The change of gravitational potential due to moving mass particle creates gravitational varying extra potential at the observation point. So for a moving mass particle there will be a dynamic change in extra potential at the observation point. It is postulated that the changing potential will be transformed into electromagnetic field. In a four dimensional space time continuum the electromagnetic field tensors are constructed with the help velocity dependents extra potential. The others also show good agreement with the order of observed value. Specially the Neutron star,-very highly dense with temperature 10^11 Kelvin and mainly consist of neutron and few percentage of proton and electron, although as a whole globally electrically neutral forbids one to set up ion-core-dynamo theory of magnetism. The very high value of magnetic field of a neutron star thus can be explained over the theory of dynamic gravitational electromagnetism.