ϵ\frac{d^{2}ψ}{dt^{2}} = −ψ  (1)
q \frac{∂p}{∂t} = −∇.J_{p} − qR  (2)

or

ϵ\frac{d^{2}ψ}{dt^{2}} = −ψ  (3)
q \frac{∂p}{∂t} = −∇.J_{p} − qR  (4)