In experimental studies, you will be required to keep a certifiable notebook that documents your work. Rarely are electronic notebooks as robustly accepted as certifiable compared to pen written notebooks. Exceptions may exist when they have been well implemented.
In theoretical studies, you will be challenged to develop a logical work flow to your analysis or derivations. Rarely are such developments done well "just in your head" without some form of note taking. Here again, you return to keeping a notebook, logbook, or journal to record your thoughts. Unlike with experimental studies, you may be under a less firm guideline that you must keep your journal in a written form rather than an electronic format.
I hope this helps you address your question. Otherwise, you will need to provide more specifics.
It depends on the definition for "Physics". The definition is being changed always continuously, and the definition depends on individuals. This comment is applicable is to all subjects.