results of midterm ii - physics.wisc.edu · physics 202, lecture 17 today’s topics inductance (ch...
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76-89 A
70-75 AB
61-69 B
55-60 BC
40-54 C
< 40 D
Results of Midterm II
Physics 202, Lecture 17Today’s Topics
Inductance (Ch 30) Reminder of Faraday’s and Lenz’s Laws Self Inductance Mutual Inductance
Energy Stored in B Field
LR Circuits
Review: Faraday’s Law of Induction Faraday’s Law in plain words: When the magnetic
flux through an area is changed, an emf is produced along the closed path enclosing the area.Quantitatively:
dt
dB
!"=#
A
B
θ
! •=" AB dB
conventional direction of ε
Note the - sign
Review: Lenz’s Law Lenz’s law in plain words: the induced emf always
tends to work against the original cause of flux change
Cause of dΦB/dt “Current” due to Induced ε will:
Increasing B generate B in opposite dir.Decreasing B generate B in same dir.
Relative motion subject to a force in oppositedirection of relative motions
The magnetic flux due to selfinductance is proportional to I:
The induced emf is proportionalto dI/dt:
When the current in a conducting device changes,an induced emf is produced in the opposite directionof the source current self inductance
Self Inductance
L: Inductance, unit: Henry (H)
Exercise: Calculate Inductance of a Solenoid
show that for an ideal solenoid:
(see board)
Reminder: magnetic field inside the solenoid (Ch 28)!
L =µ0N2A
l
Area: A
# of turns: N
Mutual Inductance For coupled coils: ε2 = - M12 dI1/dt
ε1 = - M21 dI2/dt
Can prove (not here):M12=M21=M
M: mutual inductance(unit: also Henry) ε2 = - M dI1/dt
ε1 = - M dI2/dt
Examples of Coupled Coils (Transformers)
Question: Why use iron core?
Energy Stored in a Magnetic Field
When an inductor of inductance L is carrying a currentchanging at a rate dI/dt, the power supplied is
The work needed to increase the current in an inductorfrom zero to some value I
Energy in an Inductor Energy stored in an inductor is U= ½ LI2
This energy is stored in the form of magnetic field:energy density: uB = ½ B2/µ0 (recall: uE= ½ ε0E2)
Compare: Inductor: energy stored U= ½ LI2 Capacitor: energy stored U= ½ C(ΔV)2
Resistor: no energy stored, (all energy converted to heat)
Basic Circuit ComponentsComponent Symbol Behavior in circuit
Ideal battery, emf ΔV=V+-V- =εResistor ΔV= -IR
Realistic Battery (Ideal) wire ΔV=0 (R=0, L=0, C=0)Capacitor ΔV=V- - V+ = - q/C, dq/dt =I
Inductor ΔV= - LdI/dt(Ideal) Switch L=0, C=0, R=0 (on), R=∞ (off)Transformer
Future TopicsDiodes,Transistors,…
rε
An inductor and are resistance constitute a LR circuit Any inductor has a resistance, R R could also include any other additional
resistance When the current starts to flow a voltage drop will
occur a the resistance and the inductance Once current stabilizes, reaches maximum of
LR Circuit
Exercise: Turn on LR Circuit
Apply Kirchhoff loop rule
!
I =V0
R(1" e
"t
L /R )
Exercise: Turn on LR Circuit (cont)
Note: the time constant is τ=L/RQuiz: What is the current when t=∞ ?
!
I =V0
R(1" e
"t
L /R )
Exercise: Turn off LR Circuit
Apply Kirchhoff loop rule
!
I = I0e"
t
L /R
Exercise: Turn off LR Circuit (cont)
Note: the time constant is τ=L/RQuiz: What is the current when t=∞ ?
!
I = I0e"
t
L /R