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AC Circuits & Phasors

Which phasor shows a current that is positive and increasing? D Which phasor shows a current that is negative with an increasing magnitude (i.e. getting more negative?) B

X = “reactance”; the effective combination of Xs in a circuit is called “impedance.”

At high frequencies, the reactance of what circuit element decreases At high frequencies, the reactance of what circuit element decreases? A] resistor B] capacitor C] inductor D] none decrease E] all decrease Low reactance is like low resistance… small voltage drop for a large current.

An AC voltage source drives a sinusoidal current through two resistors. The amplitude of the sinusoidal voltage across the top resistor is 4 V. The amplitude of the sinusoidal voltage across the bottom resistor is 3 V. What is the amplitude of the sinusoidal voltage provided by the source? A] 0 V D] 7 V B] 1 V E] 12 V C] 5 V

An AC voltage source drives a sinusoidal current through a resistor and an inductor in series. The amplitude of the sinusoidal voltage across the top resistor is 4 V. The amplitude of the sinusoidal voltage across the bottom inductor is 3 V. What is the amplitude of the sinusoidal voltage provided by the source? A] 0 V D] 7 V B] 1 V E] 12 V C] 5 V

An AC voltage source drives a sinusoidal current through a capacitor and a resistor in series. The amplitude of the sinusoidal voltage across the top capacitor is 4 V. The amplitude of the sinusoidal voltage across the bottom resistor is 3 V. What is the amplitude of the sinusoidal voltage provided by the source? A] 0 V D] 7 V B] 1 V E] 12 V C] 5 V

An AC voltage source drives a sinusoidal current through a capacitor and an inductor in series. The amplitude of the sinusoidal voltage across the top capacitor is 4 V. The amplitude of the sinusoidal voltage across the bottom inductor is 3 V. What is the amplitude of the sinusoidal voltage provided by the source? A] 0 V D] 7 V B] 1 V E] 12 V C] 5 V

Ampere’s law This completes “Maxwell’s Equations” 16

Electromagnetic “Discontinuities” Must Propagate at a speed

Our wavefront satisfies both “Gauss’s laws” because there is no enclosed charge or current, and fields on opposite sides of the box are the same.

There is a changing B flux as the wavefront moves by There is a changing B flux as the wavefront moves by. This changing flux must be equal to the line integral of the E field. Only the back edge (gh) contributes to this line integral.

There is a changing E flux also There is a changing E flux also. This gives another reqd relation between E and B.

Accelerating Charges Radiate http://www. its. caltech

Accelerating Charges Radiate

Coulomb’s law can’t describe the “kinked” E field Coulomb’s law can’t describe the “kinked” E field. We got it from connecting field lines (Gauss’ law!) + geometry. So, while Gauss “derived” his law from Coulomb, Gauss’ Law is better. It’s always true, while Coulomb’s law is only true for unaccelerated charges.

But what’s it good for? http://www.youtube.com/watch?v=Qil3iFpgWdM 2:33

Quiz: What are you doing over break (2 pts for any answer): A] Maybe a physics problem a week or so, just to stay in top mental condition B] I won’t pick up a physics book C] I will use my expertise from Physics 161 to re-animate dead tissue. All answers got 2 points, but C is of course the best answer.