What can you do with a Ph.D in physics?

Let me start my answer by asking you a couple questions. Why do you want a physics PhD in the first place? And why do you want to study EE/CS as an undergrad (from your wording, it sounds like that part is still in the future, too) if the ultimate goal is a physics PhD?

Generally speaking, degrees are a means to an end, not ends themselves. (Among other things, they tend to be hugely expensive!) So the "normal"--and more cost-effective--approach is to decide what kinds of things you'd like to do as a career, based not only on your academic skills and interests but also on things like where such jobs are located, how many of them exist and/or are likely to exist in the future, how easy it is to switch to a different career (perhaps closely related, perhaps not) if the opportunity or need arises, whether you're more of an introvert or extrovert, whether you like working on your own or in close collaboration with others, whether you want to lead or teach or write or invent or build or discover, etc.

Once you've figured out at least a few of those things (doesn't have to be all!), then you can work backward and figure out what types of schooling you'll need. Keep in mind also that your interests may very well change after you've dived into a subject, and not necessarily one in your major field of study. An elective like scuba diving might lead you to marine biology or the geophysics of underwater vents, for example.

You should also be aware that getting into grad school in a physics program is likely to be difficult with only an undergrad engineering degree. Doctoral programs generally have quite a few prerequisites in terms of undergrad course work, including year-long courses in classical mechanics, statistical mechanics, quantum mechanics, electromagnetism, etc., along with the mathematics courses to support all of those (freshman calc, multivariate calc, ordinary and partial differential equations, etc.). In most cases I believe it's far easier to go from undergrad physics to grad EE or CS than the other way around.

All of that said, physics careers outside academia include working at a national laboratory (e.g., Fermilab, Brookhaven, Oak Ridge, Sandia, LBL, LLNL, Argonne), at NASA (JPL, Ames, Johnson, Kennedy, Lewis, Langley, etc.), at an astronomical observatory, at a volcano observatory, etc., or in industry helping build rockets and spacecraft, doing materials science (e.g., looking for new kinds of superconductors, energy-efficient coatings, batteries, solar cells), building atomic-level microscopes, optimizing aircraft wing and engine design, figuring out how proteins fold and how microorganisms move and adapt to their surroundings, inventing non-invasive ways to measure very small effects and/or modify tiny components inside living organisms, and simulating all of the above on computers.

And some of us end up in industry doing purely non-physics work; the analytical skills one learns as a physicist happen to translate quite well into engineering careers. (One does need to learn to suppress the research instinct, though; getting results is more important in engineering than understanding every last detail or investigating every last corner of the data.)

Common job titles for physics and engineering physics bachelor's degree recipients include:

• Accelerator Operator.
• Applications Engineer.
• Data Analyst.
• Design Engineer.
• High School Physics Teacher.
• IT Consultant.
• Lab Technician.
• Laser Engineer.

Does that mean you have to choose one of these fields? Of course not. You could become a CEO or even a financial analyst. Many have even pursued a career in the private sector after receiving their PHD. There are so many routes you could choose. You are the only person who sets that limitation and makes that final decision in the end. I expect you will do that extremely well if this is your area of focus!