Maxwell's equations describe the relationship between electric and magnetic fields. Gauss' law states that the divergence of the electric flux density equals the electric charge density. Gauss' magnetism law states that the divergence of the magnetic flux density is always zero. Faraday's law describes how a changing magnetic field generates an electric field. Ampere's law shows the relationship between electric current and the surrounding magnetic field. Maxwell unified electricity, magnetism, and light through his equations, which can be written in differential or integral form and describe fields in free space or harmonically varying fields.