Modern Physics is a very important constituent of Physics portion of the IIT JEE.
It is interesting as well as usually fetches many questions in the JEE. It includes
topics like Nuclear Fission and Fusion which are easy to master. These topics are
quite fascinating but involve concepts which must be understood properly.
Modern Physics for IIT JEE refers to the Physics based on the two major branches:
relativity and quantum mechanics. Classical Physics refers to the traditional Physics
which was based on the concepts before coming up of Modern Physics. There were various
theoretical and experimental paradoxes that forced thinking out of the traditional
path. Modern physics is generally encountered when dealing with extreme conditions.
Quantum mechanical effects appear in circumstances dealing with "lows"
(low temperatures, small distances), while relativistic effects tend to appear when
dealing with "highs" (high velocities, large distances), the "middles"
being classical behavior. The Classical Physics was indeed in accord with common
sense. Modern Physics has in fact come over that and imparts a better understanding
of nature. Modern Physics in IIT JEE syllabus is the most scoring part.
Nuclear Fission and Fusion: Nuclear Fission
and Fusion are two different kinds of energy releasing reactions. In these reactions,
the energy is released from high- powered atomic bonds between the particles present
in the nucleus. The two processes are quite opposite in nature. While Fission involves
the splitting of an atom into two or more atoms, in Fusion, two or more smaller
atoms combine to form a larger atom. We discuss both the processes one by one.
Nuclear Fission:Nuclear Fission is the process
of splitting atoms. It is a process in nuclear physics in which the nucleus of the
atom into smaller nuclei as fission products along with some by-produce particles.
Fission hence may be termed as a form of elemental transmutation.
The by-products comprise free neutrons and photons which are generally in the form
of gamma rays in addition to other nuclear fragments such as beta particles and
alpha particles. Fission is an exothermic reaction which means there is a release
of huge amount of energy when it takes place. Fission of heavy elements releases
considerable amount of useful energy either in the form of gamma rays or as kinetic
energy of the fragments. This energy may be used for nuclear power or for the explosion
of nuclear weapons.
The sum of the masses of these fragments is less than the original mass. This
gap in the mass which is around 0.1 percent of the original total mass
has been converted into energy according to Einstein's equation.
Nuclear Fusion: In simple words, Fission refers
to the process in which two or more atoms combine to form a larger atom. Nuclear
energy can also be released by fusion of two light elements (elements with low atomic
numbers). The power that fuels the sun and the stars is nuclear fusion. In a hydrogen
bomb, two isotopes of hydrogen, deuterium and tritium are fused to form a nucleus
of helium and a neutron. It may also be defined as the process in which multiple
nuclei join together to form a heavier nucleus. It is accompanied by the release
or absorption of energy depending on the masses of the nuclei involved.
Radioactive Decay of Substances:
Radioactive decay, as the word suggests refers to the decay to attain stability.
It refers to the loss of particles from an unstable atom in order to attain more
stability. The unstable elements emit some particles from their nucleus to gain
stability and this process is termed as radio-activity. For elements, uniformity
is produced by having an equal number of neutrons and protons which determines and
henceforth directs the nuclear forces to keep the nuclear particles inside the nucleus.
There may be cases when a particle becomes more frequent than another and hence
creates an unstable nucleus. The unstable nucleus then releases radiation in order
to gain stability.
This radio-active decay can occur in five forms:
As stated above, each decay emits some specific particle which also changes the
type of product produces. The nuclei produced from the decay are called the daughter
nuclei. The type of decay also determines the number of neutrons and protons found
in the daughter nuclei. Let us consider an example of a radioactive substance.
The stable Beryllium contains 4 protons and 5 neutrons in its nucleus. A lighter
isotope of beryllium is also available which contains 4 protons and only 3 neutrons,
which gives a total mass of 7 amu. This isotope decays into Lithium-7 through electron
capture. A proton from Beryllium-7 captures a single electron and becomes a
neutron. This reaction produces a new isotope (Lithium-7) that has the same
atomic mass unit as Beryllium-7 but one less proton which stabilizes the element.
Beta decay occurs when the neutron to proton ratio
is too great in the nucleus and causes instability. In basic beta decay, a neutron
is turned into a proton and an electron. The electron is then emitted. Here's
a diagram of beta decay with hydrogen-3:
Half-Life is a very common term associated with radio-active decay. It cannot
be easily detected when a single radioactive atom will decay. But, we can get an
idea about the time required for half a large number of identical radioactive atoms
to decay. This time is called the half-life.
Structure of Atom and Nucleus:
An atom is made up of three subatomic particles: protons, neutrons and electrons.
The protons and neutrons are placed inside the nucleus. The nucleus is at the center
of the atom. The electrons keep on moving in orbits around the nucleus. The nature
of the atom is determined by the number of protons. The protons carry positive charge,
while electrons are negatively charged. Neutrons, as the name suggests are neutral
and do not carry any charge. If the nucleus contains 17 protons, then the atom is
chlorine. An atom of oxygen contains 8 protons in its nucleus.
The nucleus is the dense central core of the atom which contains both the protons
as well as the neutrons. Electrons are outside the nucleus in energy levels. Protons
have a positive charge, neutrons have no charge, and electrons have a negative charge.
An atom is said to be neutral of it has the same number of protons and electrons.
The neutrons can vary in number in the atom of a particular element. Atoms of the
same element that have differing numbers of neutrons are called isotopes.
The formula for finding the atomic number of an elemnet is given by
Atomic Number = Number of electrons – Number of Protons