In electromagnetism, displacement current density is the quantity ∂D/∂t appearing in Maxwell's equations that is defined in terms of the rate of change of D, the electric displacement field.

What is the formula of displacement current density?

where J_d = ε₀∂E/∂t is called the displacement current density. The Maxwell term is then equal to μ₀J_d. Setting aside the adjective 'displacement' for the moment, this terminology appears to be reasonable because Equation 7.32 shows that J_d has the same units as the current density J.

What is displacement current density and derive an expression for it?

J is the conducting current density. JD is the displacement current density. Here, ρ is the electric charge density. Thus, JD=∂D∂t displacement current density is necessary to balance RHS with LHS of the equation.