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Tue. Nov. 9, 2009 Physics 208, Lecture 20 1
From last time…
Inductors in circuits
Inductors Flux = (Inductance) X (Current)
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Φ = LI
Vbatt R
L
I Vb
Va
Voltage drop across inductor Constant current
No voltage difference Current changing in time
Voltage difference across inductor
Tue. Nov. 9, 2009 Physics 208, Lecture 20 2
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ΔVL =Vb −Va = −L dIdt
RL Circuit
What is voltage across L just after switch closed?
Tue. Nov. 9, 2009 Physics 208, Lecture 20 3
Before switch closed, IL = 0 Current through inductor cannot ‘jump’ Just after switch closed, IL= 0.
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VL = −L dIdt
A. VL = 0
B. VL= Vbattery
C. VL= Vbattery / R
D. VL= Vbattery / L
Kirchoff’s loop law:
VR + VL = Vbattery
R and L in series, IL=0 IR=0, VR=0
Tue. Nov. 9, 2009 Physics 208, Lecture 20 4
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VL t = 0( ) = −Vbattery = −L dILdt
⇒dILdt
=Vbattery
L
IL
IL(t)
Time ( t ) 0
0
Slope dI / dt = Vbattery / L
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IL t = 0( ) = 0
IL instantaneously zero, but increasing in time
Just a little later…
A short time later ( t=0+Δt ), the current is increasing …
Tue. Nov. 9, 2009 Physics 208, Lecture 20 5
IL(t)
Time ( t ) 0
0
Slope dI / dt = Vbattery / L
A. More slowly
B. More quickly
C. At the same rate
IL>0, and IR=IL
VR≠0, so VL smaller
VL= -LdI/dt, so dI/dt smaller
Switch closed at t=0
Tue. Nov. 9, 2009 Physics 208, Lecture 20 6
IL
IL(t)
Time ( t ) 0
0
Initial slope
What is current through inductor in equilibrium, a long time after switch is closed?
Later slope
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dIdt
=Vbattery
L€
dIdt
=Vbattery
L− IL t = Δt( )R
A. Zero
B. Vbattery / L
C. Vbattery / R
Equilibrium: currents not changing
dIL / dt =0, so VL=0
VR=Vbattery
IL = IR =Vbattery / R
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Vbattery
R
2
Tue. Nov. 9, 2009 Physics 208, Lecture 20 7
RL summary
I(t)
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I t( ) = I∞ 1− e− t /(L /R )( ) = I∞ 1− e
−t /τ( )
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I∞ =Vbattery /Rτ = L /R = time constant
I(t)
Switch closed at t=0
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I∞ =Vbattery /R
Question What is the current through
R1 immediately after the switch is closed?
Tue. Nov. 9, 2009 Physics 208, Lecture 20 8
L R1
R2
A. Vbattery / L
B. Vbattery / R1
C. Vbattery / R2
D. Vbattery / (R1+R2)
E. 0
IL cannot ‘jump’. IL=0 just after closing switch.
All current flows through resistors.
Resistor current can jump.
Thinking about electromagnetism
Many similarities between electricity, magnetism Some symmetries, particularly in time-dependence
Tue. Nov. 9, 2009 Physics 208, Lecture 20 9
Electric Fields
Arise from charges Capacitor, Q=CV
Arise from time-varying B-field Inductor, Faraday effect
Magnetic Fields
Arise from currents Inductor, Φ=LI
Arise from time-varying E-field
Tue. Nov. 9, 2009 Physics 208, Lecture 20 10
Maxwell’s unification Intimate connection
between electricity and magnetism
Time-varying magnetic field induces an electric field (Faraday’s Law)
Time-varying electric field generates a magnetic field
This is the basis of Maxwell’s unification of electricity and magnetism into Electromagnetism
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∇ × E = − 1
c∂ B ∂t
∇ × B = 1
c∂ E ∂t
In vacuum:
Tue. Nov. 9, 2009 Physics 208, Lecture 20 11
• A Transverse wave.
• Electric/magnetic fields perpendicular to propagation direction
• Can travel in empty space
f = v/λ, v = c = 3 x 108 m/s (186,000 miles/second)
Tue. Nov. 9, 2009 Physics 208, Lecture 20 12
The EM Spectrum
Types are distinguished by frequency or wavelength
Visible light is a small portion of the spectrum
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Tue. Nov. 9, 2009 Physics 208, Lecture 20 13
Sizes of EM waves Visible light
typical wavelength of 500 nm = = 0.5 x 10-6 m = 0.5 microns (µm)
AM 1310, your badger radio network, has a vibration frequency of 1310 KHz = 1.31x106 Hz
What is its wavelength?
A. 230 m
B. 0.044 m
C. 2.3 m
D. 44m Tue. Nov. 9, 2009 Physics 208, Lecture 20 14
A microwave oven irradiates food with electromagnetic radiation that has a frequency of about 1010 Hz. The wavelengths of these microwaves are on the order of
A. kilometers
B. meters
C. centimeters
D. micrometers
Quick Quiz
!
" = c / f =3#10
8m /s
1010/s
= 3cm
Tue. Nov. 9, 2009 Physics 208, Lecture 20 15
Mathematical description
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Bo = Eo /c
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E = E o cos kz −ωt( )
B = B o cos kz −ωt( )
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E ⊥ B
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k =2πλ
, ω = 2πf
z
x
y
Propagation direction =
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E × B
Tue. Nov. 9, 2009 Physics 208, Lecture 20 16
EM Waves from an Antenna
Two rods are connected to an ac source, charges oscillate between the rods (a)
As oscillations continue, the rods become less charged, the field near the charges decreases and the field produced at t = 0 moves away from the rod (b)
The charges and field reverse (c) The oscillations continue (d)
Tue. Nov. 9, 2009 Physics 208, Lecture 20 17
Detecting EM waves
FM antenna AM antenna
Oriented vertically for radio waves
Tue. Nov. 9, 2009 Physics 208, Lecture 20 18
Transatlantic signals
Gulgielmo Marconi’s transatlantic transmitter
Capacitor banks Induction
coils Spark gap
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Tue. Nov. 9, 2009 Physics 208, Lecture 20 19
Transatlantic receiver Left to right: Kemp, Marconi, and Paget pose in front of a kite that
was used to keep aloft the receiving aerial wire used in the transatlantic radio experiment.
Tue. Nov. 9, 2009 Physics 208, Lecture 20 20
Energy and EM Waves Energy density in E-field Energy density in B-field
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uE = εoE2 r,t( ) /2
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uB = B2 r,t( ) /2µo
Total
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uTot = εoE2 /2 + B2 /2µo
= εoE2 /2 + E 2 /2c 2µo = εoE
2 r,t( ) = B2 r,t( ) /µo
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uTot = εoE2 = εoEo
2 cos2 kz −ωt( ) moves w/ EM wave at speed c
Tue. Nov. 9, 2009 Physics 208, Lecture 20 21
Power and intensity in EM waves Energy density uE moves at c
Instantaneous energy flow = energy per second passing plane = This is power density W/m2
Oscillates in time Time average of this is Intensity =
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cεoEmax2 /2 = cBmax
2 /2µo€
cuTot = cεoE2 = cεoEo
2 cos2 ωt( )
Tue. Nov. 9, 2009 Physics 208, Lecture 20 22
Example: E-field in laser pointer
3 mW laser pointer.
Beam diameter at board ~ 2mm
Intensity =
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10−3Wπ 0.001m( )2
= 318W /m2
How big is max E-field?
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cεoEmax2 /2 = 318W /m2
Emax =2 318W /m2( )
3×108m /s( ) 8.85 ×10−12C2 /N ⋅m2( )= 489N /C = 489V /m
Tue. Nov. 9, 2009 Physics 208, Lecture 20 23
Spherical waves Sources often radiate EM wave in all directions
Light bulb The sun Radio/tv transmission tower
Spherical wave, looks like plane wave far away Intensity decreases with distance
Power spread over larger area
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I =Psource4π r2
Source power
Spread over this surface area
Tue. Nov. 9, 2009 Physics 208, Lecture 20 24
Question A radio station transmits 50kW of power from its
antanna. What is the amplitude of the electric field at your radio, 1km away.
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I =50,000W4π 1000m( )2
= 4 ×10−3W /m2
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cεoEmax2 /2 = 4 ×10−3W /m2
Emax =2 4 ×10−3W /m2( )
3×108m /s( ) 8.85 ×10−12C2 /N ⋅m2( )=1.73N /C =1.73V /m
A. 0.1 V/m
B. 0.5 V/m
C. 1 V/m
D. 1.7 V/m
E. 15 V/m
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Tue. Nov. 9, 2009 Physics 208, Lecture 20 25
The Poynting Vector Rate at which energy flows through a unit area perpendicular
to direction of wave propagation
Instantaneous power per unit area (J/s.m2 = W/m2) is also
Its direction is the direction of propagation of the EM wave
This is time dependent Its magnitude varies in time Its magnitude reaches a maximum at the
same instant as E and B
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S = 1
µo
E × B ≡ Poynting Vector
Tue. Nov. 9, 2009 Physics 208, Lecture 20 26
Radiation Pressure Saw EM waves carry energy They also have momentum When object absorbs energy U from EM wave:
Momentum Δp is transferred
Result is a force
Pressure = Force/Area = €
Δp =U /c ( Will see this later in QM )
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F = Δp /Δt =U /Δtc
= P /c
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prad =P /Ac
= I /c
Radiation pressure on perfectly absorbing object
Power
Intensity
Tue. Nov. 9, 2009 Physics 208, Lecture 20 27
Radiation pressure & force EM wave incident on surface exerts a radiation pressure prad (force/area) proportional to intensity I.
Perfectly absorbing (black) surface:
Perfectly reflecting (mirror) surface:
Resulting force = (radiation pressure) x (area) €
prad = I /c
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prad = 2I /c
Tue. Nov. 9, 2009 Physics 208, Lecture 20 28
Question A perfectly reflecting square solar sail is 107m X 107m. It has
a mass of 100kg. It starts from rest near the Earth’s orbit, where the sun’s EM radiation has an intensity of 1300 W/m2.
How fast is it moving after 1 hour?
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prad = 2I /c
Frad = prad A = 2IA /c =2 1300W /m2( ) 1.145 ×104m2( )
3×108m /s= 0.1N
a = Frad /m =10−3m /s2
v = at = 10−3m /s2( ) 3600s( ) = 3.6m /s
A. 100 m/s B. 56 m/s C. 17 m/s D. 3.6 m/s E. 0.7 m/s
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