The IB physics extended essay is all about showing the IBO that you know how to write a research paper.
You need to choose a research question which is interesting enough to you which will motivate you to read research papers on it.
The extended essay needs many resources in the bibliography. Six would be a minimum number. Aim for double digits.
The IB physics internal assessment may have only one independent variable with one five by five table but the extended essay may have two or more independent variables with two or more five by five tables. In other words you may conduct more labs and obtain more data when completing a physics extended essay than a physics internal assessment.
Ask your teacher for samples of the extended essay. If he/she can’t provide any for you then go online and try to find some. If that doesn’t work then e-mail me at email@example.com and I can send you a few.