In this paper we present a description of the Nernst electric field E generated by forced flux tube motion due to a temperature gradient backward differenceT. Depending on the value of backward differenceT with respect to a critical gradient backward differenceTcrit, we find for backward differenceT/backward differenceTcritmuch less-than 1 a regime E ∝ backward differenceT, in close analogy to the regime of thermally activated flux flow in transport current measurements (characterized by an electric field proportional to the transport current). The theory for backward differenceT<backward differenceTcrit is applied to an analysis of experimental results obtained for polycrystalline Tl2Ba2CaCu2Ox films. From the temperature dependence of the Nernst field we derive activation energies for flux depinning.