an insightful explanation required

[mujee]

hey guys
i want to know how does a series LC circuit operate. what i am confused about is that if ac is applied, capacitor would start to conduct but inductor would block it ..like for example during the positive half cycle of ac capacitor would try to charge up but at the same time the inductor connected in series to capacitor would block any change..so how would current actually flow in this circuit when one component wants to conduct and the other blocks it completely.. how would all the process happen

please dont give me the mathematical solution that when impedances of both become equal they would be in resonance. i need physical explanation of this phenomenon
have asked in various forums and every one ends up with same explanation of impedances being equal with help of phasor diagrams blah blah blah

can anyone explain what actually happens in the circuit

 

[echo47]

If someone told you only "a capacitor passes AC, and an inductor blocks AC", then it's no wonder that you are confused. The statement is too simplistic. It omits critical variables such as the values of the components, and the frequency of the AC.

A resonant system requires math to explain exactly what's happening, but I'll try some non-mathematical hand-waving.

Capacitors and inductors are energy storage devices. A capacitor stores energy in an electric field. An inductor stores energy in a magnetic field. In an LC resonant circuit, the energy transfers back and forth between the capacitor and the inductor.

Maybe a "water circuit" analogy will help you. Imagine a loop of pipe filled with water (the circuit). Water flow is equivalent to electric current. Water pressure is equivalent to voltage. Pretend that the water has insignificant inertia. Now insert an elastic diaphragm (capacitor) into the pipe. Water can't flow through the diaphragm, but a pressure difference stretches the diaphragm, storing energy in the elastic. Now also insert a flywheel impeller (inductor) into the pipe. Water can't flow through the impeller until a pressure difference gradually causes it to spin, storing energy in its rotating mass.

Now apply a momentary external force to the water in the circuit. Notice how the water, diaphragm, and impeller begin oscillating back and forth. The pulse of energy that you applied is now transferring back and forth between the diaphragm and the impeller. The frequency of oscillation depends on the values of those two devices.

A real system will have some friction (resistance), so the energy will gradually dissipate as heat, gradually decreasing the amplitude of the oscillation. If you apply a continuous series of external pulses at the circuit's resonant frequency, you can maintain continuous oscillation. If you apply the pulses at the wrong frequency, the oscillation will tend to stall. Same thing happens in an LC resonant circuit.

 

[mujee]

well yeah you are right..we have been told that capacitor conducts ac completely and inductor blocks it
this concept seems to work fine in parallel resonant LC circuits...but it just cant explain the series LC circuit kuz in parallel when ac is applied across both of them, caparcitor charges and inductor resists it..and by the time capacitor starts discharging inductor starts conducting which inturn charges up the capacitor. so it oscillates

but when it comes to series circuit. it just doesnt help at all and thats what i am confused about..how does a series circuit actually conduct current

 

[echo47]

Oh, ok!
Series and parallel LC circuits are similar, except for the location of the dissipating resistor, and the equations that describe their behavior.
If you are referring to just an L and C in series, with their ends floating in empty space, then that's not a complete circuit - no current can flow.

 

[Kral]

mujee,
It is helpfule to get back to basics: The current thru a capacitor is proportional to the rate of change of voltage. The voltage across an inductor is proportional to the rate of change of current. Imagine a series circuit that consists of an ideal capacitor, and an ideal inductor. Further, suppose that the circuit is oscillating sinusoidally at the resonant frequency. When the capacitor voltage reaches its maximum positive value, the rate of change of voltage is at its minimum, so the current is zero. Under these conditions, the rate of change of current is at its maximum negative value, so the inductor voltage is at its maximum negative value. The two voltages add to zero.
Regards,
Kral

 

[mujee]

thanks kral
this is exactly the kind of explanation i was looking for...thanks alot