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 parallel 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 parallel to the magnetic field.
Reason (R) : In a moving coil galvanometer, the plane of the coil is always parallel to the magnetic field.
Show Hint
A radial magnetic field ensures that the deflecting torque remains constant and independent of the rotation angle (\(\theta = 90^\circ\) relative to the area normal), which creates a linear scale on the galvanometer display (\(I \propto \alpha\)). The soft iron core serves a dual purpose: it makes the field radial and boosts the field strength.
Updated On: Jul 2, 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.
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The Correct Option is D
Solution and Explanation
Concept:
A moving coil galvanometer measures small electric currents. It works on the principle that a current-carrying loop placed in an external magnetic field experiences a deflecting magnetic torque.
• Deflecting Torque Formula: The magnetic torque \(\tau\) acting on a coil of \(N\) turns, area \(A\), carrying current \(I\) in a field \(B\) is given by: \[ \tau = NIAB\sin\theta \] where \(\theta\) is the angle between the normal to the area of the coil (\(\vec{A}\)) and the magnetic field vector (\(\vec{B}\)).
• Role of the Soft Iron Core: Cylindrical pole pieces combined with a central cylindrical soft iron core are used to create a radial magnetic field. Soft iron has a high magnetic permeability (\(\mu_r \gg 1\)), which concentrates the magnetic field lines.
Step 1: Analyzing the Assertion (A).
The Assertion states that the soft iron core only shapes the field into a radial pattern and does not alter its intensity. Soft iron is a ferromagnetic material with high magnetic permeability, meaning it readily conducts magnetic field lines. Placing a soft iron core inside the galvanometer layout concentrates the surrounding magnetic flux lines into the core area. This increases the field intensity (\(B\)) inside the air gap, making the galvanometer far more sensitive. Since the core significantly enhances the strength of the magnetic field, the Assertion statement is completely false.
Step 2: Analyzing the Reason (R).
The Reason states that the plane of the coil is always perpendicular to the magnetic field. In a moving coil galvanometer, the cylindrical pole pieces and soft iron core produce a radial magnetic field. In a radial field configuration, the magnetic field lines run parallel to the plane of the coil for all angular orientations of the coil. This keeps the angle between the field lines and the normal to the coil's plane at \(\theta = 90^\circ\) at all times (\(\sin 90^\circ = 1\)), maximizing the torque (\(\tau = NIAB\)). Since the field lines lie parallel to the plane of the coil (making them perpendicular to the coil's normal vector), the Reason statement is also false. Since both individual statements are false, this matches Option (D).
• Deflecting Torque Formula: The magnetic torque \(\tau\) acting on a coil of \(N\) turns, area \(A\), carrying current \(I\) in a field \(B\) is given by: \[ \tau = NIAB\sin\theta \] where \(\theta\) is the angle between the normal to the area of the coil (\(\vec{A}\)) and the magnetic field vector (\(\vec{B}\)).
• Role of the Soft Iron Core: Cylindrical pole pieces combined with a central cylindrical soft iron core are used to create a radial magnetic field. Soft iron has a high magnetic permeability (\(\mu_r \gg 1\)), which concentrates the magnetic field lines.
Step 1: Analyzing the Assertion (A).
The Assertion states that the soft iron core only shapes the field into a radial pattern and does not alter its intensity. Soft iron is a ferromagnetic material with high magnetic permeability, meaning it readily conducts magnetic field lines. Placing a soft iron core inside the galvanometer layout concentrates the surrounding magnetic flux lines into the core area. This increases the field intensity (\(B\)) inside the air gap, making the galvanometer far more sensitive. Since the core significantly enhances the strength of the magnetic field, the Assertion statement is completely false.
Step 2: Analyzing the Reason (R).
The Reason states that the plane of the coil is always perpendicular to the magnetic field. In a moving coil galvanometer, the cylindrical pole pieces and soft iron core produce a radial magnetic field. In a radial field configuration, the magnetic field lines run parallel to the plane of the coil for all angular orientations of the coil. This keeps the angle between the field lines and the normal to the coil's plane at \(\theta = 90^\circ\) at all times (\(\sin 90^\circ = 1\)), maximizing the torque (\(\tau = NIAB\)). Since the field lines lie parallel to the plane of the coil (making them perpendicular to the coil's normal vector), the Reason statement is also false. Since both individual statements are false, this matches Option (D).
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Reason (R): The coil of a galvanometer is suspended in a uniform radial magnetic field.
- A galvanometer can be converted into an ammeter of desired range by connecting a:
Galvanometer:
A galvanometer is an instrument used to show the direction and strength of the current passing through it. In a galvanometer, a coil placed in a magnetic field experiences a torque and hence gets deflected when a current passes through it.
The name "galvanometer" is derived from the surname of Italian scientist Luigi Galvani, who in 1791 discovered that electric current makes a dead frog’s leg jerk.
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