Page 37 - Physics - XII
P. 37

Q14.  What do you mean by fi gure of merit of a galvanometer.
          Ans.  The amount of current required to produce a defl ection of one division in the galvanometer is called
                 fi gure of merit of a galvanometer.

           Q15.  How is an ammeter used in a circuit?
          Ans.  An ammeter is always connected in series in an electric circuit.

           Q16.  How will you use a galvanometer for measuring current?                                     (NCERT)
          Ans.  In order to measure current of higher order, a very low resistance known as “shunt” resistance is
                 connected in parallel to galvanometer, so that most of the current passes through the shunt. In this
                 way, a galvanometer is converted into ammeter.
           Q17.  (a)   Out of galvanometer, ammeter and voltmeter which has the highest resistance and which

                      has the lowest? Explain.
                 (b)  Which of the two meters has lower resistance – a milliammeter or a microammeter?
                                                                                                            (NCERT)
          Ans.  (a)   The voltmeter has the highest resistance so that when we connect it in parallel with a resistance,
                     negligible current passes through it. The ammeter has the lowest resistance so that when we
                     connect it in series in a circuit, it should not reduce much of the original current to be measured.
                 (b)   A milliammeter has current range more than that of microammeter. So, a milliammeter has
                     lower resistance than a microammeter.

           Q18.  What are the factors on which sensitivity of a galvanometer depends?                       (NCERT)
          Ans.  The sensitivity of a galvanometer depends on the following factors:
                   (i)  Number of turns in the coil (N)
                  (ii)  Magnetic fi eld (B)
                 (iii)  Area of coil (A)
                  (iv)  Torsional constant of springs

           Q19.  Should an ideal voltmeter have infi nite resistance?
          Ans.  Yes, a voltmeter must have infi nite resistance, but practically this is not possible.
           Q20.  What is meant by range of a voltmeter?
          Ans.  It is a maximum value of the potential diff erence which can be measured by it.

           Q21.  Internal resistance of the cell is taken to be zero. This implies that we have to use a freshly
                 charged accumulator in the experiment or use a good battery eliminator. If the internal
                 resistance is fi nite, how will it aff ect the result?                                       (NCERT)
          Ans.  The fi gure of merit of galvanometer varies directly as the e.m.f. of the battery. Hence, in case of
                 fi nite internal resistance of a battery, the fi gure of merit decreases.

           Q22.  Is it possible to fi nd the galvanometer resistance by taking 1/3 defl ection? If so, what changes
                 would be required in the formula for calculation of value of G.                            (NCERT)
          Ans.  Yes, 1/3 defl ection method can be used and in this case, the G is equal to twice the value of shunt
                 resistance (s) as:
                              I /I = S/(G + S) = 1/3
                               g
                 ⇒         G + S = 3S

                 ⇒             G = 2S


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