Transcript Lecture 29

Agenda
• This Week
– Ch. 27-29
– Skip Lab, No Quiz
• Next Week
– Finish 29, No Quiz
– Review
• Following
– Exam II on Monday (2 weeks)
– April 6th [Or previous Friday if prefer]
• Sections
– 27.1-27.8
– 28.1-28.5, skim28.8: No worry about “how” Mag fields arise
– 29.1-29.5
Today
• Work done by B fields
• Induction
• Bad News
– No Sub for Sub
• Good News
• Quizzes
– Will curve so that Average is > or = Exam Avg
– Exam Avg between C+ & B– Essentially, add ~ 5 pts to quiz average
What Happens Here?
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B Field Points Into Screen
What is the trajectory for this particle?
Positive Charge, Constant Velocity Right?
Initial: Force points ? …
Down
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All Perpendicular:
Circular Motion
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FB
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Velocity, Force & Field Always Perpendicular
When Force always points inwards… Circular Motion
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Circular Motion
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Force points inwards
Perpendicular to velocity
Force & speed constant
FC = mv2/r
Here: FB = FC
Right Hand Rule says Force points
inwards
• What says force is constant?
Examine Energy
• Does energy of particle change because
of B field?
• Does B field do work on particle?
• EF = EI + WB
• PEF + KEF = PEI + KEI + WB
• PE Same (why not?)
• DKE = WB
• So for speed to be constant, WB = zero
What is WB?
• So for speed to be constant, WB = zero
WB   FB dx
WB   FB cos   dx
• What is angle between magnetic force & motion?
All Perpendicular:
Circular Motion
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FB
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Velocity, Force & Field Always Perpendicular
When Force always points inwards… Circular Motion
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What is WB?
• So for speed to be constant, WB = zero
WB   FB dx
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WB   FB cos   dx
What is angle between magnetic force & motion?
90o  cos(90o) = 0
Dot product is zero  WB = 0
Always?
Chapter 29: Induction
• What do these things have in common?
– Credit Cards
– Electric Guitars
– Electric Motors
– Electric Generators
• See Title of Page…
Chapter 29: Induction
• Charges make E fields
– E fields from V?
• Moving charges make B fields
– Can B fields make E fields?
– Dr. Seuss?
• What do “moving” B fields make?
– Voltage
– Voltage makes charges move
– Moving Charges make B fields….
“Induction”
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Changing B fields “induce” a voltage
Voltage creates moving charges
Which have B fields of their own
These B fields attempt to cancel out the
change
• Simple: Things don’t like change
• Laziness is the rule – even if it takes work
Changing Fields?
Actually “Effect of Field”
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Important thing is actually Magnetic Flux
Flux? [See Gauss’s Law…]
Total field passing thru some Area
How much B field affects something
Changing Fields?
Actually “Effect of Field”
• Important thing is actually Magnetic Flux
• Flux? [See Gauss’s Law…]
dB  B dA
B   B dA
B  B A  BA cos  
If B constant
Principle
• Magnetic systems are Luddites
• Do NOT like change
• If possible, will act to oppose a change in
magnetic flux
• Math: VIND = - dB/dt
• A change in magnetic flux will induce a
voltage that will attempt to create moving
charges to oppose this change.
Example
Less words
Area of uniform magnetic field (pointing into screen)
A closed loop of wire is hanging out inside this field
The field begins to increase in magnitude.
What happens?
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Example
Less words
Area of uniform magnetic field (pointing into screen)
A closed loop of wire is hanging out inside this field
The field begins to increase in magnitude.
What happens?
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Voltage induced in loop
Creates current
Current opposed flux change
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Example
Less words
Area of uniform magnetic field (pointing into screen)
A closed loop of wire is hanging out inside this field
The field begins to decreases in magnitude.
What happens?
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Example
Less words
Area of uniform magnetic field (pointing into screen)
A closed loop of wire is hanging out inside this field
The field begins to increase in magnitude.
What happens?
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Voltage induced in loop
Creates current
Current opposed flux change
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Example
Less words
Area of uniform magnetic field (pointing into screen)
An open loop of wire is hanging out inside this field
The field begins to increase in magnitude.
What happens?
Voltage induced in loop
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Would creates current
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Current would opposed flux change
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Induction rarely wins
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Attempting to keep flux constant
Nearly all materials have some resistance
Induced motion electrons lose energy
Eventually Induced current reduces to
zero
• Unless superconductor
– Magnetic Fields cannot penetrate
superconductor
– Levitation…
Next Week
• Monday
– More interesting bits of induction
– Complex Examples
• Tuesday
– Maybe short lab
– No quiz
– Problem solving?
• W&F
– Wrap up induction, review for exam
Agenda
• This Week
– Ch. 27-29
– Skip Lab, No Quiz
• Next Week
– Finish 29, No Quiz
– Review
• Following
– Exam II on Monday (2 weeks)
– April 6th [Or previous Friday if prefer]
• Sections
– 27.1-27.8
– 28.1-28.5, skim28.8: No worry about “how” Mag fields arise
– 29.1-29.5