How many branches in physics?

Answer 1

What do you mean by branches?

Classical Mechanics
Optics
Thermodynamics
Electro-magnetism
Quantum Mechanics
Plasma Physics
Particle Physics
Biophysics
Astrophysics
are some branches of physics. Of course the number of branches will depend on how specific you want to get - and that can end up in a semantic argument about which "branches" are subsumed under which other branch. So don't worry about counting the number - but you can find out what sorts of topics and general groupings of physics there are.

Wikipedia provides a list of physics branches and subbranches.

Of course - each of these branches dovetails with other of the branches at some level. Some like to believe that all of the "branches" are manifestations of one, over-arching, unifying physical law.

Enjoy.

Answer 2

Only two that are commonly recognized: applied physics and theoretical physics. Applied physics applies the laws of physics to real-world things and events. Theoretical physics comes up with models and equations describing physical things and events, but those models and equations have yet to be tested and verified. Typically, the proved theories from theoretical physics are the basis for the applied physics that applies those proved theories.

A prime example is E = mc^2, which was the result of one of Einstein's theories. When chain reactions in an atomic pile were observed as proof to this theory, the Manhattan Project was formed to build an atomic bomb based on it. Einstein's theory was applied to making the bomb.

You may get answers that list a bunch of other branches of physics, but they are not likely universally accepted as branches. And many of the so-called applied physics branches could be designated engineering fields. There is a fine line between applied physics and some fields of engineering. In any case, all the physics branches will fit under the applied and theoretical categories.