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Physics Test - 9
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Physics Test - 9
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  • Question 1/10
    4 / -1

    Directions: From the statements of Assertion and Reason mark the correct answer from the options.

    (a) If both assertion and reason are true, and reason is the correct explanation of assertion.

    (b) If both assertion and reason are true, but reason is not the correct explanation of assertion.

    (c) If assertion is true, but reason is false.

    (d) If both assertion and reason are false.

    Assertion: Diamagnetic materials can exhibit magnetism.

    Reason: Diamagnetic materials have permanent magnetic dipole moment.

    Solutions

    Some materials when placed in an external magnetic field, acquire a very low magnetism in a direction opposite to the field. These materials when brought near the end of a strong magnet, they get repelled. These materials are diamagnetic and this property of materials is called the diamagnetism. This property is generally found in materials in which there are even number of electrons in their atoms (or molecules) and pairs are formed with two electrons. In each pair, the spin of one electron is in direction opposite to the spin of other electron. Hence, the electrons of each pair completely cancel the magnetic moment of each other. Thus, the net magnetic moment of each atom of such material is zero. Hence, option (c) is true.

  • Question 2/10
    4 / -1

    For a certain radioactive substance, it is observed that after 4 hours, only 6.25% of the original sample is left undecayed. Which of the following statements is incorrect?

    Solutions
    6.25%=6.25100=116=1(2)4. Hence 4 hours are
    equal to 4 half lives. There fore, the half life of the
    substance is 1 hour, which is choice (1). The decay constant =ln(2) half life =ln(2) per hour, which is
    choice (3).
     Mean life =1 decay constant =1ln(2) hour =10.693 hour 
    Hence choice (2) is incorrect. After further 4 hours (i.e. after 8 hours), 1(2)8= 1256=100256%=0.39% of the substance remains
    undecayed, which is choice (4). Thus the only incorrect choice is ( 2 ).
  • Question 3/10
    4 / -1

    A police car horn emits sound at a frequency of 240 Hz when the car is at rest. If the speed of sound is 330 m/s, the frequency heard by an observer who is approaching the car at a speed of 11 m/s, is

    Solutions
    Observer
    v0=11m/sv=330m/s Source vs=0π=330+11330×240=248Hz
  • Question 4/10
    4 / -1

    The magnetic susceptibility of a material of a rod is 449. Permeability of vacuum is 4π x 10-7 henry/metre. The absolute permeability of the material of the rod (in henry/metre) is

    Solutions
    Here, χ=μr1
    χ=μμ01
    499=μ4π×1071
    μ=500×4π×107
    =2π×104 henry/metre.
  • Question 5/10
    4 / -1

    A Carnot engine works as a refrigerator between 250 K and 300 K. If it receives 750 calories of heat from the reservoir at the lower temperature, then the amount of heat rejected at the higher temperature is

    Solutions
    As we have, Q1Q2=T1T2Q1Q21=T1T21
    Q1Q2Q2=T1T2T2
    Q1Q2750=300250250=50250
    Q1=50250×750+750=900cal
  • Question 6/10
    4 / -1

    The threshold frequency for a metallic surface corresponds to an energy of 6.2 eV and the stopping potential for a radiation incident on this surface is 5 V. The incident radiation lies in

    Solutions
    From Einstein's photoelectric equation
    or
    hv=hv0+eV0=6.2+5=11.2eVhcλ=11.2eVλ=hc11.2eV=6.6×1034×3.0×10811.2×1.6×1019=1.1049×107=1104.9
    This incident radiation lies in ultra violet region.
  • Question 7/10
    4 / -1

    A long horizontal rod has a bead, which can slide along its length and is initially placed at a distance L from one end A of the rod. The rod is set in angular motion about A with constant angular acceleration α . If the coefficient of friction between the rod and the bead is and gravity is neglected, then the time after which the bead starts slipping is

    Solutions
    The radius of the circular motion of the bead is r=L. The linear acceleration of the
    bead is a =αr=αL
    If m is the mass of the bead, then
    Force acting on the bead =ma=mαL
    Reaction force acting on the bead is R=mαL This bead starts slipping when frictional force between the bead and the rod becomes equal to centrifugal force acting on the bead, i.e.
    μR=mv2r
    or μmαL=mrω2=mLω2(v=rω)
    or μα=ω2=(αt)2(ω=αt)
    or μα=α2t2 or t=μα
    Thus, the correct choice is (1) .
  • Question 8/10
    4 / -1

    Directions: The following question contains 4 choices (A), (B), (C) and (D), out of which ONLY ONE is correct

    In the experiment to determine the speed of sound using a resonance column,

    Solutions

    Correct Answer : prongs of the tuning fork are kept in a vertical plane

  • Question 9/10
    4 / -1

    The figure shows variation of photocurrent with anode potential for a photosensitive surface for three different radiations. Let Ia, Iband Icbe the intensities and va, vband vcbe the frequencies for the curves a, b and c, respectively. Then,

    Solutions
    From the graph, we note that the saturation current is same for curves b and c but different for curve a. Therefore, intensities of b and c will be equal but different from that of a i.e. IaIb but
    Ib=Ic. As stopping potential is same for curves a and b, hence va=vb. Thus answer (a) is
    correct.
  • Question 10/10
    4 / -1

    The bob of a simple pendulum is a spherical hollow ball filled with water. A plugged hole near the bottom of the oscillating bob gets suddenly unplugged. During observation, till water is coming out, the time period of oscillation would

    Solutions

    When water begins to come out, the centre of mass begins to lower. There is an increase in the effective length and hence an increase in the time period : When water flows out completely, the centre of mass begins to shift back to the geometrical centre of the follow bob. So, there is a decrease in the effective length and hence a decrease in time period. When water flows out completely, the centre of mass is again at the geometrical centre. Now, the time period is the same as in the beginning.

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