Competitive Exams: Physics MCQs (Practice-Test 9 of 35)

  1. An electron having kinetic energy T is moving in a circular orbit of radius R perpendicular to a uniform Magnetic induction B. If kinetic energy is doubled and magnetic induction is tripled, The radius will become

    1. R '9/4

    2. R '3/2

    3. R '2/9

    4. R '4/3

  2. If in the first Bohr orbit of a hydrogen atom the total energy of the electron is-21.76 × 10 − 19 J, then its potential energy will be

    1. -43.52 × 10 − 19 J

    2. -21.76 × 10 − 19 J

    3. -10.88 × 10 − 19 J

    4. -13.60 × 10 − 19 J

  3. Consider the following substances, which are frequently used as a source of electrons:

    1. Magnesium

    2. Calcium

    3. Tungsten

    4. Platinum

    The correct sequence of the above substances in increasing order of their work functions is

    1. 1, 2, 3, 4

    2. 2, 1, 3, 4

    3. 2, 1, 4, 3

    4. 1, 2, 4, 3

  4. The de-Broglie wavelength of material particles which are in thermal equilibrium at temperature T is

    1. h/sqrt 2mkT

    2. h/sqrt 2 mkT

    3. h-mkT

    4. h/sqrt2 kT

  5. The phase velocity (Vp) and the group velocity (Vg) of a de-Broglie wave, in free space (speed of light is c) are related as

    1. Vp/Vg = sqrt2

    2. Vp. Vg = C2

    3. Vp/Vg = 0.5

    4. Vp Vg = 2C2

  6. In Compton scattering, the incident photon will lose maximum energy if it is scattered at an angle of:

    1. 0

    2. 60

    3. 90

    4. 180

  7. In one of the radioactive decay chains we come across the element 22688 Ra. The final product of the chain is 20682 Pb. The number of a and b-is emitted

    1. 3 a and 6b-

    2. 4 a and 5b-

    3. 5 a and 4b-

    4. 6 a and 6b-

  8. At time t. = 0, N1 nuclei of decay constant l1 and N2 nuclei of decay constant l2 are mixed. The decay rate of the mixture is

    1. N1 N2 e- (l1 + l2) t

    2. (N1/N2) e- (l1 − l2) t

    3. (N1 N2 e-l1t + N2 l2 e-l 2t)

    4. N1/N2 e- (l1 + l2) t

  9. The nuclear reaction is an example of

    1. nuclear chain reaction

    2. successive disintegration

    3. pair production

    4. artificial radioactivity

  10. Which one of the following particles is a lepton?

    1. Neutron

    2. Proton

    3. Electron

    4. Lambda

  11. Match List I with List II and select the correct answer:

    List-I (Process) List-II (Approximate Energy released in mV)
    1. Fission of one uranium nucleus

    2. Formation of one helium nucleus by Fission

    3. Formation of a compound nucleus

    4. Formation of a deuteron by the

    1. Energy 2

    2. Energy 8

    3. Energy 200 by the capture of a neutron

    4. Energy 28 captures of a neutron by a proton

    • A
    • B
    • C
    • D
      • 3
      • 4
      • 2
      • 1
      • 2
      • 4
      • 1
      • 3
      • 4
      • 2
      • 3
      • 1
      • 3
      • 1
      • 2
      • 4
  12. Under space charge condition with the plate voltage Eb = 100 V and the permanence K = 104 (In S. I unit), the plate current in a diode will be

    1. 10 − 6 mA

    2. 10 mA

    3. 102 mA

    4. 106 mA