free electron is...
You sometimes create a spark when you touch a doorknob after shuffling your feet on a carpet. Why? The air
always has a few free electrons that have been kicked out of atoms by cosmic rays. If an electric field is present, a
free electron is accelerated until it collides with an air molecule. It will transfer its kinetic energy to the molecule,
then accelerate, then collide, then accelerate, collide, and so on. If the electron’s kinetic energy just before a
collision is 2.0×10−18 J or more, it has sufficient energy to kick an electron out of the molecule it hits. Where there
was one free electron, now there are two! Each of these can then accelerate, hit a molecule, and kick out another
electron. Then there will be four free electrons. In other words, as shows below, a sufficiently strong electric field
causes a “chain reaction” of electron production. This is called a breakdown of the air. The current of moving
electrons is what gives you the shock, and a spark is generated when the electrons recombine with the positive
ions and give off excess energy as a burst of light.
Part A
The average distance an electron travels between collisions is 2.0 μm . What acceleration must an electron
have to gain 2.0×10−18 J of kinetic energy in this distance?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
a
= = 1.1×1018
Also accepted: = 1.10×1018
, = 1.1×1018
Part B
What force must act on an electron to give it the acceleration found in part A?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
F
= = 1.0×10−12
Also accepted: = 1.00×10−12 , = 1.0×10−12
Part C
What strength electric field will exert this much force on an electron? This is the breakdown field strength.
Note: The measured breakdown field strength is a little less than your calculated value because our model of
the process is a bit too simple. Even so, your calculated value is close.
Express your answer to two significant figures and include the appropriate units.
ANSWER:
E
= = 6.3×106
= 6.25×106
Also accepted: = 6.3×106
, Part D
Suppose a free electron in air is 0.70 cm away from a point charge. What minimum charge is needed to cause
a breakdown and a spark as the electron moves toward the point charge?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
q
= Also accepted: = 34
= 34.0
, = 34
Problem 25.59
Description: Two m point charges on 1.0­m­long threads repel each other after being equally charged, as
shown in the figure. Assume that theta degree(s). (a) What is the charge q?
Two 2.6 g point charges on 1.0­m­long threads repel each other after being equally charged, as shown in the figure.
Assume that θ = 22 ∘ .
Part A
What is the charge q ?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
q
= 0.80
= Also accepted: , = ­0.80
= ­0.801
, , = 0.801
= 0.80
Problem 25.66
Description: An electric field E_vec=100,000 imath_unit N/C causes the 5.0 g point charge in the figure to
hang at a 20 degree(s) angle. (a) What is the charge on the ball?
An electric field E⃗ =
100, 000 ^
ı N/C
causes the 5.0 g point charge in the figure to hang at a 20∘ angle.
Part A
What is the charge on the ball?
Express your answer to two significant figures and include the appropriate units.
ANSWER:
q
= 180
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