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Physics Test - 71

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Physics Test - 71
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  • Question 1
    1 / -0
    A circular loop of radius \(R\), carrying current \(I\), lies in the \(X-Y\) plane with its center at the origin. The total magnetic flux through the \(X-Y\) plane is:
    Solution

    Since here total flux entering equals the total flux leaving and hence net flux equals zero.

  • Question 2
    1 / -0
    What should be the angular speed with which the earth have to rotate on its axis so that a person on the equator would weight \(\frac{3}{5}\)th as much as present?
    Solution

    Actual weight on the equator \(=W=mg\)

    where \('m'\) is the mass and \('g'\) is the gravity.

    According to the given condition,

    Weight on the equator \(=W^{\prime}=mg^{\prime}\)

    Weight on the equator \(=W^{\prime}=\frac{3}{5} mg \ldots \ldots \ldots .\) \((i)\)

    We know that, \(\lambda=0\) at the equator.

    \(mg^{\prime}=mg-m R \omega^{2} \cos \lambda\)

    \(\Rightarrow \frac{3}{5} mg=mg-m R \omega^{2} \cos 0(\because \lambda=0)\)

    \(\Rightarrow \frac{3}{5} mg=mg-m R \omega^{2}(1)(\because \cos 0=1)\)

    \(\Rightarrow \frac{3}{5} mg=mg-m R \omega^{2}\)

    \(\Rightarrow m R \omega^{2}=mg-\frac{3}{5} mg\)

    \(\Rightarrow m R \omega^{2}=(1-\frac{3}{5}) mg\)

    \(\Rightarrow mR \omega^{2}=\frac{2}{5} mg\)

    \(\Rightarrow R \omega^{2}=\frac{2}{5} g\)

    \(\Rightarrow \omega^{2}=\frac{2 g}{5 R}\)

    \(\Rightarrow \omega^{2}=\frac{(2 × 10)}{\left(5 × 6.4 × 10^{5}\right)}\)

    \( \because R=\) Radius of Earth \(=6400 ~km=\)\(\left.6.4 × 10^{6} ~m\right)\)

    \(\Rightarrow \omega=\sqrt{\left(6.25 × 10^{-7}\right)}\)

    \(\Rightarrow \omega=7.8 × 10^{-4} ~rad / sec\)

  • Question 3
    1 / -0

    If \(Y, K\) and \(\eta\) are the values of Young's modulus, bulk modulus and modulus of rigidity of any material respectively. Choose the correct relation for these parameters.

    Solution

    We know that \(Y=3 K(1-2 \sigma)\)

    Here,

    \(\sigma=\) Poisson's ratio

    \(\sigma=\frac{1}{2}\left(1-\frac{Y}{3 K}\right)\)

    Also,

    \(Y=2 \eta(1+\sigma) \)

    \( \sigma=\frac{Y}{2 \eta}-1\)

    From equation (i) and (ii), we have

    \( \frac{1}{2}\left(1-\frac{Y}{3 K}\right)=\frac{Y}{2 \eta}-1 \)

    \( \Rightarrow 1-\frac{Y}{3 K}=\frac{Y}{\eta}-2 \Rightarrow \frac{Y}{3 K}=3-\frac{Y}{\eta} \)

    \( \Rightarrow \frac{Y}{\eta-Y} \)

    \( \Rightarrow \frac{\eta Y}{3 K}=3 \eta-Y\)

    \(\Rightarrow K=\frac{\eta Y}{9 \eta-3 Y}\)

  • Question 4
    1 / -0

    On interchanging the resistances, the balance point of a meter bridge shifts to the left by 10 cm. The resistance of their series combination is 1kΩ. How much was the resistance on the left slot before interchanging the resistances?

    Solution

    \(\text{R}_1+\text{R}_2= 1000\)

    \(\Rightarrow \text{R}_2 = 1000 - \text{R}_1\)

    On balancing condition

    \(\text{R}_1 (100-1) = (1000 - \text{R}_1)1 ......\text{(i)}\)

    On Interchanging resistance balance point shifts left by 10cm

    On balancing condition

    \(\left(1000-R_1\right)(110-1)=R_1(1-10) \)

    \(\text { or, } R_1(1-10)=\left(1000-R_1\right)(110-1).......\text{(ii)}\)

    Dividing eqn (i) by (ii)

    \(\frac{100-1}{1-10}=\frac{1}{110-1} \)

    \(\Rightarrow(100-1)(110-1)=1(1-10) \)

    \(\Rightarrow 11000-1001-1101+1^2=1^2-101 \)

    \(\Rightarrow 11000=2001\)

    or, \(1=55\)

    Putting the value of ' 1 ' in eqn (i)

    \(\mathrm{R}_1(100-55)=\left(1000-\mathrm{R}_1\right) 55 \)

    \(\Rightarrow \mathrm{R}_1(45)=\left(1000-\mathrm{R}_1\right) 55 \)

    \(\Rightarrow \mathrm{R}_1(9)=\left(1000-\mathrm{R}_1\right) 11 \)

    \(\Rightarrow 20 \mathrm{R}_1=11000 \)

    \(\therefore \mathrm{R}_1=550 \mathrm \Omega\)

  • Question 5
    1 / -0

    Consider the circuit shown in the above figure. With switch \({S}_{1}\) closed and the other two switches open, the circuit has a time constant \(\tau_{{C}} .\) With switch \({S}_{2}\) closed and the other two switches open, the circuit has a time constant \(\tau_{{L}}\). With switch \({S}_{3}\) closed and the other two switches open, the circuit oscillates with a period \({T}\). Find the \(t\)

    Solution
    When switch \({S}_{1}\) is closed and the others are open, the inductor is essentially out of the circuit and what
    remains is an RC circuit.
    The time constant is \(\tau_{{C}}={R C} .\) When switch \({S}_{2}\) is closed and the others are open, the capacitor is
    essentially out of the circuit. In this case, what we have is an LR circuit with time constant \(\tau_{{L}}={L} / {R}\).
    Finally, when switch \({S}_{3}\) is closed and the others are open, the resistor is essentially out of the circuit and what remains is an \({LC}\) circuit that oscillates with period \({T}=2 \pi \sqrt{{L} C} .\) Substituting \({L}={R} \tau_{{L}}\) and \({C}=\)
    \(\tau_{{C}} / {R},\) we obtain, \({T}=2 \pi \sqrt{\tau_{{C}} \tau_{{L}}}\)
  • Question 6
    1 / -0

    What is the effect on the interference fringes in Young's double slit experiment if the width of the source slit is increased?

    Solution

    When the widths of the two slits are increased, the fringes become brighter. However, the width of each slit should be considerably smaller than the separation between the slits. When the slits become so wide that this condition is not satisfied, the interference pattern disappears i.e., The fringes become less distinct.

  • Question 7
    1 / -0

    A laser device produces amplification in the:

    Solution

    A laser device produces amplification in the Ultraviolet or visible region.

    Laser, a device that stimulates atoms or molecules to emit light at particular wavelengths and amplifies that light, typically producing a very narrow beam of radiation. The emission generally covers an extremely limited range of visible, infrared, or ultraviolet wavelengths.

  • Question 8
    1 / -0

    Which of the following process is used to do maximum work done on the ideal gas if the gas is compressed to half of its initial volume?

    Solution

    The \(P-V\) diagram of adiabatic process, isothermal process and isobaric process. 

    Work done in process= area enclosed by \(P-V\) diagram with volume axis.Since area under the curve is maximum for adiabatic process, so work done on the gas will be maximum for adiabatic process.

  • Question 9
    1 / -0

    The universal constant of gravitation _________.

    Solution

    The universal constant of gravity is not one that depends on the nature of the medium in which the bodies are placed.

    The gravitational constant can be defined as the constant relating the force exerted on the objects to the mass and distance between the objects. The value of the universal gravitation constant is found to be \(\mathrm{G}=6.673 \times 10^{-11} \mathrm{Nm}^{2} / \mathrm{kg}^{2}\).

  • Question 10
    1 / -0

    A spherical surface of radius of curvature \(R\) separates air (refractive index \(1.0\)) from glass (refractive index \(1.5\)). The centre of curvature is in the glass. A point object \(P\) placed in air is found to have a real image \(Q\) in the glass. The line \(PQ\) cuts the surface at point \(O\) and \(PO\) = \(OQ\). The distance \(PO\) is equal to

    Solution
    Given, \(PO=OQ\)
    \(\Rightarrow u=v\)
    We know
    \(\frac{\mu_{2}}{v}-\frac{\mu_{1}}{u}=\frac{\mu_{2}-\mu_{1}}{R}\)
    \( \Rightarrow \frac{\mu_{2}}{v}-\frac{\mu_{1}}{u}=\frac{\mu_{2}-\mu_{1}}{R} \)
    \( \Rightarrow \frac{\mu_{2}+\mu_{1}}{~V}=\frac{\mu_{2}-\mu_{1}}{R}\)
    \( \Rightarrow \frac{1.5+1}{v}=\frac{1.5-1}{R} \)
    \( \Rightarrow \frac{2.5}{v}=\frac{0.5}{R} \)
    \( \Rightarrow v=\frac{2.5 R}{0.5}\)
    \(v=5 R\)
    \( OP=OQ=u=v=5R\)
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