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Modern Physics Test - 4
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Modern Physics Test - 4
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  • Question 1/10
    4 / -1

    A radioactive sample consists of two distinct species having equal number of atoms initially. The mean life time of one species is τ and that of the other is 5τ. The decay products in both cases are stable. A plot is made of the total number of radioactive nuclei as a function of time. Which of the following figures best represent the form of this plot?

    Solutions

        ....(i)

    Adding (i) and (ii) we get

    N = N1 + N2 = N0 (e -t/τ+e-t/5τ) 

    (a) is NOT the correct option as there is  a time τ for which N is constant which means for time τ there is no process of radioactivity which does not makes sense. (b) and (c) shows intermediate increase in the number of radioactive atom which is IMPOSSIBLE as N will only decrease exponentially.

  • Question 2/10
    4 / -1

    The potential difference applied to an X-ray tube is 5kV and the current through it is 3.2mA. Then the number of electrons striking the target per second is

    Solutions

    KEY CONCEPT :

    No. of electrons striking the target per second

  • Question 3/10
    4 / -1

    A Hydrogen atom and a Li++ ion are both in the second excited state. If ℓ H and ℓ Li are their respective electronic angular momenta, and EH and ELi their respective energies, then

    Solutions

  • Question 4/10
    4 / -1

    The half-life of 215At is 100 μ s. The time taken for the radioactivity of a sample of 215At to decay to 1/16th of  its initial value is

    Solutions

    A = A0 (1/2)n; n = number of half lives.

  • Question 5/10
    4 / -1

    Which of the following processes represents a γ -decay ?

    Solutions

    In γ-decay, the atomic number and mass number do not change.

  • Question 6/10
    4 / -1

    The electric potential between a proton and an electron is given by  where r0 is a constant. AssumingBohr’s model to be applicable, write variation of rn with n, n being the principal quantum number?

    Solutions

    Given potential energy between electron  and proton

    But this force acts as centripetal force

    By Bohr’s postulate, mvr  = nh/2π    ...(ii)

    From (i) and (ii),

  • Question 7/10
    4 / -1

    If the atom 100Fm257 follows the Bohr model and the radius of 100Fm257 is n times the Bohr radius, then find n.

    Solutions

    KEY CONCEPT : For an atom followig Bohr ’s model, the radius is given by

     = Bohr’s radius and m = orbit number.
    For Fm, m = 5 (Fifth orbit in which the outermost electron is present)

  • Question 8/10
    4 / -1

    For uranium nucleus how does its mass vary with volume?

    Solutions

    KEY CONCEPT : We know that radius of the nucleus R = R0A1/3, where A is the mass number.

    Volume ∝ mass. 

  • Question 9/10
    4 / -1

    A nucleus with mass number 220 initially at rest emits an α -particle. If the Q value of the reaction is 5.5 MeV, calculate the kinetic energy of the α -particle

    Solutions

    By conservation of momentum, p1 = p2

    Solve equation (i) and (ii)

  • Question 10/10
    4 / -1

    In a photoelectric experiment anode potential is plotted against plate current.

    Solutions

    From the graph it is clear that A and B have the same stopping potential and therefore, the same frequency.
    Also, B and C have the same intensity.

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