Although physicists study a wide variety of phenomena, there are certain theories that are used by all physicists. Each of these theories has been tested in numerous experiments and proven to be a correct approximation of nature within its domain of validity. For example, the theory of classical mechanics accurately describes the motion of objects, provided that they are much larger than atoms and move much slower than the speed of light. While these theories have long been well-understood, they continue to be areas of active research—for example, a remarkable aspect of classical mechanics known as chaos was discovered in the 20th century, three centuries after its original formulation by Isaac Newton (1642–1727). The "central theories" are important tools for research into more specialized topics, and all physicists are expected to be literate in them.

Typical thermodynamic system - heat moves from hot (boiler) to cold (condenser) and work is extracted
Classical mechanics is a model of the physics of forces acting upon bodies. It is often referred to as "Newtonian mechanics" after Newton and his laws of motion. Classical mechanics is subdivided into statics, which models objects at rest, kinematics, which models objects in motion, and dynamics, which models objects subjected to forces. It is superseded by relativistic mechanics for systems moving at large velocities near the speed of light, quantum mechanics for systems at small distance scales, and relativistic quantum field theory for systems with both properties. Nevertheless, classical mechanics is still very useful, because it is much simpler and easier to apply than these other theories, and it has a very large range of approximate validity. Classical mechanics can be used to describe the motion of human-sized objects (such as tops and baseballs), many astronomical objects (such as planets and galaxies), and certain microscopic objects (such as organic molecules.)
Electromagnetism -- called also Electromagnetics -- is the physics of the electromagnetic field, a field that results from the presence and motion of charged particles and exerts forces on them. The sub-discipline of electrodynamics describes the behavior of moving charged particles interacting with electromagnetic fields. Electromagnetism encompasses various real-world electromagnetic phenomena. In fact, light is an oscillating electromagnetic field that is radiated from accelerating charged particles. Aside from gravity, most of the forces in everyday experience are ultimately a result of electromagnetism.
Thermodynamics is the branch of physics that deals with the action of heat and the conversions from one to another of various forms of energy. Thermodynamics is particularly concerned with how these affect temperature, pressure, volume, mechanical action, entropy, and work. Statistical mechanics, a related theory, is the branch of physics that analyzes macroscopic systems by applying statistical principles to their microscopic constituents. It can be applied to calculate the thermodynamic properties of bulk materials from the properties of individual molecules, which is the basis of statistical thermodynamics.
The theory of relativity is:
a physical theory which is based on the two postulates: (1) that the speed of light in a vacuum is constant and independent of the source or observer and (2) that the mathematical forms of the laws of physics are invariant in all inertial systems-- called also special relativity, special theory of relativity. It leads to the assertion of the equivalence of mass and energy and of change in mass,dimension, and time with increased velocity.
an extension of special relativity to include transformations between non-inertial frames-- also called general relativity or the general theory of relativity. It is formulated using differential geometry and interprets gravity as a distortion of spacetime caused by the presence of mass or energy.
Quantum mechanics is the branch of mathematical physics treating atomic and subatomic systems and their interaction with radiation in terms of observable quantities. It is based on the observation that all forms of energy are released in discrete units or bundles called quanta. Quantum mechanics provides a physical theory of matter that is based on the concept of the possession of wave properties by elementary particles, affords a mathematical interpretation of the structure and interactions of matter on the basis of these properties, and incorporates within it quantum theory and the uncertainty principle -- called also wave mechanics. Remarkably, quantum theory typically permits only probable or statistical calculation of the observed features of subatomic particles, understood in terms of wavefunctions. The discovery of quantum mechanics in the early 20th century revolutionized physics, and quantum mechanics is fundamental to most areas of current research.