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6:00–7:00PM THU 11 MAR: Second Exam (Chapters 24.9-28.8 / HW04-06) Room Lockett 6. Physics 2102 Jonathan Dowling. Lecture 13: THU 04 MAR 10. Aurora Borealis. Magnetic fields Ch.28.1–5. “I’ll be back…. How Do You Use Magnetic Fields in Your Everyday Life!?. - PowerPoint PPT Presentation
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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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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!
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