A sheet of aluminium foil of negligible thickness is placed between the plates of a parallel plate capacitor. How will its capacitance be affected if:a)the foil is electrically insulated?b)the coil is connected to the upper plate with the help of connecting wires?

1. the capacitor now bcomes like -----|  |  |------ or two caps. in series. with the dist. between them now = d/2 where d= original distance between the plates w/o foil.hence each capacitors now become of cap.=2C_original addin up in series they give back a cap.=original cap.hence no effect. bt if the foil wud have been even a bit thick the cap wud have inc. due to intro. of a dielectric with dielectric const.= infinity
2. if the coil is connected to upper plate the cap. becomes 2 times since d=d_original/2.(hint: the al foil begins to act as a plate nullyfying the effect of upper plate)

The system will be equivalent to two identical capacitors connected in series in which two plates have separation half the original separation. So new capacitance--- c`=2cAs c is inversely proportional to d.So c and c` are in series ==> applying the formula of series combination b/w capacitors we get c which is original capacitor.When foil is connected to upper plate,it reduces to capacitor whose separation reduces to half original one .C`=2xc