Week 5

Basic Facts

The total energy of a system of point charges is the energy required to assemble the system. For example, if you have three charges Q1, Q2, Q3 then the total energy is

U=Q₂V₁₂ + Q₃V₁₃ + Q₃V₂₃ where V₁₂ is the potential at particle 2 due to charge 1.

An electric dipole, dipole moment p placed in an electric field E has a potential energy
U = -p•E and a torque τ=p×E

In addition to its integral form, Gauss's Law can be expressed in differential form as
∇•E = ρ 
           ε₀
which allows you to compute the charge density ρ causing a particular field.

 

Useful Facts

An electric dipole, dipole moment p placed in an electric field E experiences a force
F = -p^.gradE so that there is no force in a uniform electric field.