6:00–7:00PM THU 11 MAR: Second Exam (Chapters 24.9-28.8 / HW04-06) Room Lockett 6

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6:00–7:00PM THU 11 MAR:Second Exam (Chapters 24.9-28.8 / HW04-06)Room Lockett 6

Lecture 13: THU 04 Lecture 13: THU 04 MAR 10 MAR 10 Magnetic fields Magnetic fields Ch.28.1–5

Physics 2102

Jonathan Dowling

“I’ll be back….

Aurora Borealis

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How Do You Use Magnetic Fields in Your

Everyday Life!?

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Electric vs. Magnetic FieldsElectric vs. Magnetic Fields

Electric fields are created:• microscopically, by electric charges (fields) of elementary particles (electrons, protons)• macroscopically,by adding the field of many elementary charges of the same sign

Magnetic fields are created :• microscopically, by magnetic “moments” of elementary particles (electrons, protons, neutrons)• macroscopically, by

• adding many microscopic magnetic moments (magnetic materials); or by• electric charges that move (electric currents)

Magnetic Field Direction

FROM North Poles

TO South Poles

B

+

–

Compare to ElectricField Directions

E

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Ritchie’s Rule for Magnets

Opposite Poles Attract

Like Poles Repel

We know that an electric fields exists because it accelerates electric charges, with a force independent of the velocity of the charge, proportional to the electric charge: FE = qE

We know that a magnetic field exists because it accelerates electric charges in a direction perpendicular to the velocity of the charge, with a magnitude proportional to the velocity of the charge and to the magnitude of the charge: FB= q v x B

Magnetic forces are perpendicular to both the velocity of chargesand to the magnetic field (electric forces are parallel to the field).

Since magnetic forces are perpendicular to the velocity,they do no work! (W=F · r)

Speed of particles moving in a magnetic field remains constantin magnitude, ONLY the direction changes. Kinetic energy is constant! (no work).

Magnetic vs. Electric Magnetic vs. Electric ForcesForces

Magnetic vs. Electric ForcesMagnetic vs. Electric Forces

€

rF E = q

r E

€

rF E = q

r E

Electric Force on Charge Parallel to E:

Electric Force on Charge Parallel to E:

€

rF B = q

r v ×

r B

€

rF B = q

r v ×

r B

Magnetic Force on Charge Perpendicular to B and v.

Magnetic Force on Charge Perpendicular to B and v.

+q

€

rF E

€

rF E

€

rE

€

rE

€

rF B

€

rF B

€

rB

€

rB

+q

€

rv

€

rv

Definition of Magnetic FieldDefinition of Magnetic Field

€

rE =

r F Eq

€

rE =

r F Eq

Definition of Electric Field:Definition of Electric Field:

€

B =

r F Bq

r v

€

B =

r F Bq

r v

Definition of Magnetic Field:Definition of Magnetic Field:

€

Units : B =Newton

Coulomb ⋅ meter/sec( )

⎡

⎣ ⎢

⎤

⎦ ⎥=

Newton

Coulomb/sec( ) ⋅meter

⎡

⎣ ⎢

⎤

⎦ ⎥=

Newton

Ampere ⋅meter

⎡

⎣ ⎢

⎤

⎦ ⎥=

N

A ⋅m

⎡ ⎣ ⎢

⎤ ⎦ ⎥

€

Units : B =Newton

Coulomb ⋅ meter/sec( )

⎡

⎣ ⎢

⎤

⎦ ⎥=

Newton

Coulomb/sec( ) ⋅meter

⎡

⎣ ⎢

⎤

⎦ ⎥=

Newton

Ampere ⋅meter

⎡

⎣ ⎢

⎤

⎦ ⎥=

N

A ⋅m

⎡ ⎣ ⎢

⎤ ⎦ ⎥

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Thompson Experiment& The Discovery of the Electron

Forces Balance: v=E/B

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L

€

E ≠ 0, B = 0; qE = FE = ma

a = FE /m = qE /m

L = vt; y = 12 at 2; Solve : y =

qEL2

2mv 2

y

II: E≠0, B=0

I: E=0, B=0

III: vB=E

€

v = E /B; y =qEL2

2mv 2

y =qEL2B2

2mE 2=

qL2B2

2mE

€

m

q=

L2B2

2yE

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The Hall Effect — Charge Flow in Conductors is From Electrons:Benjamin Franklin’s Biggest Blunder!

Hall Voltage for Positive Charge Carriers

Fm = vdBsin90°

The current expressed in terms of the drift velocity is:

At equilibrium:

A = WdFe = qE = qV/W