moving heater

Hi,

I am trying to model heat transfer to a fluid flowing in a rectangular duct. The duct is heated at the bottom wall through a rectangle step function such that only a short length of the wall is heated. I am trying to make this step pulse move with time, i.e a different section is heated at each instant. When I specify the step function limits in terms of 't' I get an 'unknown variable' error. Is there a way to do this in Comsol?

Thanks.