Question

why do capacitors look like open circuit at low frequency in mid band model?

Answer 1

If you think about it, a capacitor will charge to the DC value across it thereby acting as a barrier to DC. Therefore when choosing a capacitance you make it so it blocks low frequency and yet pass the frequency of interest. When coupling amplifiers a coupling capacitor is used to separated there respective DC bias and still pass the frequency of interest.

Answer 2

capacitor has reactance of XC=1/2*PI*f*C where f is equal to da frequency . now if u consider a signal of low frequency than what happens is XC tends to infinity as 1/0 is infinity.. so infinity resistance is present and so it becomes an open circuit... if wrong plz correct

Answer 3

True, I tried to explain without the mathematical expression so as to get a general feel for how capacitors work. When considering the band of frequency that a RC network will pass you must consider it according to the equivalent time constants and find what is known as the 3 dB points that are used to set a margins for the pass band.

Answer 4

what sort of materials is the capacitor made of...does it store electricity, photons. or some other emf frequency...

Answer 5

The basic capacitor is two metallic plates separated by a insulating material so apparent current passing through the capacitor actually builds up charge on the capacitor.
Basic Equation
CV = Q
C capacitance
V voltage across the capacitor
Q the charge differential across the capacitor
Differential
C (dV/dt) = dQ/dt = i