12th Class Physics Sample Paper Physics Sample Paper-6

  • question_answer
    (i) What do you mean by current sensitivity of a moving coil galvanometer. On what factor does it depend?
    (ii) State two reasons why a galvanometer cannot be used as such to measure high order current in a given circuit?
    (iii) The current sensitivity of a moving coil galvanometer is 5 div/mA and voltage sensitivity is 20 div/volt. Find the resistance of the galvanometer.
    Or
    (i) Write any three characteristics, a ferromagnetic substance should possess, if it is to be used to make a permanent magnet. Give one example of such a material.
    (ii) The susceptibility of a magnetic material is - 0.085. Identify the magnetic type of the material.
    (iii) A Rowland ring of mean radius 15 cm has 3500 turns of wire wound on a ferromagnetic core of relative permeability 800. What is the magnet field (B) in the core for a magnetising current of 1.2 A?

    Answer:

    (i) Current sensitivity of a moving coil galvanometer Current sensitivity of a moving coil galvanometer is defined as the deflection of the coil per unit current flowing through it. Current sensitivity,          \[({{I}_{s}})=\frac{\theta }{I}=\frac{NAB}{C}\]

    Current sensitivity of a moving coil galvanometer depends on
    (a) Number of turns (N) It increases with increase in number of turns in the coil.
    (b) Area of cross-section of coil (A) It increases with increase in area of cross-section of the coil.
    (c) Magnetic field strength (B) It increases with increase in magnetic field strength.
    (d) Torsional rigidity of suspension (C) It increases with decrease of torsional rigidity of suspension.
    (ii) A galvanometer cannot be used as such to measure current due to following two reasons.
    (a) The resistance of galvanometer is small and if high order current passes through it, then it may get damaged.
    (b) A galvanometer is very sensitive device and gives full scale deflection for a current of the order of microampere. Therefore, it cannot measure an electric current of the high order of ampere.
    (iii) Given,          \[{{I}_{s}}=5\,div/mA=5\times {{10}^{3}}div/A,\] \[{{V}_{s}}=20div/V\] We know that resistance,  \[R=\frac{{{I}_{s}}}{{{V}_{s}}}=\frac{5\times {{10}^{3}}}{20}\] [by Ohm's law] \[\Rightarrow \]               \[R=250\,\Omega \] Or
    (i) A ferromagnetic substance should possess the following three characteristics to be used to make a permanent magnet.
    (a) High permeability
    (b) High retentivity
    (c) High coercivity
    For example, Steel and cobalt.
    (ii) As susceptibility of the substance is negative. Therefore, it is a diamagnetic substance. (iii) A rowland ring is a circular ring of a magnetic material over which is wound a torroidal solenoid. The magnitude of the magnetic field in the core is given by \[\because \]       \[B=\frac{\mu \,nl}{2\pi r}\] and       n = 3500 is the number of turns Now relative permeability,            \[{{\mu }_{r}}=\frac{\mu }{{{\mu }_{0}}}\] \[\Rightarrow \]   \[\mu ={{\mu }_{0}}{{\mu }_{r}}=4\pi \times {{10}^{-7}}\times 800\] \[\therefore \] \[B=4\pi \times {{10}^{-7}}\times 800\times \frac{3500\times 1.2}{2\times 3.14\times 15\times {{10}^{-2}}}\]                                     [given r = 15 cm] \[\Rightarrow \]   B = 4.48 T


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