Understanding the Equivalent Circuit Model for a MOSFET Circuit

[frutti]

hi
i have a mosfet circuit question
since my prof. didn't explained much about the mosfet I am pre lost..
during the lecture..he show us two circuit diagram..
the mosfet circuit
and this regular circuit..

and he ask us why the above equivalent circuit model is a valid representation of the mosfet circuit.
and i don't know how this regular circuit is related to mosfet circuit..
please explain to me..
thanx

by the way.. in the mosfet circuit
c is the wiring capacitance and C_GS of following gate

 

[cepheid]

A MOSFET can act like a switch. When the gate voltage is low (below a certain threshold), the drain and source are not electrically connected, and so the transistor does not conduct (this is equivalent to the switch being open). When the gate voltage is high (above a certain threshold), there is a conductive pathway between drain and source (which is equivalent to the switch being closed). But it won't be a perfect conductor. There will be certain resistance between drain and source that appears when the transistor is "on" and conducting. This resistance is represented by R_ON. I shouldn't say more.