If one has a BS in physics and a MS in engineering how hard is it to obtain a phd in physics?
PhDs in physics take a great deal of dedication and work no matter what your prior qualifications are.
If you're asking about if it's harder to get accepted: no.
If you're asking if it's easier to complete: no.
On one hand, you have at least shown some talent in completing graduate coursework (and possibly a thesis). That's good to selection committees. You also presumably have the undergraduate background needed for the graduate coursework in physics. But they'll still care about grades in physics coursework, what that coursework was, and the physics GRE.
On the other hand, your graduate coursework was in engineering... not physics. There will be new required coursework in physics. You MIGHT get out of some elective requirements. But the core of the Ph.D. degree is the research you do.... and often the elective classes are pleasant 2-3-times-a-week distractions from that work, which will be otherwise consuming.
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