ac circuit part 2

RL-CIRCUIT
The impedance of an RL circuit is the total opposition to AC current flow caused by the resistor (R) and the reactance of the inductor (XL).
The equation for the impedance of an RL circuit is:

where:
Z = the total impedance in ohms
XL = the inductive reactance in ohms
R = the resistance in ohms
Although you can use an ordinary ohmmeter to measure resistance, there are no common lab instruments for directly measuring reactance and impedance. For all practical purposes, then, you must calculate reactance and impedance from other circuit values that are more readily available.

In practical RL circuits, you can readily determine or directly measure the values of VT, f, R, and L. There is more than enough information among these items to calculate the values of XL and Z.
RC-CIRCUIT
A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors driven by a voltage or current source. A first order RC circuit is composed of one resistor and one capacitor and is the simplest type of RC circuit.
RC circuits can be used to filter a signal by blocking certain frequencies and passing others. The four most common RC filters are the high-pass filter, loSeries RLC Circuit
If an AC emf given by is used to drive current through a resistor, a capacitor, and an inductor connected in series, then the current through each element must be the same. The voltages across the various elements obey the rules given above, and the sum of these voltages must, by the loop theorem, be equal to the applied emf. This sum must be taken at a particular instant of time, which is complicated because each voltage difference will be at a different part of its cycle.

Note that the amplitude of I is given by , which looks about like the DC relation I = V / R. The quantity Z is called the impedance, it has units of ohms, and it plays the same role in AC circuits as resistance does in DC circuits. Unlike the resistance, however, it depends on the driving frequency, so the current that flows in the circuit depends sensitively on the driving frequency. Be careful, however; this formula for the impedance only applies to the series RLC circuit. Each different circuit has its own impedance formula.
Resonance: Consider the series RLC circuit discussed above. The formula for the current makes it easy to see how things should be adjusted to get as much current as possible from a given driving emf : simply make Z as small as possible. If the circuit value of R is fixed (as is usually the case) then the only way to get more current is by fiddling with and . And it is clear that the smallest value of Z will be obtained when , which a little algebra shows is equivalent to . But this simply says that things should be adjusted so that the driving frequency is equal to the natural frequency of the circuit (without the correction due to resistance). So, if the driving frequency is near the natural frequency, very large currents can result. When a circuit is driven near its natural frequency, we say that it is being driven at resonance. And the formula for Z shows that the smaller the resistance of the circuit, the larger the response at resonance will be. This is what makes the radio tuner work. The antenna of the radio picks up radio signals from every station in the area, but only the station whose frequency matches the natural frequency of the tuning circuit will cause large currents to flow in the circuit. These currents, when amplified, are the ones that produce the sound you hear. If the circuit is not properly tuned, then it may pick up two stations equally well, an effect you have probably heard many times.
w-pass filter, band-pass filter, and band-stop filter.