# IIT JEE Physics : Modern Physics – previous 15 years questions & answers> Part I

This post contains the objective & passage questions asked in Modern Physics in the IIT JEE Physics paper during the last 15 years. Pat II of the post will deal with the answers. ( pre – final draft )

M O D E R N   P H Y S I C S

Section.A            Fill in the blanks

1.In a ………. Biased p-n junction, the net flow of holes is from the n region to the p region.        ( IIT JEE 1993 )

2. A potential difference of 20 kV is plied across and X – ray tube.  The minimum wavelength of X-rays generated is ………….Å .( IIT JEE 1996 )

3.Consider the following reaction:  H² 1+  H²1 =  4He2 + Q.  Mass of the deuterium atom = 2.0141 m. This is a nuclear …… reaction in which the energy    Q released is …….. MeV. ( IIT JEE 1996 )

4. The wavelength of Kα X – rays produced by an X-ray tube is 0.76 Å.  The atomic number of the anode material of the tube is ……..(IIT JEE 1996 )

5.The recoil speed of a hydrogen atom after it emits a photon is going from n = 5 state to n = 1 state is …….. m/s. ( IIT JEE 1997)

Section B       Objective questions –  ( Only one option is correct )

1. A star initially has 1040 deuterons.  It produces energy via the processes 1H2 + 1H2 → 1H3 + p and 1H2 + 1H3 → 2He4 + n.  If the average power radiated by the star is 1016 W, the deuteron supply of the star is exhausted in a time of the order of -The mass of the nuclei are as follows: M(H2) = 2.014 amu ; M(n) = 1.008 amu; M(p) = 1.008 amu; M (He4) = 4.001 amu.  ( IIT JEE 1993 )

1. 106 s
2. 108 s
3. 1012 s
4. 1016 s

2. Fast neutrons can easily be slowed down by:  ( IIT JEE 1994 )

1. The use of lead shielding
2. Passing them through heavy water
3. Elastic collisions with heavy nuclei
4. Applying a strong electric field

3. Consider α- particles, β – particles and γ- rays having an energy of 0.5MeV.  In increasing order of penetrating powers, the radiations are : ( IIT JEE 1994 )

1. α,β,γ
2. α,γ,β
3. β,γ,α
4. γ,β,α

4. An artificial satellite moving in a circular orbit around the Earth has a total (kinetic + potential) energy = E0. Its potential energy is:(IIT JEE 1997)

1. –E0
2. 1.5 E0
3. 2E0
4. E0

5. The Kα X-ray emission line of tungsten occurs at l = 0.021 nm.  The energy difference between K and L levels in this atoms is about:( IIT JEE 1997)

1. 0.51 MeV
2. 1.2 MeV
3. 59 keV
4. 13.6 eV

6. The electrical conductivity of a semiconductor increases when em radiation of wavelength shorter than 2480 nm is incident on it.  The band gap (in eV) for the semiconductor is:  ( IIT JEE 1997 )

1. 0.9
2. 0.7
3. 0.5
4. 1.1

7. The dominant mechanisms for motion of charge carriers in forwards and reverse biased silicon p-n junctions are: ( IIT JEE 1997 )

1. Drift in forward bias, diffusion in reverse bias
2. Diffusion in forward bias, drift in reverse bias
3. Diffusion in both forward and reverse bias
4. Drift in both forward and reverse bias

8. Masses of two isobars 29Cu64 and 30Zn64 are 63.9298 u and 63.9292 u respectively.  It can be concluded from these data that:( IIT JEE 1997 )

1. Both the isobars are stable
2. Zn64 is radioactive, decaying to Cu64 through β-decay
3. Cu64 is radioactive, decaying to Zn64 through γ-decay
4. Cu64 is radioactive, decaying to Zn64 through β-decay

9. As per Bohr model, the minimum energy (in eV) required to remove an electron from the ground state of doubly ionized LI atom (Z = 3) is: ( IIT JEE 1997 )

1. 1.51
2. 13.6
3. 40.8
4. 122.4

10. The circuit shown in the figure contains two diodes each with a forward resistance of 50 W and with infinite backward resistance.  If the battery voltage is 6V, the current through the100 W resistance (in amperes) is: ( IIT JEE 1997 )

1. Zero
2. 0.02
3. 0.03
4. 0.036

11. Which of the following statements is not true?   ( IIT JEE 1997 )

1. The resistance of intrinsic semiconductors decreases with increase of temperature
2. Doping pure Si with trivalent impurities give p-type semiconductors
3. The majority carriers in n-type semiconductors are holes
4. A p-n junction can act as a semiconductor diode

12. The maximum kinetic energy of photoelectrons emitted from a surface when photons of energy 6 eV fall on it is 4 eV.  The stopping potential in volts is: ( IIT JEE 1997 )

1. 2
2. 4
3. 6
4. 10

13. The half –life of 131 I is 8 days.  Given a sample of 131 I at time t = 0, we can assert that –  ( IIT JEE 1998 )

1. No nucleus will decay before t = 4 days
2. No nucleus will decay  before t = 8 days
3. All nuclei will decay before t = 16 days
4. A given nucleus may decay at any time after t = 0

14. In a p-n junction diode not connected to any circuit:   ( IIT JEE 1998 )

1. The potential is the same everywhere
2. The p-type side is at higher potential than the n-type side
3. There is an electric field at the junction directed from the n-type side to the p –type side
4. There is an electric field at the junction directed from the p-type side to the n–type side

15. X-rays are produced in an X-ray tube operating at a given accelerating voltage.  The wavelength of the continuous X-rays has values from:(IIT JEE 1998 )

1. 0 to α
2. λ min to α where λ min > 0
3. 0 to λ max where λ max < α
4. λ min to λ max  where 0 < λ min  < λ maxα

16. The work function of a substance is 4.0 eV.  The longest wavelength of light that can cause photoelectron emission from this substance is approximately: ( IIT JEE 1998 )

1. 540 nm
2. 400 nm
3. 310 nm
4. 220 nm

17. Binding energy per nucleon versus mass number curve for nuclei is shown in figure.  W,X,Y and Z are four nuclei indicated on the curve.  The process that would release energy is:  ( IIT JEE 1999 )

1. Y→ 2Z
2. W → X + Z
3. W → 2Y
4. X → Y + Z

18. Order of magnitude of density of uranium nucleus is (mp = 1.67 x 10-27) kg):  ( IIT JEE 1999 )

1. 1020 kg/m3
2. 1017 kg/m3
3. 1014 kg/m3
4. 1011 kg/m3

19. 22Ne nucleus, after absorbing energy, decays into two α-particles and an unknown nucleus.  The unknown nucleus is: ( IIT JEE 1999 )

1. Nitrogen
2. Carbon
3. Boron
4. Oxygen

20. In hydrogen spectrum the wavelength of H α  line is 656 nm; whereas in the spectrum of a distant galaxy H α line wavelength is 706 nm.  Estimated speed of galaxy with respect to earth is:  ( IIT JEE 1999)

1. 2 x 108 m/s
2. 2 x 107 m/s
3. 2 x 106 m/s
4. 2 x 105 m/s

21. A particle of mass M at rest decays into two particles of masses m1 and m2 having non-zero velocities.  The ratio of the de-Broglie wavelengths of the particles λ1/ λ2 is:  ( IIT JEE 1999 )

1. m1/m2
2. m2/m1
3. 1
4. √m2/ √m1

22. Which of the following is a correct statement?   ( IIT JEE 1999 )

1. Beta rays are same as cathode rays
2. Gamma rays are high energy neutrons
3. Alpha particles are singly ionized helium atoms
4. Protons and neutrons have exactly the same mass

23. The half – life period of a radioactive element x is same as the mean life time of another radioactive element y.  Initially both of them have the same number of atoms.  Then:  ( IIT JEE 1999 )

1. x and y have the same decay rate initially
2. x and y decay at the same rate always
3. y will decay at a faster rate than x
4. x will decay at faster rate than y

24. Electrons with energy 80 keV are incident on the tungsten target of an X-ray tube.  K-shell electrons of tungsten have 72.5 keV energy.  X-rays emitted by the tube contain only:  ( IIT JEE 2000 )

1. a continuous X-ray spectrum (Bremsstrahlung) with minimum wavelength of ≈ 0.155 Å.
2. a continuous X-ray spectrum (Bremsstrahlung) with all wavelengths
3. the characteristic X-ray spectrum of tungsten
4. a continuous X-ray spectrum (Bremsstrahlung) with a minimum wavelength of ≈ 0.155 Å and the characteristic X-ray spectrum of tungsten.

25. Imagine an atoms made up of proton and a hypothetical particle of double the mass of the electron but having the same charge as the electron.  Applying the Bohr atom model and consider all possible transitions of this hypothetical particle to the first excited level.  The longest wavelength photon that will be emitted has wavelength λ  (given in terms of the Rydberg constant R for the hydrogen atom) equal to: ( IIT JEE 2000 )

1. 0/5 R
2. 36/5 R
3. 18/5 R
4. 4/R

26. Two radioactive materials X1 and X2 have decay constants 10 λ  and λ respectively.  If initially they have the same number of nuclei, then the ratio of the number of  nuclei of X1 to that of X2 will be 1/e after a time:  ( IIT JEE 2000 )

1. 1/10 K
2. 1/11λ
3. 11/10 λ
4. 1/9 λ

27. The electron in a hydrogen atom makes a transition from an excited state to the ground state. Which of the following statements is true? ( IIT JEE 2000 )

1. Its kinetic energy increases and its potential and total energy decreases
2. Its kinetic energy decreases, potential energy increases and its total energy remains the same
3. Its kinetic and total energy decreases and its potential energy increases
4. Its kinetic, potential and total energy decreases.

28. The electron emitted in beta radiation originates from: ( IIT JEE 2001 )

1. Inner orbits of atom
2. Free electrons existing in nuclei
3. Decay of a neutron in a nucleus
4. Photon escaping from the nucleus

29. The transition form the state n = 4 to n = 3 in a hydrogen like atom results in ultraviolet radiation.  Infrared radiation will be obtained in the transition: ( IIT JEE 2001 )

1. 2 → 1
2. 3 → 2
3. 4 → 2
4. 5 → 4

30. The intensity of X-rays from a Coolidge tube is plotted against wavelength λ  as shown in the figure.  The minimum wavelength found is λc and the wavelength of the Kα line is λk.  As the accelerating voltage is increased: ( IIT JEE 2001 )

• λk – λc increases
• λk – λc decreases
• λk increases
• λk decreases                        ( to continue ……… )