Question

A parallel-plate capacitor has square plates that are 8.20 cm on each side and 4.80 mm apart. The space between the plates is completely filled with two square slabs of dielectric, each 8.20 cm on a side and 2.40 mm thick. One slab is Pyrex glass and the other slab is polystyrene. If the potential difference between the plates is 88.0 V, find how much electrical energy (in nJ) can be stored in this capacitor.

Answer 1

The relative permittivity of the pyrex glass is 4.7 and of polystyrene is 2.55.

Note : The above values are taken from wikipedia. Values may differ in your case. So, change the final calculation accordingly.

The capacitance is given by

The given capacitor can be imagenedas two capacitors connected in series with half the saperation.

So, the capacitance of these two capacitors are

and

The final capacitance (in series combination) is

Now the energy can be calculated using

So, the energy stored in capacitor when connected with 88 V source is 1.587 x 10^-7 J.