Moving Coil Galvanometer, Class XII Physics, Electromagnetism (Important Questions)

Описание: 1. What is a radial magnetic field? What is its importance in moving coil galvanometer?
Radial magnetic field, as the name suggests, is a field that acts radially. It keeps the magnetic field always parallel to the plane of the rectangular coil at all positions. In other words, the magnetic field remains perpendicular to the area vector (normal) of the coil throughout its motion, which means the angle between the magnetic field and the normal is always 90°. Because of this, the torque acting on the coil (which depends on the number of turns, magnetic field, current, and area of the coil) remains maximum and does not change with the angle of deflection α. Since the torque does not depend on α, the deflection becomes directly proportional to the current. Hence, the scale of the galvanometer is linear and uniform.

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2. Of what material is the spring, with which the coil of a moving coil galvanometer is suspended, made? Why?
The suspension spring of a moving coil galvanometer is made of phosphor bronze. This material is used because it is highly elastic, has a very low torsional constant, and does not lose its elasticity easily. It is also non-magnetic, so it does not disturb the magnetic field inside the galvanometer. Due to its low torsional constant, even a small current can produce a noticeable deflection, making the instrument highly sensitive.

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3. The coil of a moving coil galvanometer is wound on a metallic frame. What is the function of the metallic frame?
The coil of a moving coil galvanometer is wound on a light metallic frame, usually made of aluminium. The metallic frame provides mechanical support to the coil and maintains its shape. More importantly, when the coil moves in the magnetic field, eddy currents are induced in the metallic frame. These eddy currents oppose the motion of the coil and provide electromagnetic damping. This damping helps the pointer to come to rest quickly without oscillating, thereby ensuring accurate and steady readings.

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4. Increasing the current sensitivity of a moving coil galvanometer may not necessarily increase its voltage sensitivity. Why?
Current sensitivity is defined as the deflection produced per unit current and depends on factors such as the number of turns, magnetic field strength, area of the coil, and torsional constant. Voltage sensitivity, however, is the deflection produced per unit voltage and depends not only on current sensitivity but also on the resistance of the coil. When the current sensitivity is increased by increasing the number of turns of the coil, the resistance of the coil also increases because the length of the wire increases. Since voltage sensitivity is inversely proportional to resistance, an increase in resistance may offset the increase in current sensitivity. Therefore, increasing current sensitivity does not necessarily increase voltage sensitivity.


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