Joule heating using AC voltage

Can I simulate the following phenomenon in COMSOL 4.1?

When AC voltage is applied across a silicon beam, the temperature on the beam will change due to Joule heating and the resulting thermal force will make the beam expand and contract.

I have successfully applied DC voltage across the beam using stationary solver and see the expansion of the beam. But I do not know how to apply AC voltage in this case. Can anyone help? Thanks.

James