Arlenna has a post on the physical and mental challenges that teaching poses to instructors.
I can easily relate to what she writes. My own teaching schedule is fairly demanding this term, including both a graduate course as well as a large introductory physics class for life science majors (200 registered students). One of the things that frustrate many a college professor is the lack of appreciation for the effort required to teach a large freshmen course, not only on the part of those who themselves have never taught, but also of university administrators and even of faculty colleagues (typically those who are most skillful at avoiding the most onerous teaching duties).
“What’s the big deal ? The material is simple, just go, shut your brain off, teach for an hour and you are done”. That is what I have heard many a time, including, as I stated above, from some colleagues who would know better, if they themselves had ever been involved in freshmen teaching. There are a number of reasons why things are not nearly as straightforward as the above, superficial remarks would suggest.
First of all, the fact that the “material is simple” does not mean that teaching it is simple. In fact, very often it proves quite a challenge to present in a clear and compelling way topics which might seem elementary but are not, especially those constituting the intellectual foundation of a field, i.e., whose comprehension is crucial to mastering more advanced subjects later on. Especially in physics, where everything is related, learning cannot occur by looking at individual subjects as if they were disconnected from the rest (and forgetting them after the midterm exam). A student who does not possess a solid understanding of concepts such as kinematics, Newton’s law of action and reaction, potential energy, etc., is inexorably ill-equipped to tackle later on electricity and magnetism, waves, atomic physics and so on.
I find myself spending way more time preparing my lectures for freshmen physics, than graduate quantum mechanics. The vastly different levels of preparedness of the students in the audience, make it particularly difficult a task to strike a proper balance between rigor and precision of the presentation, and accessibility of the material. Much easier is to teach a graduate course, as students are not only more experienced and familiar with the basic terminology, but also tend to take a much more active role in the learning process.
But classroom teaching, which tends to be fairly tiring per se (when I return from a lecture I typically need at least half an hour just to reorder my thoughts) is just one part of the overall endeavor, and not even the main one.
I am not referring to the grading of homework, which in a class like this one is mostly done by teaching assistants . A professor in charge of a course is expected to tutor students outside lecture hours. For courses with low enrollment, additional help with course material can be offered during office hours, typically 2-3 a week, scheduled at regular times. But when enrollment exceeds fifty or so, office hours are merely symbolic, as most students will not be able to come to the office at those times, due to conflicts with other classes, work and/or other obligations. Obviously, an instructor must make a bona fide effort to accommodate the needs of the students; in practice, this means that one will be essentially “on call” during the entire term, willing to schedule meetings with students, often on a short (e.g., two hours) notice, anytime between 9 am and 7 pm Monday through Friday, and occasionally during the weekend. To that one must add the e-mails from students seeking help at random times (roughly 5-10 a day), to which a professor must respond with reasonable timeliness.
Let us get one thing straight: all of the above is not only perfectly doable, it is also an integral and most rewarding part of the profession, and something that each college teacher should be willing and eager to do, or (s)he should really look for another job. However, there is no question that it takes a significant toll on one’s research program. This term, most of the research work in my group will be carried out by my postdoc and graduate student, to whom I can devote a couple hours a week at the most. Thus, if this term teaching assignment were to become the norm for me, I think it would be a matter of a couple years for my research program to be reduced to a mere part-time activity. It is largely not a matter of how many, but which courses one is requested to teach. I have been teaching at the undergraduate and graduate levels for over a decade, and can comfortably state that I would have much more time at my disposal this term if I were to teach, e.g., two graduate courses.
This is, I believe, the main reason underlying the tendency of many physics departments at research universities to assign large freshmen courses to part-time, adjunct instructors, i.e., not to cut too significantly into the research time of its faculty, especially junior ones, of whom much is expected in terms of scientific progress, publications, funding success etc. It is an open secret that most college faculty, especially at research universities, are not keen on teaching freshmen classes, precisely because they tend to be time-consuming and mentally and physically draining.
The heavy use of adjuncts is a practice to which many academics and students object, for a number of valid reasons. I think that regular, full-time faculty ought to shoulder the greatest part of the effort of teaching large freshmen courses. Clearly, however, it is imperative that this duty be equitably distributed among research-active faculty. Conversely, faculty whose research program is dwindling, or has dried out, should be asked to give a more substantial contribution to the teaching of the most time-consuming classes.
 I am going to have to grade my midterm and final exams, and am dreading doing that, but it is admittedly something that the instructor has to do once a term and no more.