HELIUM

source:news papers

WHY RANK OF MATRIX?

Generally we will find the rank of matrix.Most of us dont know why do we find.One application of determining rank of matrix is in solving networks problems.Network consists of several meshes.There we write current equations in the form of matrix and will find the rank of  matrix.If the current flowing in one mesh is depending on another mesh current then the rank of the matrix will be less than the order of the matrix.

Conditions of a two port network

The conditions that have to be obeyed by the two port network are
1) the components present inside the network should be passive elements and dependent sources only
2) The current directions at input and outputside must be into the network.

Classical and wave-mechanical model of electron

For large scale phenomenon,such as electronic trajectories in a vaccum tube,the classical model yields accurate results.For small scale systems,however such as an electron in an atom or in a crystal,the classical model treated by newtonian mechanics gives result which do not agree with experiment.To describe such subatomic systems properly it is found necessary to attribute to the electron a wavelike property which imposes restrictions on the exactness with which the electronic motion can be predicted.

Why the temperature coefficient of resistance in metals is positive while it is negative in semiconductors?

SEMICONDUCTOR:
we know that in semiconductor at room temperature the number of charge carriers available are very less.But as temperature increases the number of charge carriers also increases which in turn decreases the resistance.
so the resistance of semiconductor decreases with increase in temperature.
hence semiconductors have negative temperature coefficient.

METAL:
At room temperature metals have more number of charge carriers.generally the temperature has no effect on number of charge carriers.but with increases in temperature the mean free path of electrons decreases.that means as temperature increases the electrons in conduction band will collide each other during their journey(scattering effect).
so resistance will increase with increase in temperature.
hence metals have positive temperature coefficient.

note: In semiconductor the scattering effect is less when compared to the increase of charge carriers with temperature.
In metals the scattering effect is more.

How time constant is equal to (L/R)?

We know that the series RLC circuit has time constant T=L/R The units on LHS is sec.But the unit on RHS is henry/ohm.so how these two are equal?

we know that  L= V/(di/dt)

and R= V/I

L/R={V/(di/dt)}/(V/i)

so th units of L/R is seconds.

Why silicon is preferred over germanium in manufacturing transistors?

For manufacturing the transistors we have majorly two semiconductor materials.They are silicon and germanium.But silicon is mostly preffered over germanium.

              The first transistor was made up of germanium only.since germanium has lower barrier poterntial when compared to that of silicon.The barrier potential of silicon is 0.7Vwhile germanium has 0,3V.
But we use silicon for manufacturing BJTs because,
1)silicon is thermally stable even at high temperatures while the germanium is not that much stable.
2)germannium has more leakage current
3)silicon is the most abundant element in the world.Thats why it is very cheap to manufacture.