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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

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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-n-chug approach

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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

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€

r F g =G

m1m2

r2 r F e =k

q1q2

r2

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

– differences in magnitude and direction

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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”

– mediates the force between charges– stores energy due to interactions

€

r g =

Nkg

r E =

NC

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II - Potential & Energy

• Continue analogy with gravitational field– Relate electric equipotentials to contour lines on

a topographic map

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S1

S2

S3

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S5

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S10

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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

– Changes in potential energy depend on field strength, change in position and a property of the object

€

Vg =r g h Ve =

r E d

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II - Potential & Energy

• Formulas are developed in context of energy storage and transfer

€

ΔE =r F ⋅Δ

r x ΔE =qΔV

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II - Potential & Energy

• Formulas are developed in context of energy storage and transfer

• Rearrangement yields essential relationships

€

r F q

⋅Δr x =V ⇒ V =

r E d

€

ΔE =r F ⋅Δ

r x ΔE =qΔV

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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

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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

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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?

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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 deflectedout from magnetic fieldwhen current runs throughthe wire

• RH palm rule• Sets up lab with current balance

Oh Nooo!

€

r F ⊥I

r l and

r B

F ∝ I

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IV - Magnetism

• Small motor projectapplication of Lorentzforce on loop

• Loop rotates to alignB 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