CBSE 2025, Set 3

Question

(a) (i) With the help of a labelled diagram, explain the principle of working of a moving coil galvanometer. Write the purpose of using (1) radial magnetic field, and (2) soft iron core, in it.
(ii) Define current sensitivity of a galvanometer. "Increasing the current sensitivity may not necessarily increase the voltage sensitivity." Give reason.

Edvaya · Accepted Answer

The correct answer is Subjective. **(i) Moving Coil Galvanometer:**

![Moving Coil Galvanometer](https://www.edvaya.com/images/cbse/physics/2025/set3/q31a-moving-coil-galvanometer-answer.webp)

**Principle:** A current-carrying coil placed in a uniform magnetic field experiences a torque. This deflecting torque is proportional to the current flowing through the coil.
When a current $I$ flows through a coil of $N$ turns, area $A$, in a magnetic field $B$, the deflecting torque is $	au = NIAB \sin 	heta$. 
This torque rotates the coil until it is balanced by the restoring torque $k\phi$ of the suspension spring, where $k$ is the torsional constant and $\phi$ is the angle of twist.
At equilibrium: $NIAB = k\phi \Rightarrow \phi = \left(\frac{NAB}{k}ight)I$

**Purpose of Specific Components:**
1. **Radial Magnetic Field:** It ensures that the plane of the coil is always parallel to the magnetic field ($	heta = 90^\circ$ and $\sin 	heta = 1$) irrespective of its position. This provides maximum torque and makes the scale **linear** ($I \propto \phi$).
2. **Soft Iron Core:** It is highly permeable, so it concentrates the magnetic field lines, increasing the strength of the magnetic field $B$. This increases the sensitivity of the galvanometer.

**(ii) Current and Voltage Sensitivity:**
* **Current Sensitivity ($I_s$):** It is defined as the deflection produced per unit current flowing through the galvanometer.
 $$I_s = \frac{\phi}{I} = \frac{NAB}{k}$$
* **Reasoning:** Voltage sensitivity ($V_s$) is the deflection per unit voltage: $V_s = \frac{\phi}{V} = \frac{\phi}{IR} = \frac{I_s}{R}$, where $R$ is the resistance of the galvanometer.
 If we increase current sensitivity by increasing the number of turns $N$, the length of the wire increases, which proportionally increases the resistance $R$ of the coil. 
 Since $I_s$ and $R$ increase in the same proportion, their ratio ($V_s$) remains unchanged. Thus, increasing current sensitivity does not necessarily increase voltage sensitivity.

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Physics > Magnetism Induction And Alternating Current > Moving Charges And Magnetism

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