Transcript Beyond Mechanics - Modeling Instruction Program

Electricity & Magnetism
• Unit I - Charge Behavior and Interactions
– Charge produces and responds to an electric field
• Unit II - Electric Potential
– An electric field can store energy
• Unit III - Circuits
– The electric field can cause bulk charge flow in
conducting materials
• Unit IV - Magnetism
– Charge flow produces and responds to a new field
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What’s the same?
• Many of the labs and activities are very
familiar
–
–
–
–
Sticky tape, Coulomb’s Law
Mapping electric potential
Ohm’s Law, resistance in series and parallel
Mapping magnetic field
• Many of the exercises are familiar
2
So, what’s different?
• Develop models to account for observed
phenomena
• Use these models throughout the entire set
of materials
– Diagrammatic representations, causal
mechanisms stressed over QPS using plug-nchug approach
3
I- Charge and Field
• Emphasis on atomic model of matter
– charge is a fundamental property of matter
(like mass)
– charge carriers are microscopic constituents of
matter
– behavior of charged objects results from uneven
charge distribution
4
I- Charge and Field
• Compare and contrast electrical and
gravitational forces
– force arises from fundamental property of
objects (mass vs charge)
– inverse square relationships between point
particles
m1m2
q1q2
Fg  G
r
2
Fe  k
r
2
– differences in magnitude and direction
5
I - Charge & Field
• Emphasis placed on origin and nature of the
field
– arises from uneven distribution of charge
– strength is the force per unit “something”
N
N
g
E
kg
C
– mediates the force between charges
– stores energy due to interactions
6
II - Potential & Energy
• Continue analogy with gravitational field
– Relate electric equipotentials to contour lines on
a topographic map
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S16
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S14
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S12
S11
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S9
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S7
S6
S5
S4
S3
S2
S1
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II - Potential & Energy
• Continue analogy with gravitational field
– Relate electric equipotentials to contour lines on a
topographic map
– Potential is property of position in field
Vg  gh
Ve  Ed
– Changes in potential energy depend on field strength,
change in position and a property of the object
8
II - Potential & Energy
• Formulas are developed in context of energy
storage and transfer
E  F  x
E  qV
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II - Potential & Energy
• Formulas are developed in context of energy
storage and transfer
E  F  x
E  qV
• Rearrangement yields essential relationships
F
 x  V  V  Ed
q
10
III - Circuits
• Emphasis on causal mechanisms rather than
application of Ohm’s and Kirchoff’s Laws
– field responsible for bulk flow of charge in
conductors
– ∆V accounted for by differences in charge
density
• Study of circuits no longer disconnected
from field and potential developed earlier
11
III - Circuits
• Influences
– Sherwood and Chabay article
• “A Unified Treatment of Electrostatics and Circuits”
• surface charge distribution responsible for field
– CASTLE curriculum
• unequal initial flow rates can cause compression or
depletion of charge
12
III - Circuits
• Paradigm labs examine what’s happening in the
wires as well as in resistors
– use voltage probe to measure ∆V in wire during transient
– make charge distribution maps to account for ∆V
• infer existence of field
– must also exist in wires if charge is to flow
– field strength dependent on charge distributions
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III - Circuits
• Sketch the charge distribution and field in the intervals
midway through the discharge of the capacitor through the
long bulb.
• Sketch a diagram that you feel describes the charge
distribution, wire size and field midway through the
discharge of the capacitor through the round bulb. In what
ways are the diagrams similar? How are they different? 14
IV - Magnetism
• Moving charge produces and responds to a
magnetic field
• 1st lab - mapping field produced by charge
moving in a wire
– RH curl rule
– field strength increases with current and
decreases with distance
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IV - Magnetism
• Compare/contrast with electric field
– E-field - static charge, lines originate from (+)
and terminate on (-)
– B-field - moving charge, lines form closed
loops
• Magnetic domains in “permanent magnets”
result of electron spin
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IV - Magnetism
• Demo to show how force acts
on wire carrying current.
• Mr BIl’s swing is deflected
out from magnetic field
when current runs through
the wire
Oh Nooo!
FIl andB
FI
• RH palm rule
• Sets up lab with current balance
17
IV - Magnetism
• Small motor project
application of Lorentz
force on loop
• Loop rotates to align
B fields
• Sets stage for introduction of flux and induction
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Materials are a work-in-progress
• Magnetism least well-developed of the
E&M units
• Instructional notes in all units need work to
more coherently develop story line
• Feedback from Modeling teachers should
help
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