The longer you are in your position, the more you will be asked to do on behalf of the department. This can run the gamut from mentoring new faculty, serving on various departmental committees to serving on university-wide committees.
Below are resources that might help you deal with deciding on which committees to serve and how to make the most of those opportunities when you do accept the assignment (APS does not endorse or assume responsibility for the information posted on these web sites).
Resources from APS Archive