NEET-XII-Physics

P1 year:2019

with Solutions - page 3

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  • Qstn #31
    α-particle consists of :
    (1) 2 protons and 2 neutrons only
    (2) 2 electrons, 2 protons and 2 neutrons
    (3) 2 electrons and 4 protons only
    (4) 2 protons only
    digAnsr:   1
    Ans : ( 1 )
    Sol. α-particle is nucleus of Helium which has two
    protons and two neutrons.
  • Qstn #32
    The speed of a swimmer in still water is
    20 m/s. The speed of river water is 10 m/s and
    is flowing due east. If he is standing on the
    south bank and wishes to cross the river along
    the shortest path the angle at which he should
    make his strokes w.r.t. north is given by :
    (1) 30° west
    (2) 0°
    (3) 60° west
    (4) 45° west
    digAnsr:   1
    Ans : ( 1 )
    Sol.
    SR
    RG
    V 20 m/s
    V 10 m/s
    =
    =
    V
    SG
    N

    S
    W E
    V
    SR
    V
    RG
    = +
    SG SR RGV V V
     =
    RG
    SR
    V
    sin
    V
     = 10sin
    20
     = 1sin
    2
     = 30° west9
  • Qstn #33
    A particle moving with velocity ``\vec{V}``
    is acted by three forces shown by the vector
    triangle PQR.
    The velocity of the particle will :
    m33.png
    (1) Increase
    (2) Decrease
    (3) Remain constant
    (4) Change according to the smallest force ``\vec{QR}``
    digAnsr:   3
    Ans : ( 3 )
    Sol.
    P
    R Q
    As forces are forming closed loop in same
    order
    So, =netF 0
    &implies; =dvm 0
    dt
    &implies; =v constant
  • Qstn #34
    Two particles A and B are moving in uniform
    circular motion in concentric circles of radii
    ``r_A`` and ``r_B`` with speed ``v_A`` and ``v_B``
    respectively. Their time period of rotation is
    the same. The ratio of
    angular speed of A to that of B will be :
    (1) ``r_A : r_B``
    (2)``v_A : v_B``
    (3)``r_B : r_A``
    (4) 1 : 1
    digAnsr:   4
    Ans : ( 4 )
    Sol.
    rA
    r
    B
    T
    A
    = T
    B
    = T
    ω =
    A
    A
    2
    T
    ω =
    B
    B
    2
    T
    ω
    = = =
    ω
    A B
    B A
    T T
    1
    T T
  • Qstn #35
    A block of mass 10 kg is in contact against the
    inner wall of a hollow cylindrical drum of
    radius 1 m. The coefficient of friction between
    the block and the inner wall of the cylinder is
    0.1. The minimum angular velocity needed for
    the cylinder to keep the block stationary when
    the cylinder is vertical and rotating about its
    axis, will be : (g = ``10m/s^2`` )
    (1) ``\sqrt10`` rad/s
    (2)``\frac {10}{2\pi} rad/s``
    (3) 10 rad/s
    (4) 10 ``\pi`` rad/s
    digAnsr:   3
    Ans : ( 3 )
    Sol.
    N mrω2
    f
    L
    mg
    For equilibrium of the block limiting friction
    f
    L
    ≥ mg
    &implies; μN ≥ mg
    &implies; μmrω2 ≥ mg
    ω≥
    μ
    g
    r
    ω =
    μmin
    g
    r
    ω =
    min
    10
    0.1 1
    = 10 rad/s
  • Qstn #36
    Two parallel infinite line charges with linear
    charge densities +``\lambda`` C/m and -``\lambda`` C/m are
    placed at a distance of 2R in free space. What
    is the electric field mid-way between the two
    line charges?
    (1) Zero
    (2)``\frac{2\lambda}{\pi\varepsilon _0 R}`` N/C
    (3)``\frac{\lambda}{\pi\varepsilon _0 R}``N/C
    (4)``\frac{\lambda}{2\pi\varepsilon _0 R}``N/C
    digAnsr:   3
    Ans : ( 3 )
    Sol. -
    -
    -
    -
    -
    -
    -
    -
    -
    -
    +
    +
    +
    +
    +
    +
    +
    +
    -+
    (1)(2)
    R E2
    E
    1
    X-X
    Electric field due to line charge (1)
    =
    
    1
    0
    ˆE i N /C
    2 R
    Electric field due to line charge (2)
    =
    
    2
    0
    ˆE i N /C
    2 R
    = +
    net 1 2
    E E E
    =
     +
     
    0 0
    ˆ ˆi i
    2 R 2 R
    =

    
    0
    î N /C
    R
  • Qstn #37
    Two point charges A and B, having charges +Q
    and -Q respectively, are placed at certain
    distance apart and force acting between them
    is F. If 25% charge of A is transferred to B,
    then force between the charges becomes :
    (1) F
    (2)``\frac {9F}{16}``
    (3)``\frac {16F}{9}``
    (4)``\frac {4F}{3}``
    digAnsr:   2
    Ans : ( 2 )
    Sol.
    A B
    r
    +Q -Q
    2
    2
    kQ
    F
    r
    =
    If 25% of charge of A transferred to B then
    =  = =  + =
    A B
    Q 3Q Q 3Q
    q Q and q Q
    4 4 4 4
    r
    q
    A
    q
    B
    A B
    1 2
    kq q
    F
    r
    =
    2
    1 2
    3Q
    k
    4
    F
    r
     
     
     =
    1 2
    9 kQ
    F
    16 r
    =
    1
    9F
    F
    16
    =
  • Qstn #38
    A small hole of area of cross-section 2`` mm^2`` is
    present near the bottom of a fully filled open
    tank of height 2 m. Taking g = 10 m/``s^2``, the rate
    of flow of water through the open hole would
    be nearly
    (1) 12.6 × ``10^{-6} m^3/s``
    (2) 8.9 × ``10^{-6} m^3/s``
    (3) 2.23 × ``10^{-6} m^3/s``
    (4) 6.4 × ``10^{-6} m^3/s``
    digAnsr:   1
    Ans : ( 1 )
    Sol.
    2 m
    a = 2 mm
    2
    Rate of flow liquid
    = =Q au a 2gh
    =    6 22 10 m 2 10 2 m/s
    = 2 × 2 × 3.14 × 10-6 m3/s
    = 12.56 × 10-6 m3/s
    = 12.6 × 10-6 m3/s
  • Qstn #39
    m39 png
    The correct Boolean operation represented by
    the circuit diagram drawn is :
    (1) AND
    (2) OR
    (3) NAND
    (4) NOR
    digAnsr:   3
    Ans : ( 3 )
    Sol. From the given logic circuit LED will glow,
    when voltage across LED is high.
    R
    +6 V
    B
    A
    Truth Table
    A
    0
    0
    1
    1
    B
    0
    1
    0
    1
    Y
    1
    1
    1
    0
    This is out put of NAND gate.
  • Qstn #40
    In which of the following processes, heat is
    neither absorbed nor released by a system?
    (1) Isothermal
    (2) Adiabatic
    (3) Isobaric
    (4) Isochoric
    digAnsr:   2
    Ans : ( 2 )
    Sol. In adiabatic process, there is no exchange of
    heat.
  • Qstn #41
    A 800 turn coil of effective area
    0.05 ``m^2 ``is kept perpendicular to a magnetic
    field 5 × ``10^{-5}`` T. When the plane of the coil
    is rotated by 90° around any of its coplanar axis in
    0.1 s, the emf induced in the coil will be:
    (1) 2 V
    (2) 0.2 V
    (3) 2 × ``10^{-3} V
    (4) 0.02 V
    digAnsr:   4
    Ans : ( 4 )
    Sol. Magnetic field B = 5 × 10-5 T
    Number of turns in coil N = 800
    Area of coil A = 0.05 m2
    Time taken to rotate ▵t = 0.1 s
    Initial angle 
    1
    = 0°
    Final angle 
    2
    = 90°
    Change in magnetic flux ▵
    = NBAcos90° - BAcos0°
    = - NBA
    = - 800 × 5 × 10-5 × 0.05
    = - 2 × 10-3 weber
    ▵=

    e -
    t
      = =
    3( )2 10 Wb
    0.02 V
    0.1 s
  • Qstn #42
    The radius of circle, the period of revolution,
    initial position and sense of revolution are
    indicated in the fig.
    m42.png
    y - projection of the radius vector of rotating
    particle P is :
    (1) y(t) = -3 cos2``\pi``t, where y in m
    (2) y(t) = 4 sin``\frac {(\Pi t)}{2}``, where y in m
    (3) y(t) = 3 cos``\frac {(3\Pi t)}{2}``, where y in m
    (4)3 cos``\frac {(\Pi t)}{2}``, where y in m
    digAnsr:   4
    Ans : ( 4 )
    Sol. At t = 0, y displacement is maximum, so
    equation will be cosine function.
    y
    P t = 0
    T = 4 s
    x
    T = 4 s
      ω = = =2 2 rad/s
    T 4 2
    y = a cosωt
    =y 3cos t
    2
    12
  • Qstn #43
    A parallel plate capacitor of capacitance 20 μF
    is being charged by a voltage source whose
    potential is changing at the rate of 3 V/s. The
    conduction current through the connecting
    wires, and the displacement current through
    the plates of the capacitor, would be,
    respectively.
    (1) Zero, 60 μA
    (2) 60 μA, 60 μA
    (3) 60 μA, zero
    (4) Zero, zero
    digAnsr:   2
    Ans : ( 2 )
    Sol. Capacitance of capacitor C = 20 μF
    = 20 × 10-6 F
    Rate of change of potential
      = 
     
    dV
    3 v/s
    dt
    q = CV
    =dq dVC
    dt dt
    i
    c
    = 20 × 10-6 × 3
    = 60 × 10-6 A
    = 60 μA
    As we know that i
    d
    = i
    c
    = 60 μA
  • Qstn #44
    In an experiment, the percentage of error
    occurred in the measurement of physical
    quantities A, B, C and D are 1%, 2%, 3% and 4%
    respectively. Then the maximum percentage of
    error in the measurement X,
    where X =``\frac {A^2B^{\frac12}}{C^{\frac12}D^3}`` will be
    (1) (``\frac{3}{13}``)%
    (2) 16%
    (3) - 10%
    (4) 10%
    digAnsr:   2
    Ans : ( 2 )
    Sol. Given
    =
    1
    2 2
    1
    33
    A B
    x
    C D
    % error,
    ▵ ▵ ▵ =  + x A 1 B100 2 100
    x A 2 B
    ▵ ▵+  + 1 c D100 100 3 100
    3 c D
    =  +  +  + 1 12 1% 2% 3% 3 4%
    2 3
    = + + +2% 1% 1% 12%
    = 16%
  • Qstn #45
    A cylindrical conductor of radius R is carrying
    a constant current. The plot of the magnitude
    of the magnetic field. B with the distance d
    from the centre of the conductor, is correctly
    represented by the figure :
    (1) m45a.png
    (2) m45b.png
    (3) m45c.png
    (4) m45d.png
    digAnsr:   3
    Ans : ( 3 )
    Sol.
    d
    B
    R dO
    Inside (d < R)
    Magnetic field inside conductor
    μ=

    0
    2
    i
    B d
    2 R
    or B = Kd ...(i)
    Straight line passing through origin
    At surface (d = R)
    μ=

    0 i
    B
    2 R
    ...(ii)
    Maximum at surface
    Outside (d > R)
    μ=

    0 i
    B
    2 d
    or  1B (Hyperbolic)
    d13