Mosfet LC oscillator
Ringing...In your second circuit, once C is charged to V, there's still current flowing in L.  In an ideal world, that will continue to pump C up to 2V, at which point the current reverses and it goes back down to zero.  It would repeat indefinitely in an ideal world, but in the real world the AC will be damped out by resistive losses--so your circuit starts again in the other direction every cycle.