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Qustion on lossless network

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cybcad

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It's well known that in the lossless network the input power is equal to the sum of output power and reflected power ,and no power will be lossed or stored in the network. but i know that the capacitor or inductance has capability of store energy at the resonance frequency, where the energy come from?
anyone can tell me why?

THX
 

Who ever told you: "no power will be...stored in the network"?

Analogies are useful. A lossless network is to electricity as a bucket without any holes in it is to water. You can fill up the bucket, and if no water splashes out, and as there are no holes, then no water leaks out. So is there water "stored" in the lossless bucket at some time later? Sure is.
 

hi biff44
storing energy in the lossless network should be finished at the original moment when the network begin to work, that is storing energy is instantaneous , is it true?
 

I think electrical system is slightly different than mechanical system. For a lossless electrical system, the net power (in and out) may not be 0 at a specific time. Howver, the time average (for one cycle of a harmonic) will be 0. For a time harmonic case, it should reach the steady state instantaneously. In the first moment, some energy goes into the losseless network and make the mechanism working. After that moment, the time average of the net power will be 0. A typical example is a capacitor. It needs some initial charging time. After it is steady, then the energy just goes in and out with the time average as 0. Regards.
 

cybcad, maybe you would enjoy this little experiment. Go get a high voltage capacitor capacitor, charge it up to 200 volts or so, let it sit there a minute (to make sure all the "power" is not stored in it anymore, wet your fingers, and grab onto the leads. Then come back here and tell us if there was any "power" stored in it.

(Don't actually do this, I do not want you to die).

The same (painful) experience could be had by you with a resonant theoretically lossless LC pair that had 300 volt peak sine wave briefly attached to it before you grabbed on.
 

The sum of powers is for the long term steady state solution. Once the original transient is finished, additional power into the network follows the sum rule.

Most of these network rules are for steady state. A quarter wave transmission line impedance matching device has reflected power for the time that it takes the bouncing wave (reflections at each end) to build up to the steady state condition.
 

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