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Everyone has encountered the term "Physics" in various contexts at least once during his/her life experience. Young children and teenagers know Physics as a school subject that offers a generic insight on how the world works; other people consider Physics simply as a natural science that seeks to explain, in a logical way, the relationship between various natural phenomena. The overwhelming majority of people however, consider Physics as a set of natural laws or phenomena explained through experiments and formulae. These varying perceptions are due to experience and understanding but what is Physics really?
In vocabulary, Physics is defined in the following way:
Physics is the branch of science concerned with the nature and properties of matter and energy.
In a broader sense, Physics deals with matter, energy and their interactions. Hence, it is a natural science based on experiments, measurements and mathematical analysis with the purpose of finding quantitative laws for everything - from the micro world (the world that we cannot see as it is composed by very small particles) to the planets, solar systems and galaxies that occupy the macro cosmos.
However, the most important goal of Physicists is to make old, new and emerging discoveries in Physics applicable in daily life. They aim to make Physics relevant and practical so that people may benefit from these discoveries and as a result, their life become easier and happier than before.
In this section, the methods used in science (especially in Physics) for discovering new thing are discussed. All discoveries in Physics are made in one of the following ways:
A scientific study can only be useful if we use accurate measurements. Before making any measurement we have to choose a predefined quantity as a standard of measurement. For example, if we want to measure the length of something, we have to use a predefined unit for length, to calculate the weight of an object we have first to take a predefined unit of weight and express the result in its terms and so on.
When reading Physics guides and material, you will likely have encountered the terms 'Modern Physics' and 'Classical Physics', if not, and for those who may be new to these terms: Classical Physics comprises all laws and discoveries made up to the end of 19th century, a period in which many discoveries related to micro-world were made. All these discoveries and those made later belong to Modern Physics.
Scientific development has brought, as a result, the enrichment of Physics with many new concepts and laws. Therefore, Physicists found it reasonable to divide Physics into several parts (branches), so that they may orient their energies and abilities towards a specific branch to which they are more specialized. Regardless of their specific abilities, nowadays it is impossible for a scientist to dedicate themself to Physics as a whole due to the huge amount of knowledge which has been acquired by humanity. That's why it is better to divide Physics into branches, which albeit autonomous, have a lot of relationships between them in the generic sense. This separation also allows scientists to focus their energies on developing and exploring specific problems, by increasing specialisation, scientists can push the boundary in certain fields and increase the holistic knowledge of physics for future generations.
A simplified scheme that represents the most important Physics branches (but not all) and (some) sub-branches is shown below.
Let's take a closer look at all branches and sub-branches shown in the above figure and how each fits within modern physics and classical physics.
Classical Mechanics is a sub-branch of Classical Physics that deals with motion and forces. It further divides into Kinematics (the branch of Classical Mechanics that deals specifically with motion without analysing its causes) and Dynamics (the branch of Classical Mechanics that deals with both motion and its causes, i.e. forces).
Optics, as a sub-branch of Classical Physics, deals with the properties of light. Geometrical Optics is concerned with the light path and images formed from it while Physical Optics is concerned with the physical properties of light such as Reflection, Refraction, Interference, Diffraction, Dispersion, Polarization, etc.
Electromagnetism deals with the behaviour of charged particles and their interaction. When only the electric nature of mater is taken into consideration, we are studying Electricity, and in cases when we are interested on the magnetic nature of matter, this part is covered by Magnetism. Electricity is further divided into Electrostatics (electric charges at rest) and Electrodynamics (electric charges in motion). Magnetism on the other hand, is further divided into Simple Magnetism (it involves magnets, compasses, earth magnetic field, etc.) and Electromagnetism, from which the main branch has taken the name (it involves the interaction between the electric and magnetic fields and forces).
Waves deals with the regular vibration that occur in a given medium (usually in space but not exclusively) due to a disturbance. Waves analyse both quantitatively and qualitatively the various types and properties of these vibrations.
Astrophysics deals with celestial bodies and their properties. It also gives response to questions related to the origin, history, actual behaviour and the future of universe.
Thermodynamics is concerned with the interaction between the particles of substances caused by the heat energy supplied or taken out from those substances.
On the other hand, the description of Modern Physics branches is as follows:
Quantum Mechanics deals with very, very, small particles (much smaller than the atom itself) whose exact location cannot be fully determined. Therefore, a probabilistic approach is used to give response to all questions arisen in this regard. In few words, in classical mechanics, objects exist in a specific place at a specific time. However, in quantum mechanics, objects instead exist in a haze of probability; they have a certain chance of being at point A, another chance of being at point B and so on.
Elementary Particles deals with subatomic particles that in general are not stable. By definition, a particle is regarded as elementary only if there is no evidence that it is made up of smaller constituents.
Nuclear Physics deals with the particles contained within the nuclei of atoms and their behaviour when they are forced to change their actual state if struck by very energetic particles.
Atomic Physics is the branch of Modern Physics that is concerned with atoms as isolated systems composed by electrons and atomic nuclei. It is mainly concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change.
Relativity is a theory that was first formulated by Einstein. Nowadays it constitutes one of the main branches of Modern Physics, which explains many phenomena that cannot be explained through the standard Physics. Einstein has supported the idea that motion must be defined relative to a frame of reference and that space and time are relative, rather than absolute concepts. Relativity consists of two principal parts: general theory of Relativity and Special Theory of Relativity for which we will discuss later.