Two parallel plates

PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Electric fields in matter · Electric fields in matter

The Boundary-Value Problem, Ch Energy and force in dielectrics

Problem

Two parallel plates, each of area

S = 0.030\,\mathrm{m^2}

, are charged to a free charge

Q = 5.0\times10^{-9}\,\mathrm{C}

and then disconnected from any source, so that

Q

is fixed. There is vacuum between them at their present separation

x = 1.5\,\mathrm{mm}

. ((a)) Write the stored energy

W_{\rm free}(x)

for this isolated pair of plates. ((b)) Use virtual work at fixed charge to find the force between the plates, stating what is held fixed.