## Sunday, May 4, 2008

### Past JEE Objective Questions Modern Physics - Part 1

1982
1. The shortest wavelength of X-rays emitted from an X-ray tube depends on

a. the current in the tube
b. the voltage applied to the tube.
c. the nature of gas in the tube.
d. the atomic number of the target material.

2. the threshold wave length for photoelectric emission from a material is 5200 Angstrom. Photoelectrons will be emitted when this material is illuminated with monochromatic radiation from a

a. 50 watt red lamp.
b. 1 watt neon lamp.
c. 50 watt infrared lamp.
d. 1 watt untraviolet lamp.

1983
3. Beta rays emitted by a radioactive material are

b. electrons orbiting around the nucleus
c. charged particles emitted by the nucleus
d. neutral particles

4. If elements with principal quantum number n>4 were not allowed in nature, thenumber of possible elements would be

a. 64
b. 32
c. 4
d. 64

5. Consider the spectral line resulting from the transition n = 2 --> n =1 in the atoms and ions given below. The shortest wavelength is produced by

a. hydrogen atom
b. single ionized helium
c. deuteron atom
d. doubly ionized lithium

6. The equation 4[1,1]H+ --> {4,2]He2+ +2e++26 MeV represents

a. β decay
b. γ decay
c. fusion
d. fission

7. To produce characteristic X-rays using Tungsten target in an X-ray generator, the accelerator voltage should be greater than -----------volts and the energy of the characteristic radiation is ---------eV. (The binding energy of the innermost electrons in tungsten is 40 Kev.)

8. The radioactive decay rate of a radioactive element is found to be 10³ disintegrations/second at a certain time. If the half-life of the element is one second, the decay rate after one second is ------------- and after three seconds is -----------------.

9. Say whether the statement is true or false.
A small metal ball is suspended in a uniform electric field with the help of an insulation thread. If a high energy X-ray beam falls on the ball, the ball will be deflected in the direction of the field.

1984

10

11. Identify the correct statement(s).

In the Bohr model of the hydrogen atom
a. the radius of the nth orbit is proportional to n².
b. the total energy of the electron in the nth orbit is inversely proportional to n.
c. the angular momentum of the electron in an orbit is an integral multiple of h/2π .
d. the magnitude of the potential energy of the electron in any orbit is greater than its kinetic energy.

12. the maximum kinetic energy of electrons emitted in the photoelectric effect is linearly dependent on the ------------------- of the incident radiation.

13.

1985
14. A proton moving with a constant velocity passes through a region of space without any change in its velocity. If E and B represent the electric and magnetic fields respectively, this region of space may have

a. E = 0,B =0
b. E = 0,B ≠0
c. E≠0, B = 0
d. E≠0, B≠0

15. The X-ray beam coming from an X-ray tube will be

a. monochromatic
b. having all wavelengths smaller than a certain maximum wavelengths.
c. having all wavelengths larger than a certain minimum wavelength.
d. having allwavelengths lying between a minimum and a maximum wavelength.

16. IN the uranium radioactive series the initial nucleus is [238,92]U and the final nuclues is [206,82]Pb. When the uranium nucleus decays to lead, the number of α-particles emitted is ------------ and the number of β-particles emitted is ---------.

17. Planck's constant has the dimensions -----------------.

1986

18. the mass number of a nucleus is

a. is always less than its atomic number.
b. always more than its atomic number.
c. sometimes equal to its atomic number.
d. sometimes more than and sometimes equal to its atomic number.

19. When the number of of electrons striking the anode of an X-ray tube is increased, the ----------- of the emitted X-rays increases, while when the speeds of the electrons striking the anode are increased, the cut-off wavelength of the emitted X-rays ----------------.

20. When Boron nucleus [10,5]B is bombarded by neutrons, α particles are emitted. The resulting nuclues is of the element ------------ and has the mass number ---------.

21. Atoms having the ---------- but different ----------- are called isotopes.

22. State whether the statement is true or false.
In a photoelectric emission process, the maximum energy of the photoelectrons increases with increasing intensity of the incident light.