Low-field AC susceptibility studies of bulk and single crystals samples of
Bi2Sr2CaCu2O8+δ (Bi-2212) have been carried out in the frequency range up to
1 KHz. At constant AC field amplitude, the dissipative component of AC
susceptibility shifts to higher temperatures with increasing AC field frequency.
Moreover, the thermal activation energy (Ea) for flux creep in the crystals
decreases with increasing AC field amplitude up to 800 Am-1. These results are
explained in terms of the effect of flux creep and Lorentz force (J×Β) in high Tcsuperconductors.