Writing about a scientific topic gives the instructor an excellent way to measure student comprehension. In writing tasks, the instructor can also observe student thinking abilities of a scientific nature. Yet it is common that physics curricula do not explicitly use writing tasks in their assessment schemes, even though almost all physicists implicitly judge student writing in problem solving and problem solving is the "gold standard" for student achievement in physics. Therefore, it would be good to design a model for writing in physics with an assessment tool that is consonant with existing implicit criteria of physicists. We developed a writing model for our online physical science course in 2005. We describe the model's rubrics that we numerically rate using information content, or entropy, of the marking process. With entropy, we can evaluate and refine each rubric, in a manner accessible to any teacher using a rubric.