net electric force exerted by a charge Q

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Clicker Questions for NEXUS/Physics
Electric Forces
A note on usage:
The clicker slides in this booklet are meant
to be used as stimuli to encourage class discussion.
They are intended for use in a class that attempts
to help students develop a coherent and sophisticated
understanding of scientific thinking.
They are NOT intended as items to test whether
students are “right or wrong” or “know” the correct
answer by one-step recall if enough cues are given.
This has a number of instructional implications
that are reviewed in general on the next four slides.
The individual slides also contain annotations
discussing their intended use.
Usage: 1
• Feedback
One of the most important values of a clickerresponse system is to provide instructors with
some understanding of what students are thinking.
Good clicker questions can be highly revealing
(and surprising). But the critical fact is not that the
students make mistakes but to use those mistakes
to probe their thinking and find out why.
This raises the importance of a rich subsequent
discussion well above “letting the students know
what the right answer is.”
Usage 2:
• Student-student interactions
The critical value for student learning occurs
in what happens after a clicker question has
obtained a mixed response from the students.
The standard next cue is, “Find someone
who disagreed with the answer you chose
and see if you can convince them.”
After a minute or two of discussion, a second click
may show students having moved dramatically
towards the correct answer. A brief call for who
changed their answer and why can lead to a
useful exchange. When they have not moved
significantly, more discussion is called for.
Usage: 3
• Incompletely specified questions
Some items have questions that are simple if idealized
assumptions are made, subtler if they are not. Part of
the discussion of these items are intended to include
issues of modeling, idealizations, and hidden
assumptions.
• Questions where answers are not provided.
In these items, the intent is to have students come up
with potential answers and have the instructor collect
them and write them on the board.
Occasionally, especially at the beginning of a class, it may
take some time before students are willing to contribute
answers. It can help if you have some prepared answers
ready, walk around the class, and put up the answers as if
they came from the students. This can help students get
more comfortable with contributing.
Usage: 4
• Cluster questions
Some questions are meant to be used as part of a
group of questions. In this case, resolving the answers
to individual questions is better left until the entire
group is completed. The value of the questions are
often in the comparison of the different items and in
having students think about what changes lead to what
differences and why.
• Problem solving items
In these items (indicated by a pencil cluster logo), the
intent is to have students work together to solve some
small problem. After a few minutes, ask the groups to
share their answers, vote on the different answers
obtained, and have a discussion.
A student suspends a small piece of aluminum foil by a
light insulating thread, holds the foil between her fingers
for a moment, then releases it. The aluminum foil is
then attracted towards a charged comb. Before it
actually touches the comb, the foil most
likely_________.
A. is neutral
B. has the same charge as the comb
C. has a charge opposite to the comb’s
A small piece of aluminum foil suspended by a
light insulating thread is attracted towards a
charged comb. Suppose that instead of the
comb you used a glass rod, which is charged
oppositely to the comb. In this case, the
aluminum foil would ___________ .
A. be attracted towards the glass rod
B. be repelled by the glass rod
C. do neither
D. you can’t tell from the information given
?
A + charged object is placed near a
conductor attached to an insulating
pedestal (a). After the opposite side of
the conductor is grounded for a short
time (b), the conductor becomes
negatively charged (c). Based on this
information, we can conclude that within
the conductor
1.
2.
3.
4.
both + and - charges move freely
only - charges move freely
only + charges move freely
We can’t really conclude anything
from the information given.
Two uniformly charged spheres are firmly fastened to and
electrically insulated from frictionless pucks on an air table.
The charge on sphere 2 is three times the charge on sphere 1.
Which force diagram correctly shows the relative magnitude
and direction of the electrostatic forces
Three pithballs are suspended from thin threads.
Various objects are then rubbed against other objects
(nylon against silk, glass against polyester, etc.)
and each of the pithballs is charged by touching them
with one of these objects. It is found that pithballs 1
and 2 repel each other and that pithballs 2 and 3
repel each other. From this we can conclude that
1.
2.
3.
4.
5.
1 and 3 carry charges of opposite sign.
1 and 3 carry charges of equal sign.
All three carry the charges of the same sign.
One of the objects carries no charge.
We need to do more experiments to determine
the sign of the charges.
Two identical conductors hang from
nonconducting strings. They are given charges
q1 = Q and q2 = 3Q. After charging, the two
strings make angles of θ1 and θ2 with the vertical.
How do the angles compare?
1.
2.
3.
4.
 1 > θ2
 1 < θ2
 1 = θ2
You don’t have
enough information
to tell.
In the figure are shown four arrangements
of charge. Each charge has the same
magnitude, but some are + and some are -.
All distances are to the same scale.
In which would the magnitude of the force felt by
a positive test charge placed at P
be the largest?
A.
B.
C.
D.
E.
A
B
C
D
You can’t tell.
In the figure are shown four arrangements
of charge. Each charge has the same
magnitude, but some are + and some are -.
All distances are to the same scale.
In which would the magnitude of the force felt by
a positive test charge placed at P
be the smallest?
A.
B.
C.
D.
E.
A
B
C
D
You can’t tell.
Find the magnitude and direction of
the force the charge Q exerts on
the dipole.
In the figure below are shown four two-dimensional
arrangements of charge. Each of the charges has
the same magnitude, but some are positive and
some are negative. In each diagram a point is
labeled “P”. Rank the diagrams by the magnitude of
the force felt by a positive test charge placed at P.
A test charge (labeled q) is placed in a situation
in which it feels the electrical force from three
other charges (of opposite sign to it) labeled A,
B, and C. (The charges are on a uniform grid
as shown and the positions are to scale.)
Which of the following combinations
of forces has the greatest magnitude?
1.
2.
3.
4.
FA®q
FB®q + FC®q
FA®q + FB®q + FC®q
There is not enough
information to tell.
A test charge, q, is a distance d from a
charge Q as shown. It feels an electric
field, E0. If q were replaced by a charge
–3q, the electric field on it would
1.
2.
3.
4.
5.
6.
Change to -3E0
Change to -E0/3
Not change
Change to 3E0
Change to E0/3
Something else
A test charge, q, is a distance d from a
charge Q as shown. It feels an electric
force, F0. If q were replaced by a charge
–3q, the electric force on it would
1.
2.
3.
4.
5.
6.
Change to -3F0
Change to -F0/3
Not change
Change to 3F0
Change to F0/3
Something else
Choose the letter corresponding to the
direction of the net electric force exerted by
a charge Q on a 3-charge molecule with the
charge distribution shown if Q is positive.
Choose the letter corresponding to the
direction of the net electric force exerted
by the molecule on the charge Q
if Q is positive.
Choose the letter corresponding to the
direction of the net electric force exerted by
a charge Q on a 3-charge molecule with the
charge distribution shown if Q is negative.
Choose the letter corresponding to the
direction of the net electric force exerted
by the molecule on the charge Q
if Q is negative.
Three molecules are shown, each made
up of two positive ions (+e) and a
negative ion (-2e). What is
the direction of the net electrical
force on a positive charge
placed at the x in situation #1?
A.
B.
C.
D.
E.
F.
points to the left (←),
points to the right (→),
points up (↑),
points down (↓),
is zero (0)
Cannot be determined
from the information given.
Three molecules are shown, each made
up of two positive ions (+e) and a
negative ion (-2e). What is
the direction of the net electrical
force on a positive charge
placed at the x in situation #2?
A.
B.
C.
D.
E.
F.
points to the left (←),
points to the right (→),
points up (↑),
points down (↓),
is zero (0)
Cannot be determined
from the information given.
Three molecules are shown, each made
up of two positive ions (+e) and a
negative ion (-2e). What is
the direction of the net electrical
force on a positive charge
placed at the x in situation #3?
A.
B.
C.
D.
E.
F.
points to the left (←),
points to the right (→),
points up (↑),
points down (↓),
is zero (0)
Cannot be determined
from the information given.
Three molecules are shown, each made up of
two positive ions (+e) and a negative ion (-2e).
What is the relation between
the magnitude of the force
on a positive test charge at the x
from the negative ion B to
the magnitude of the force
from the positive ion C ?
A.
B.
C.
D.
The forces are the same;
The force from B is greater;
The force from C is greater;
Cannot be determined
from the information given.
Three molecules are shown, each made up of
two positive ions (+e) and a negative ion (-2e).
What is the relation between
the magnitude of the force
on a positive test charge at the x
from the negative ion H to
the magnitude of the force
from the two positives ion GH ?
A.
B.
C.
D.
The forces are the same;
The force from B is greater;
The force from C is greater;
Cannot be determined
from the information given.

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