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Modern Physics Test - 1
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Modern Physics Test - 1

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  • Question 1/10
    4 / -1

    The plate resistance of a triode is 3 × 103 ohms and its mutual conductance is 1.5 × 10–3 amp/volt. The amplification factor of the triode is

    Solutions

    KEY CONCEPT : We know that µ = gm × r0
    where µ = amplification factor,
    gm = mutual conductance
    r0 = plate resistance
    ∴ µ = 3 × 103 × 1.5 × 10–3 = 4.5

  • Question 2/10
    4 / -1

    The half life of radioactive Radon is 3.8 days. The time at the end of which 1/20 th of the radon sample will remainundecayed is (given log10 e = 0.4343)

    Solutions

    t1/2 = 3.8 day

    If the initial number of atom is a = A0 then after time t the number of atoms is a/20 = A. We have to find t.

  • Question 3/10
    4 / -1

    An alpha particle of energy 5 MeV is scattered through 180° by a fixed uranium nucleus. The distance of closest approach is of the order of

    Solutions

    One point charge is  uranium nucleus

    The other point charge is α particle  ∴ q2 = + 2e
    Here the loss in K.E. = Gain in P.E. (till α-particle reaches the distance d)

    = 529.92 × 10–16 m
    = 529.92 × 10–14 cm
    = 5.2992 × 10–12 cm

  • Question 4/10
    4 / -1

    Beta rays emitted by a radioactive material are

    Solutions

    β-particles are charged particles emitted by the nucleus.

  • Question 5/10
    4 / -1

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

    Solutions

    KEY CONCEPT : The maximum number of electrons in an orbit is 2n2. n > 4 is not allowed.
    Therefore the number of maximum electron that can be in first four orbits are 2 (1)2 + 2 (2)2 + 2 (3)2 + 2 (4)2
    = 2 + 8 + 18 + 32 = 60
    Therefore, possible element are 60.

  • Question 6/10
    4 / -1

    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

    Solutions

    We know that

    λ is shortest when 1/λ is largest i.e., when Z has a highervalue. Z is highest for lithium.

  • Question 7/10
    4 / -1

    The equation  represents

    Solutions

  • Question 8/10
    4 / -1

    Fast neutrons can easily be slowed down by

    Solutions

    Fast neutrons can be easily slowed down by passing them through water.

  • Question 9/10
    4 / -1

    Consider α particles, β particles and γ - rays, each having an energy of 0.5 MeV. In increasing order of penetrating powers, the radiations are:

    Solutions

    Note : The penetrating power is dependent on velocity.
    For a given energy, the velocity of γ radiation is highest and α-particle is least.

  • Question 10/10
    4 / -1

    An energy of 24.6 eV is required to remove one of the electrons from a neutral helium atom. The energy in (eV) required to remove both the electrons from a neutral helium atom is

    Solutions

    When one e– is removed from neutral helium atom, it becomes a one e– species.
    For one e– species we know

    For helium ion, Z = 2 and for first orbit n = 1.

    ∴ Energy required to remove this e–  = + 54.4 eV

    ∴ Total energy required = 54.4 + 24.6 = 79 eV

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