Lecture 24
LRC + AC Circuits
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Energy Stored in B-field:
Compared to Capacitor:
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LC Circuits
Energy shuttles
back and forth
between capacitor
and inductor
q, i, V vary sinusoidally->EM oscillations
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LC Circuits
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Analogy with block/spring
system
Energy
Energy
m->L, k->1/C
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Analogy with block/spring
system
Energy
Energy
m->L, k->1/C
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Energy Oscillations:
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Damped Oscillations: RLC
circuits
Energy gradually
dumped into
resistor: i^2*R
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Alternating Current
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Phasor Notation:
• http://www.animations.physics.unsw.edu.au/jw/
AC.html
• http://science.sbcc.edu/physics/flash/optics/
phasors4.swf
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Forced Oscillations in 3
Easy Pieces
• Resistive Load
• Capacitive Load
• Inductive Load
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Alternating Current
Purely Resistive Load:
•Phase angle (ϕ) = 0
•Current and Voltage are in phase
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Alternating Current
Purely Capacitive Load:
•Phase angle (ϕ) =-90
•Voltage “lags” current
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Alternating Current
Purely Inductive Load:
•Phase angle (ϕ) =+90
•Voltage “leads” current
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Type
R
C
L
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Resistance
R
X = 1/ωC
C
X = ωL
L
Phase of (i)
V relation
0
V=IR
leads v by 90
0
V=IX
C
lags v by 90
0
V=IX
L
Leon Theremin (1896-1993)
Soviet Inventor/ KGB Espionage Engineer
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Theremin – the classic Theremin (1920)
Burglar alarm, or "Signalling Apparatus" which
used the Theremin effect (1920s)
Electromechanical television – Nipkow disk
with mirrors instead of slots (ca. 1925)
Terpsitone – platform that converts dance
movements into tones (1932)
Theremin cello – an electronic cello with no
strings and no bow, using a plastic fingerboard,
a handle for volume and two knobs for sound
shaping (ca. 1930)[32][33][34][35]
Keyboard theremin (ca. 1930), looking like a
small piano, "with hornlike tones"[36]
Rhythmicon – world's first drum machine
(1931)
The Buran eavesdropping device (1947 or
earlier)
The Great Seal bug, also known as "The Thing"
– one of the first passive covert listening
devices; first used by the USSR for spying
(1945 or earlier)
“The Thing” or “Great Seal Bug”
Theremins device was
implanted inside a carved
wooden plaque of the US seal
given to ambassador to Soviets
as a gesture of friendship. Was
discovered by British radio
operator and reverse
engineered by MI5. Later was
presented as special UN
meeting in response to the
downing of the U2 spy plane
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