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Moving Charges and Magnetism — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsMoving Charges and Magnetism4 Marks
Question
Moving coil galvanometer operates on Permanent Magnet Moving Coil (PMMC) mechanism and was designed by the scientist D'arsonval. Moving coil galvanometers are of two types.
  1. Suspended coil.
  2. Pivoted coil type or tangent galvanometer.
Its working is based on the fact that when a current carrying coil is placed in a magnetic field, it experiences a torque. This torque tends to rotate the coil about its axis of suspension in such a way that the magnetic flux passing through the coil is maximum.
  1. A moving coil galvanometer is an instrument which:
  1. Is used to measure emf.
  2. Is used to measure potential difference.
  3. Is used to measure resistance.
  4. Is a deflection instrument which gives a deflection when a current flows through its coil.
  1. To make the field radial in a moving coil galvanometer.
  1. Number of turns of coil is kept small.
  2. Magnet is taken in the form of horse-shoe.
  3. Poles are of very strong magnets.
  4. Poles are cylindrically cut.
  1. The deflection in a moving coil galvanometer is:
  1. Directly proportional to torsional constant of spring.
  2. Directly proportional to the number of turns in the coil.
  3. Inversely proportional to the area of the coil.
  4. Inversely proportional to the current in the coil.
  1. In a moving coil galvanometer, having a coil of N-turns of area A and carrying current I is placed in a radial field of strength B.
The torque acting on the coil is:
  1. $NA^2B^2I$
  2. $NABI^2$
  3. $N^2ABI$
  4. $NABI$
  1. To increase the current sensitivity of a moving coil galvanometer, we should decrease:
  1. Strength of magnet.
  2. Torsional constant of spring.
  3. Number of turns in coil.
  4. Area of coil.
✓

Answer

  1. (d) Is a deflection instrument which gives a deflection when a current flows through its coil.
Explanation:
A moving coil galvanometer is a sensitive instrument which is used to measure a deflection when a current flows th rough its coil.
  1. (d) Poles are cylindrically cut.
Explanation:
Uniform field is made radial by cutting pole pieces cylindrically.
  1. (b) Directly proportional to the number of turns in the coil.
Explanation:
The deflection in a moving coil galvanometer, $\phi=\frac{\text{NAB}}{\text{K}}.\text{I}$ or $\phi\propto\text{N,}$ N, where N is number of turns in a oil, Bis magnetic field and A is area of cross-section.
  1. (d) NABI
Explanation:
The deflecting torque acting on the coil. $\tau_{\text{deflection}}=\text{NIAB}$
  1. (b) Torsional constant of spring.
Explanation:
Current sensitivity of galvanometer,
$\frac{\phi}{\text{I}}=\text{S}_\text{i}=\frac{\text{NBA}}{\text{K}}$
Hence, to increase (current sensitivity) S;, (torsional constant of spring) k must be decrease.

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