The parallel-plate capacitor used on each key of a computer keypad is a square 0.9cm on a side. Between the plates is a dielectric with k = 4.2. When the key is in the "up" position, the plate separation is 0.74 mm.
a) Find the initial capacitance. C = ? Express your answer to two significant figures and include the appropriate units.
b) How far must the key be depressed for a keystroke to be registered, which happens when the capacitance increases by 4.0 pF? delta x = ? Express your answer to two significant figures and include the appropriate units.