physics 2102 lecture 18 ch30: inductors & inductance ii physics 2102 jonathan dowling nikolai...
TRANSCRIPT
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Physics 2102 Physics 2102 Lecture 18Lecture 18
Ch30: Ch30:
Inductors & Inductance IIInductors & Inductance II
Physics 2102
Jonathan Dowling
Nikolai Tesla
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dA
Faraday’s LawFaraday’s Law• A time varying magnetic
FLUX creates an induced EMF
• Definition of magnetic flux is similar to definition of electric flux
Bn
dt
dEMF BΦ
−=
B
S
B dAΦ = ⋅∫rr
• Take note of the MINUS sign!!• The induced EMF acts in such a way that it OPPOSES the change in magnetic flux (“Lenz’s Law”).
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dA
Another formulation of Another formulation of Faraday’s LawFaraday’s Law
• We saw that a time varying magnetic FLUX creates an induced EMF in a wire, exhibited as a current.
• Recall that a current flows in a conductor because of the forces on charges produced by an electric field.
• Hence, a time varying magnetic flux must induce an ELECTRIC FIELD!
• But the electric field line would be closed!!?? What about electric potential difference V=∫E•ds?
Bn
dt
dsdE B
C
Φ−=⋅∫
rr
Another of Maxwell’s equations!To decide SIGN of flux, use right hand rule: curl fingers around loop C, thumb indicates direction for dA.
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Example Example A long solenoid has a circular cross-section of radius R.The current through the solenoid is increasing at a steady rate di/dt. Compute the electric field as a function of the distance r from the axis of the solenoid.
R
First, let’s look at r < R:
2
20
(2 ) ( )
( )
dBE r r
dtdi
r ndt
π π
π μ
=
=
0
2
n diE r
dt
μ=
Next, let’s look at r > R:
dt
dBRrE )()2( 2ππ =
20
2
n di RE
dt r
μ=
magnetic field lines
electric field lines
dt
dsdE B
C
Φ−=⋅∫
rr
The electric current produces a magnetic field B=μ0ni, which changes with time, and produces an electric field.The magnetic flux through circular disks Φ=∫BdA is related to the circulation of the electric field on the circumference ∫Eds.
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Example (continued)Example (continued)
r
E(r)
r = R
0
2
n diE r
dt
μ=
20
2
n di RE
dt r
μ=
magnetic field lines
electric field lines
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SummarySummary
Two versions of Faradays’ law:– A varying magnetic flux produces an EMF:
– A varying magnetic flux produces an electric field:
dt
dEMF BΦ
−=
dt
dsdE B
C
Φ−=⋅∫
rr
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Inductors are with respect to the magnetic field what capacitors are with respect to the electric field. They “pack a lot of field in a small region”. Also, the higher the current, the higher the magnetic field they produce.
Capacitance how much potential for a given charge: Q=CV
Inductance how much magnetic flux for a given current: Φ=Li
(Henry)H Ampere
mTesla][ :Units
2
≡⋅=LJoseph Henry (1799-1878)
Inductors: SolenoidsInductors: Solenoids
dt
diLEMF −=Using Faraday’s law:
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““Self”-Inductance of a solenoidSelf”-Inductance of a solenoid• Solenoid of cross-sectional
area A, length l, total number of turns N, turns per unit length n
• Field inside solenoid = μ0 n i
• Field outside ~ 0
i
Al
NNAn
2
00 μμ ==L = “inductance”
LiniNANABB ===Φ 0μ
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ExampleExample • The current in a 10 H inductor is
decreasing at a steady rate of 5 A/s.
• If the current is as shown at some instant in time, what is the magnitude and direction of the induced EMF?
• Magnitude = (10 H)(5 A/s) = 50 V • Current is decreasing• Induced emf must be in a direction
that OPPOSES this change.• So, induced emf must be in same
direction as current
(a) 50 V
(b) 50 V
i
dt
diLEMF −=