Energy density of Electric and Magnetic Fields

[iVenky]

I know that energy density of Electric and Magnetic Fields are

(ε/2)*E² and [1/(2μ)]B² respectively.

But I can't understand how we get those expressions. Would be really happy if you could clear my problem.

Thanks a lot

 

[jtbell]

See the discussion leading to equation 595 here:
http://farside.ph.utexas.edu/teaching/em/lectures/node56.html

and to equation 950 here:
http://farside.ph.utexas.edu/teaching/em/lectures/node84.html

 

[Enthalpy]

You can derive the expressions from a flat capacitor and from a solenoid, for instance.