1'Scientists Ideas: Magnetic Interactions

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Plan for todays class
1.Scientists Ideas Magnetic Interactions 2.
Demonstrations 3. Chapter 3, activity 2, p 3-27
to 3-40 4. S/Q, p 3-37 to 3-40 5. Chapter 3,
activity 2 supplement
Graduation ceremony for CEHD Final exam time
(5/14) 630 to 930 PM
Note Quiz 3 will be on Wednesday (April
1) There will be no practice exam.
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Idea M1 Magnetic interactions between two
magnets Idea M2 Magnetic interaction between a
magnet and a ferromagnetic
object Idea M3 Action at a distance Idea M4
Magnetic potential energy Idea M5 Kinds of
materials involved in magnetic interactions Idea
M6 Model of magnetism
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M3 Action at a distance A magnet can exert
forces on another magnet, or a ferromagnetic
object, without touching it. (Scientists Call
this action at a distance.) These forces can be
represented on a force diagram in the same way
as any other forces acting on the magnet
(This phenomenon of action at a distance can be
accounted for by the idea of an invisible
magnetic field of influence that extends
around a magnet. Any other magnets within this
magnetic field will feel attractive and repulsive
forces due to the influence of the field on them.)
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Idea M4 Magnetic Potential Energy In any
system of magnets (or magnets and ferromagnetic
objects) there is magnetic potential energy, the
amount of which depends on how the magnets (and
ferromagnetic objects) are arranged with respect
to each other. When the magnets (and
ferromagnetic objects) are rearranged this
magnetic potential energy may change. When this
happens in a system with no energy inputs or
outputs (a closed system), then, according to
the Law of Conservation of Energy, if the
magnetic potential energy in the closed system
increases (decreases), then the kinetic energy of
the objects in the system will decrease
(increase), and vice versa. For example, when
two carts with magnets attached push each other
apart, the energy diagram for this system would
be like this
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During time that both magnet-carts are pushing
each other further apart
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Plan for todays class
1.Scientists Ideas Magnetic Interactions 2.
Demonstrations 3. Chapter 3, activity 2, p 3-27
to 3-40 4. S/Q, p 3-37 to 3-40 5. Chapter 3,
activity 2 supplement
Graduation ceremony for CEHD Final exam time
(5/14) 630 to 930 PM
Note Quiz 3 will be on Wednesday (April
1) There will be no practice exam.
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Plan for todays class
1.Scientists Ideas Magnetic Interactions 2.
Demonstrations 3. Chapter 3, activity 2, p 3-27
to 3-40 4. Break, 140 to 155 PM 5. 220 PM,
discuss S/Q, p 3-37 to 3-40, 6. Activity 2, HW
1 and 2 7. Chapter 3, activity 2 supplement
Graduation ceremony for CEHD Final exam time
(5/14) 630 to 930 PM If you have the conflict,
please let me know during the break.
Note Quiz 3 will be on Wednesday (April
1) There will be no practice exam.
9
Plan for todays class
1.Scientists Ideas Magnetic Interactions 2.
Demonstrations 3. Chapter 3, activity 2, p 3-27
to 3-40 4. Break, 140 to 155 PM 5. 225 PM,
discuss S/Q, p 3-37 to 3-40, 6. Activity 2, HW
1 and 2 7. Chapter 3, activity 2 supplement
Graduation ceremony for CEHD Final exam time
(5/14) 630 to 930 PM If you have the conflict,
please let me know during the break.
Note Quiz 3 will be on Wednesday (April
1) There will be no practice exam.
10
S/Q, p 3-37 to 3-40 S1 In experiment 1 you
rubbed a balloon against your hair. Answer
the following questions in terms of the
model for charges in materials. a) After
rubbing, what was the type of (excess) charge
on the rubbed part of the balloon?/ hair?
Explain briefly how the balloon and hair
got charged in this way.
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After rubbing, if you brought the rubbed part of
the balloon near the rubbed part of your hair,
you found that the hair was attached to the
balloon. b) Draw a force diagram showing the
force on the rubbed hair as it was attracted
to the rubbed part of the balloon, and
explain briefly why the hair was attracted to
the balloon.
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S2 In experiment 1 you created pairs of charged
tapes by ripping them apart very quickly.
As you should have inferred earlier, the
tapes you labeled B (bottom) became
negatively charged in this process, while the
tapes you labeled T (top) became
positively charged.
Three students were discussing how they thought
the model of charges in materials could
account for how the two tapes became charged
during this process.
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Do you agree with any of them? Give your
reasoning.
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S3 Consider a system consisting of two objects,
one positively and one negatively
charged. a) As these two objects move farther
apart, does the strength of the electric
charge force increase, decrease, or remain
the same? How do you know? b) Does the value
of the electrostatic potential energy
increase, decrease or remain the same? Reasoning.
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S4 Consider a system of two charged tapes that
repel each other. Draw an I/O energy
diagram to describe the situation when the
two tapes are close together but are
starting to move away from each other. (Ignore
the fact that you are holding the ends of the
tapes in your diagram.)
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S5 In general, in a system that involves a type
of potential energy, if there are no
energy inputs to or output from the
system, what happens to that potential energy
when the K.E. of the system decreases?
increase? Why do you think there is this simple
relationship between changes in K. E. and
change in P. E.?
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S/Q P 6 to 7 S1 In each experiment, which
objects have electric charges? Do all the
objects have the same type of static charges?
Evidence? S2 How many different types of
electric charges are there? What evidence
do you have to support this? S3 How do
different types of electric charges interact
with each other? S4 Do the interactions
depend on the distance between the
objects?
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S5 Since the objects do not have to touch for
there to be an interaction, what do you
think supplies the energy for the
interactions? S6 What are the similarities and
differences between the magnetic and static
electric interactions? S7 What are the
similarities and differences between the
gravitational and static electric
interactions? S8 Draw a model that shows what
happens when the PVC pipe is rubbed with
felt or the top tape is pull off the
bottom tape.
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