Re: Is it possbile to make a compact magnetic switch?

You don’t need anything as complicated as a motor if all you wish is to 
move the flap from the closed to open position.  I motor might be used if 
there was a need to locate the flap at various intermediate positions much 
as is done with a damper in an air stream.  What you need is called 
a “Solenoid.” A solenoid is an electrical to mechanical transducer [I love 
that word.  I just means “changer.”] that converts electrical current to 
mechanical motion. 

 It is just a coil of wire with a hole in the center.  An iron bar is 
placed in that hole and held partially extended out of the hole by a weak 
spring.  When the coil is electrically energized, its magnetic field pulls 
the bar into the coil, thus creating the mechanical motion that can be 
used to open your light-weight flap.

Solenoids come in all sizes, all voltages, and either push or pull.  What 
you need would be small, and probably energized with a battery and should 
be available from any electrical or electronic supplier.  

You could refer to the manufacturer’s data to determine the answers to 
your other question, but that would only tell you the maximum values of 
current, etc. that would be needed when the solenoid was used at it’s 
highest load conditions.  When not fully loaded, those values depend 
strictly on the load, and if they are really necessary to know, would be 
determined by test.  You set up the operation configuration and measure 
the current, etc.