PHY 2049: Physics II

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PHY 2049 Physics II

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Ch. 31 Electromagnetic Oscillations

• RC, RL, LC circuits
• Driven damped Oscillations
• Resonance
• Series RLC circuits
• Power
• Transformer

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Last time

• LC circuit
• Analogous to a pendulum
• L m
• q x
• C 1/k
• Be careful with ?t in radians (not degrees)

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Alternating Currents

• A resistance

The current and voltage are in phase
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Capacitor AC response
The current leads the voltage
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Inductor AC response

• The current lags the voltage

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HITT

• A capacitor in an LC oscillator has a maximum
  potential difference of 15V and a maximum energy
  of 360 µJ. At a certain instant the energy in the
  capacitor is 40 µJ. At that instant what is the
  potential difference across the capacitor?
• A. zero B. 5V C. 10V D. 15V
• E. 20V

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A different problem

• An LC circuit has a capacitance of 30 µF and an
  inductance of 15mH. At time t 0 the charge on
  the capacitor is 10 µC and the current is 20mA.
  The maximum charge on the capacitor is
• A. 8.9 µC
• B. 10 µC
• C. 12 µC
• D. 17 µC
• E. 24 µC

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Yet different

• A 45-mH inductor is connected to a source of
  sinusoidal emf with a frequency of 400 Hz and a
  maximum emf of 20V. The maximum current is
• A. 0
• B. 0.18A
• C. 1.1A
• D. 360A
• E. 2300A
• ans B

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RLC circuit
              
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RLC circuit
              
Inductive Capacitive In resonance
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e E sin ?t i io sin (?t f) Capacitor
current leads Power I2R ermsirmscosf (cos
f R/Z)
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Resonance
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Transformers
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