is capacitor linear devics?

Answer 1

It is linear because the current and voltage relationship can be described as a linear differential equation:

I = dQ/dt = C * dV/dt

Answer 2

Ideally 3 passive elements(resistors,inductors and capacitors) are considered linear. But it's just an ideal model specially in high frequencies each of the devices show nionlinear characters explicitly

Answer 3

Yes a capacitor, inductor and resistor are linear devices.