Hey all, I'm studying for a test(cramming). I'm looking at this problem and solution and need some clarification. I can't see why they removed the resistor at t>0 and called it 'lossless'

 

I suppose "NO DAMPED" would qualify as underdamped.

 

indeed, but...

 

when we don't have a damping element,the energy has nowhere to go.so the the energy will keep going back and forth between the capa and the inductor without loss of energy so it is a loss less circuit.

ps:when we hava a resistor the energy is lost in the form of heat.

 

Because the resistor is across the supply, it is effectively isolated from the LC part of the circuit and doesn't interact with it at all. When the supply voltage drops to 0V, the voltage across the resistor drops to 0V and there is no current flowing in it. For the LC part, when the supply voltage drops to zero the left side of the cap is effectively short circuited to ground (through the 0V supply).

 

I'm not sure I am getting it. I understand how it would be lossless IF the resistor were gone. If the voltage source is replaced by a short then I understand...is that what is meant by 'LC circuit is disconnected'?

 

It is not physically disconnected, but with the voltage source now putting out 0V, the voltage source is effectively replaced by a short circuit. Afterall, a short is a two-terminal element that holds the voltage across it at 0V while permitting any current to flow through it. That's a pretty good description of a voltage source that is set to 0V.

 

I'm not sure I am getting it. I understand how it would be lossless IF the resistor were gone. If the voltage source is replaced by a short then I understand...is that what is meant by 'LC circuit is disconnected'?

Write the equations for the voltage across and the current through the LC branch and you will see that R simply isn't in them. Similarly, write the equation for the voltage across and the current through R and you will see that L and C simply aren't in them. The two branches do not interact because the voltage source maintains a voltage across each branch that is independent of the presence of the other branch.