# Ch 22, AC Circuits and EM Waves

## Key Concepts

### After completing this chapter, you should understand that --

Alternating Current (AC) varies in magnitude and direction.

the voltage from an AC voltage source can be written as

v = v(t) = Vo sin t.

in an AC circuit, the current through a resistor is directly proportional to and in phase with the voltage across the resistor.

for quantities which vary with time in an AC circuit, like voltage and current, the rms (root-mean-square) values are a useful description; they correspond to constant values in a DC circuit which would produce the same power.

the capacitive reactance of a capacitor in an AC circuit is analogous to the resistance of a resistor; its value is given by XC =  .

for a given voltage, more current will flow through a capacitor's circuit if the frequency is higher and less current if the frequency is lower.

the AC current through a capacitor is out of phase by 90° with the AC voltage across the capacitor; the current "leads" the voltage.

the inductive reactance of an inductor in an AC circuit is analogous to the resistance of a resistor; its value is given by XL = 2¹ f L .

for a given voltage, more current will flow through an inductor's circuit if the frequency is lower and less current if the frequency is higher.

the AC current through an inductor is out of phase by 90° with the AC voltage across the inductor; the current "lags" the voltage.

in determining the current through a series RCL circuit, the phase relationships between voltage across and current in the various elements is important and can be explained using phasor diagrams.

the impedance of an AC circuit is analogous to the resistance in a DC circuit.

the total impedance of a series RCL circuit is given by

Z = .

the power dissipated by an AC circuit depends on the phase relation between current and voltage.

an electric circuit may have a resonant frequency -- a frequency at which it dissipates far more power than at other frequencies.

the resonant frequency of a series RCL circuit is fo.

Maxwell's Equations show the strong, fundamental connection between Electricity and Magnetism.

a changing electric field E and/or a changing magnetic field B can cause a wave composed of oscillating strengths in the electric field E and the magnetic field B; this is called an electromagnetic (EM) wave.

examples of electromagnetic (EM) waves include light, infra-red and ultra-violet radiation, gamma rays from a nucleus, X-rays, and radio and television waves.