The I-V characteristics of Tin films in the superconducting transition have been measured when ac current was applied. The experimental results suggest that the electrical response in ac is not satisfied with the I-V equation in dc. A new equation was suggested to describe the vortex motion and the vortex pair separation in the two dimensional superconducting transition with ac current. The excess noises of Tin films in the superconducting transition have been found to depend strongly on the temperature and ac current. Comparison with the dc noise measurement shows the voltage noise density...