All basic interactions of electromagnetism can be explained in terms of four fundamental equations called Maxwell's equations, which were combined by Maxwell. The first equation is Gauss's law of electrostatics, which states that the net electric flux through any closed surface equals the net charge enclosed by the surface divided by the permittivity of the free space. This signifies that the total flux through a closed surface depends only on the charge enclosed by it. The second equation is Gauss's law of magnetism, which states that the magnetic flux through any closed surface is always zero. This implies that magnetic monopoles do not exist. The third equation is Faraday's law. This states that a changing magnetic flux produces an induced emf and an induced electric field. The induced emf in a closed loop equals the negative of the time derivative of the magnetic flux through that loop. The fourth equation is a modified form of Ampere's law and is called the Ampere-Maxwell law. For this, Maxwell added a term for the displacement current to the existing Ampere's law equation.