Here is some information about the preliminary oral exam.
Officially, the DGS chooses your committee members, but you need to come up with a reasonable committee in consultation with your adviser (see guidelines below). Ask them if they can serve on your committee, and bring the list to the DGS for approval. If you have a hard time figuring out who can be the “outside physics” member, the DGS can make suggestions.
You need a total of four members including your adviser(s). You need one experimentalist and one theorist in your field as well as your (main) adviser. The last person should be “outside physics,” meaning that they need to have a graduate faculty appointment outside of physics. Many of the physics faculty members also have another appointment outside of physics and are eligible to be this fourth person.
Committee members don't just judge you during the exams. They can be resources to you beside your adviser. If you have some sort of difficulty with your adviser, this aspect of committee members becomes more important. So choose those who could be helpful in case you need it.
In the past, the department required five committee members, but since the requirement of the Graduate School is only four, we decided to save the faculty work load by removing this requirement. However, if you feel you can benefit from an extra member, feel free to have one.
In order to become ABD as soon as possible, it is beneficial to take the oral near the end of a semester so that you can register for thesis credits during the next semester. The last date that you can take the oral and still be able to register for thesis credits is not clearly defined, but the latest safe day is before the semester starts. This leaves enough time for the Graduate School to process information about your result on the oral exam (passing with reservation is acceptable as long as the reservation is lifted in a timely manner) before the “last day to add a course without college scholastic committee approval” which is two weeks into the semester. Taking the oral exam during the first week of the semester is not recommended. On the other hand, if there is no way to schedule the oral before a semester starts, but is possible during the first week, it is worth doing it, and hope that the paperwork moves fast enough so that you can register for thesis credits that semester.
You are expected to write a paper for the oral exam, and give it to the committee members at least two weeks before the exam (some faculty members want more than two weeks to be able to give the paper due attention. Check with them well in advance about this). During the exam, you will talk about the paper for up to 20 minutes. The paper should be written concisely. The recommended length is about 10 pages (double spaced). A small difference from the recommended length is acceptable, but the length should not be more than 15 pages (double space).
The paper should deal with a research topic that you may work on for your thesis or part of it to demonstrate that you are “ready” to start research. (If you end up doing something else for your thesis, that's OK.) The paper should therefore demonstrate that you understand why it is meaningful to do such research (why it should be of interest to people or at the least other physicists) and why it is possible (probable or likely?) to find reasonable results. To this end, you should present background information about the proposed research, which may include a theoretical basis (if it's experimental research); related research which has been done and how it is related to the proposed research; what you will do differently from previous work to improve on it, if such research exists; some rough description of your proposed research; and expected results. The paper does not have to have any results from your research since it is meant to be an opportunity for you to demonstrate your aptitude to START doing research. However, some professors, particularly those in theory, like you to have done some easy research to see how well you can approach theoretical problems, and may ask you to include some of the work in the oral exam.
The total duration of the exam should not exceed 90 minutes. This time includes your initial presentation of the paper. You should prepare your presentation to be 20 minutes long so that you will not be rushed when you are asked questions later. If your presentation goes long your committee members may interrupt you, and move to the next stage of the exam. So, make sure that you can effectively review your paper in 20 minutes or less. In the oral presentation, the audience usually does not have time to really digest complex concepts, so you should use more visuals than in the paper, and discuss issues in a more intuitive way perhaps using graphics.