Lecture 12 Electromagnetic Oscillations and Alternating Current Chp. 33 Cartoon -. Opening Demo - Warm-up problem Physlet Topics –LC Circuit Qualitatively –Electrical and Magnetic energy oscillations –Alternating current –\Pure R and L, circuti –Series RLC circuit –Power and Transfomers Demos –LR circuit –Series LRC circuit

 B Where  is the rotational angular frequency of the generator  f and f= 60 Hz Axis of rotation Coil of wire

Instantaneous voltage Amplitude Angular frequency time Phase constat phase Phasor diagram

V R =RI V L = X L I L or V L = (L  I since I=I L L = 4.22mH

Impedance Z: New quantity for AC circuits. This is analogous to resistance in DC circuits

RL Circuit Example Suppose  m = 100 volts, f=1000 Hz, R=10 Ohms, L=4.22 mH, Find X L, Z, I, V R, and V l.

Power in AC circuits Instantaneous power doesn’t mean anything Need to average over time or one period of the sine wave Note

Averaging over a sine curve

Calculate Power lost in resistor from example To calculate power produced by the generator you need to take account of the phase difference between the voltage and the current. In general you can write: For an inductor P = 0 because the phase difference between current through the inductor and voltage across the inductor is 90 degrees

Series LRC circuit VLVL VCVC VRVR XL=LXL=L X C =1/(  C) ELI the ICE man

Resonance Series LRC demo

10 uF 4.25 mH

Series LCR circuit