Modern Physics 2

22. It is easier to remove out an electron from a sodium surface while it is difficult in copper surface. Why?

Each metal surface has a work function which is the minimum energy required to be given to it in order to eject an electron from an atom of its surface. Here, the work function of copper is much higher as compared to that of the sodium. More energy is required to eject an electron from copper surface than from sodium surface. Hence, it is easier to remove out an electron from a sodium surface while it is difficult in copper surface.

23. How Paschen series is originated in Hydrogen spectra? An electron is in the third excited state. How many different photon wave lengths are possible?

In the Hydrogen spectra, Paschen series is originated due to the jumping of electrons from higher state to the third energy level.

From third excited state an electron can make three possible jumps. One is from third to second, another is from third to first and last one is from third to the ground state. Each jump can give one type of wavelength. Hence, there are three different photon wavelengths are possible.

24. State and explain Einstein's mass energy relation.

Einstein gave a famous theory called theory of relativity in which he showed that the mass & energy are interrelated & interchangeable. Einstein's mass energy relation states that the energy E produced by a change of mass (∆m) is given by

where, c is the velocity of light in vacuum.

Einstein's mass energy relation explains that when there is loss of mass during any process then an equivalent amount of energy is produced.

26. Explain the term mass defect.

The difference in mass between the actual mass of the nucleus and the masses of the constituent nucleons is called mass defect. Let us consider a nucleus of mass M, atomic number Z with mass of protons M_Zand mass number A then the nucleus is made up of (A-Z) neutrons each of mass M_N. The mass defect is given as:

Mass Defect = [ZM_Z+ (A - Z)M_N -M]

27. What is Uncertainty Principle?

The dual nature of the electron means that any accurate measurement of its position would create uncertainty in the measurement of its momentum & vice-versa. This was pointed out by Werner Heisenberg in his uncertainty principle. According to Heisenberg uncertainty principle, "Simultaneous & accurate determination of both position & momentum of microscopic particle like electron is not possible". Mathematically,

where, ∆x is uncertainty in the measurement of position & ∆p is uncertainty in the measurement of momentum. This expression is known as uncertainty relation. From relation 1, if uncertainty in position is reduced, the uncertainty in momentum is increased & vice-versa. It means that certainty of determination of one property introduces uncertainty for the determination of other.

29. How do you account for the binding energy of a nucleus?

Binding energy of the nucleus is the total work or energy required to take all the nucleons apart so that they are completely separated. mass-defect is the measure of binding energy. Binding energy is defined as the mass-defect multiplied by c².

Binding energy of nucleus = mass-defect.c² = [ZM_Z+ (A - Z)M_N -M].c²

31. What is the full form of LASER? What is the principle behind the working of LASER?

The full form of LASER is light amplification by stimulated emission of radiation. The light pulse from a LASER must be monochromatic, coherent, unidirectional & intense beam of light.

When the emission of energy is made more than absorption, the amplification takes place. Energy is amplified & made coherence which produce LASER. This is the basic principle behind the working of LASER.

32. Does the gaseous discharge occur at very low pressure?

No, the gaseous discharge does not occur at very low pressure. At low pressure, the density of a gas is also very low inside the tube so there is very much less possibilities for the collision of electrons emitted from cathode with gas molecules & finally the probability of collision becomes zero. Hence, no discharge is observed at very low pressure.

33. What is nuclear fission? Give an example.

Nuclear fission is the process of splitting of a heavy nucleus into two lighter nuclei with the liberation of energy when neutron is bombarded on it. For example when a heavy nucleus like ₉₂U²³⁵ is bombarded with a slow moving neutron ₀n¹, the uranium nucleus splits into lighter nuclei (Lanthanum ₅₇La¹⁴⁸ & Bromine ₃₅Br⁸⁵) along with neutrons and large amount of energy i.e.

₉₂U²³⁵ + ₀n¹ = ₅₇La¹⁴⁸ + ₃₅Br⁸⁵ + 3₀n¹ + Q

34. Point out the difference between nuclear fission and fusion.

The difference between nuclear fusion & fission are as follows:

Nuclear fusion:	Nuclear fission;
1. Nuclear fusion is the process of fusion of two light nuclei to form a heavier nucleus with the liberation of energy.	1. Nuclear fission is the process of splitting of a heavy nucleus into two lighter nuclei with the liberation of energy.
2. It does not need bombardment of neutron.	2. It needs bombardment of neutron.
3. The energy produced is less than the energy produced during nuclear fission.	3. The energy produced is more than the energy produced during nuclear fusion.
4. This process occurs in the sun & stars.	4. This process does not occur in the sun or stars.

35. What are nuclear forces? Why is the nucleus of the atom stable?

Inside the nucleus, the protons repel each other due to electrostatic repulsion of like charges. So, for the nucleus to be stable there must exist other forces between the nucleons rather than gravitational force. These are called nuclear forces. Nuclear forces have short range, shorter than the interatomic distances and much stronger than electromagnetic interactions. They provides a net attractive force greater than the sum of gravitational and electrostatic force. So, the nucleus are stable.

36. What is meant by chain reaction?

A nuclear fission reaction is called chain reaction as the product of fission contains excess of neutrons so there is maximum probability of occurring chain reaction. Due to this chain reaction, large amount of energy can be liberated.

₉₂U²³⁵ + ₀n¹ = ₅₇La¹⁴⁸ + ₃₅Br⁸⁵ + 3₀n¹ + Q

Fission of ₉₂U²³⁵ nucleus occurs when a neutron of air bombards an atom of Uranium. Each fission will produce three neutrons having kinetic energy. Three of them having sufficient energy will again bombard the three nucleus of Uranium nucleus resulting three more fission reactions. From three fission reaction more nine neutrons are produced. The process goes on until all Uranium atoms take part in the reaction with release of huge amount of energy.

37. According to properties of charges, like charges should repel. Then, how do the protons in a nucleus stay together?

According to properties of charges, like charges should repel because Coulomb’s repulsive force between two protons acts which tends to separate protons from each other. But nuclear forces provide a net attractive force greater than the sum of gravitational and electrostatic force. So, the net effect is attractive. Hence, the protons in a nucleus stay together.

During a chain nuclear fission reaction, if the fission neutrons may become sufficient to create controlled multiple fission reaction by removing some of the fission neutrons with production of large quantity of energy is called controlled chain reaction. Here, the nuclear reaction goes on slowly without any explosion or danger. The chain reaction may be made to proceed slowly and in a controlled manner & the device for accomplishing a controlled chain reaction is called nuclear reactor.