Animated Solution for Physics - Magnetic Effects of Current: Assertion The sensitivity of a moving coil galvanometer is increased by placing a suitable magnetic material as a core inside the coil. Reason Soft iron has a high magnetic permeability and cannot be easily magnetised or demagnetised.

Imagine you are an engineer tasked with designing an instrument so sensitive that it can detect the faintest whisper of an electric current. The moving coil galvanometer is a masterpiece of classical electromagnetism, but to make it truly exceptional, we need to understand the materials that power it.

Let's start by recalling the fundamental formula for the current sensitivity of a moving coil galvanometer. Current sensitivity ($I_s$) is defined as the deflection produced per unit of current flowing through it:

Here, $N$ is the number of turns in the coil, $A$ is the cross-sectional area, $B$ is the strength of the magnetic field, and $C$ is the restoring torque per unit twist of the suspension wire. To increase the sensitivity, we can either increase the numerator ($N,A,B$) or decrease the denominator ($C$).

When we place a soft iron core inside the coil, something magical happens. Soft iron has an exceptionally high magnetic permeability (${\mu }_r$). This means it acts like a sponge for magnetic field lines, drawing them in and concentrating them in the narrow air gap where the coil rotates.

By concentrating the field lines, the soft iron core significantly increases the radial magnetic field $B$. Looking back at our sensitivity formula, an increase in $B$ directly leads to an increase in the current sensitivity $I_s$. Therefore, the Assertion given in the question is absolutely true.

Now, let's critically analyze the Reason statement. It claims that soft iron has a high magnetic permeability (which we know is true) but adds that it cannot be easily magnetized or demagnetized.

This is where the trap lies! Soft iron is specifically chosen for electromagnetic applications precisely because it has low retentivity and low coercivity. Low coercivity means that it requires very little reverse magnetic field to completely demagnetize it. In other words, soft iron can be very easily magnetized and demagnetized without significant energy loss (hysteresis loss).

Because the Reason statement falsely claims that soft iron cannot be easily magnetized or demagnetized, the Reason is false.

We have successfully deduced that the Assertion is true, but the Reason is false. This perfectly aligns with option (c). Understanding the intrinsic properties of magnetic materials like soft iron is crucial not just for solving JEE problems, but for designing real-world electrical instruments!