Past exam of the physics 2 paper of the University of Oxford / 2013 / 3 / Solution

In classical Maxwell's equations, electric charge is a source or sink of the electric field, so in vacuum $\nabla \cdot \mathbf{E}=\rho /{ϵ}_0$ can be nonzero. There is no magnetic-charge source in the same theory: Gauss's law for magnetism is $\nabla \cdot \mathbf{B}=0$. Magnetic flux entering a closed surface equals the flux leaving it. A magnetic dipole has both polarities; it is not an isolated magnetic monopole.