Does the ipad work like computers when its fully charged if we keep it plugged in does it get energy from electricity or battery ?
iPad will draw power from both the battery and the mains supply when fully charged. During intense operation, the iPad may use more than 10W, which is the maximum the USB charging port can supply, so the battery will pick up the slack. (yes, the system can drain it's battery even when plugged in)
So there is no point using it plugged in mode to increase battery life span.
By the way, what is the lifespan of ipad 3 ? I bought 2 days ago. If I use it every day and fully charge with one day intervals how long will battery last ?
According to Apple the battery should go about 1,000 cycles before dropping to about 80% of original capacity. A "cycle" is charging from 0% to 100%, so if you drop to 50% and charge it up to 100% then you've completed a half cycle. My wife's iPad 1 which she's used daily since the day it was released still has nearly the full battery capacity that it did when new.
Read up here:
http://www.apple.com/batteries/ipad.html
Sure there is. You'll drain the battery faster if you're not plugged in.So there is no point using it plugged in mode to increase battery life span.