Question:
Assertion (A) : The cylindrical soft iron core in a moving coil galvanometer only makes the magnetic field radial and does not affect the strength of the magnetic field.
Reason (R) : In a moving coil galvanometer, the plane of the coil is always perpendicular to the magnetic field.
Assertion (A) : The cylindrical soft iron core in a moving coil galvanometer only makes the magnetic field radial and does not affect the strength of the magnetic field.
Reason (R) : In a moving coil galvanometer, the plane of the coil is always perpendicular to the magnetic field.
Reason (R) : In a moving coil galvanometer, the plane of the coil is always perpendicular to the magnetic field.
Show Hint
Read statements about angles in electromagnetism extremely carefully. Maximum torque occurs when the Area Vector is PERPENDICULAR to the field, which means the Coil Plane is PARALLEL to the field.
Also, inserting a ferromagnetic material like soft iron always INCREASES the magnetic field strength due to high permeability.
Updated On: Sep 14, 2026
- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Both Assertion (A) and Reason (R) are false.
Show Solution
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The Correct Option is D
Solution and Explanation
Concept:
• A Moving Coil Galvanometer (MCG) operates on the principle that a current-carrying coil placed in a magnetic field experiences a deflecting torque.
• The magnitude of this torque is given by \( \tau = NIAB \sin(\theta) \), where \( \theta \) is the angle between the magnetic field vector and the area vector (the normal to the coil's plane).
• To ensure linear operation and maximum sensitivity, the torque must always be maximum. This requires \( \sin(\theta) = 1 \), meaning the area vector must always be perpendicular to the field lines (\( \theta = 90^\circ \)).
• Consequently, the plane of the coil itself must remain parallel to the magnetic field lines at all times.
• A radial magnetic field is used to achieve this parallel alignment regardless of the coil's rotation.
• A soft iron core is placed centrally because it is a highly permeable ferromagnetic material. It concentrates the magnetic flux lines, which significantly increases the overall magnetic field strength \( B \) in the air gap.
Step 1: Analyze the Assertion (A)
Assertion (A) claims that the soft iron core "only makes the magnetic field radial and does not affect the strength". While its cylindrical shape (along with concave pole pieces) helps create the radial field, the second part of the statement is entirely false. Soft iron is chosen specifically for its extremely high magnetic permeability (\( \mu_r \gg 1 \)). This property allows it to pull in and concentrate the magnetic flux lines, dramatically increasing the magnetic field strength (\( B \)) in the gap where the coil rotates. Because it heavily affects and increases the field strength, Assertion (A) is False.
Step 2: Analyze the Reason (R)
Reason (R) states that the plane of the coil is always perpendicular to the magnetic field.
As outlined in the concepts, for the torque to be maximum, the angle between the normal (area vector) and the field must be \( 90^\circ \).
If the normal is perpendicular to the field, it logically means the plane of the coil is parallel to the magnetic field lines. The entire engineering purpose of the radial magnetic field is to ensure that the plane of the coil is always parallel to the field lines as it rotates, not perpendicular.
Therefore, Reason (R) is fundamentally incorrect and False.
Step 3: Conclusion
Both the Assertion (A) and the Reason (R) contain clear factual errors regarding the construction and operating principles of a moving coil galvanometer. This leads us to select option (D).
• A Moving Coil Galvanometer (MCG) operates on the principle that a current-carrying coil placed in a magnetic field experiences a deflecting torque.
• The magnitude of this torque is given by \( \tau = NIAB \sin(\theta) \), where \( \theta \) is the angle between the magnetic field vector and the area vector (the normal to the coil's plane).
• To ensure linear operation and maximum sensitivity, the torque must always be maximum. This requires \( \sin(\theta) = 1 \), meaning the area vector must always be perpendicular to the field lines (\( \theta = 90^\circ \)).
• Consequently, the plane of the coil itself must remain parallel to the magnetic field lines at all times.
• A radial magnetic field is used to achieve this parallel alignment regardless of the coil's rotation.
• A soft iron core is placed centrally because it is a highly permeable ferromagnetic material. It concentrates the magnetic flux lines, which significantly increases the overall magnetic field strength \( B \) in the air gap.
Step 1: Analyze the Assertion (A)
Assertion (A) claims that the soft iron core "only makes the magnetic field radial and does not affect the strength". While its cylindrical shape (along with concave pole pieces) helps create the radial field, the second part of the statement is entirely false. Soft iron is chosen specifically for its extremely high magnetic permeability (\( \mu_r \gg 1 \)). This property allows it to pull in and concentrate the magnetic flux lines, dramatically increasing the magnetic field strength (\( B \)) in the gap where the coil rotates. Because it heavily affects and increases the field strength, Assertion (A) is False.
Step 2: Analyze the Reason (R)
Reason (R) states that the plane of the coil is always perpendicular to the magnetic field.
As outlined in the concepts, for the torque to be maximum, the angle between the normal (area vector) and the field must be \( 90^\circ \).
If the normal is perpendicular to the field, it logically means the plane of the coil is parallel to the magnetic field lines. The entire engineering purpose of the radial magnetic field is to ensure that the plane of the coil is always parallel to the field lines as it rotates, not perpendicular.
Therefore, Reason (R) is fundamentally incorrect and False.
Step 3: Conclusion
Both the Assertion (A) and the Reason (R) contain clear factual errors regarding the construction and operating principles of a moving coil galvanometer. This leads us to select option (D).
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