The exact differential for a differentiable scalar function Q defined in an open domain D ⊂ ⊂ R n is equal to d Q = ∇ ∇ Q ⋅ ⋅ d r , where ∇ ∇ Q is the gradient of Q , ⋅ ⋅ represents the scalar product, and d r is the general differential displacement vector, if an orthogonal coordinate system is used. If Q is of differentiability class C 1 (continuously differentiable), then ∇ ∇ Q is a conservative vector field for the corresponding potential Q by the definition. For three dimensional spaces, expressions such as d r = ( d x , d y , d z ) and ∇ ∇ Q = ( ∂ ∂ Q ∂ ∂ x , ∂ ∂ Q ∂ ∂ y , ∂ ∂ Q ∂ ∂ z ) can be made.