Physics Q12 – NEET-UG 2025

Home Physics Magnetism and MatterQuestion #241

A 2 amp current is flowing through two different small circular copper coils having radii ratio

1 : 2

. The ratio of their respective magnetic moments will be

Clue 1 of 3: Magnetic Moment Definition

The magnetic moment of a current loop is a measure of its strength as a magnetic source.

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● Video Walkthrough

Step 1: Recall the formula for magnetic moment✦ Active

The magnetic moment ( $M$ ) of a circular current loop is given by the formula:

M = N I A

Where $N$ is the number of turns, $I$ is the current, and $A$ is the area of the coil. For a circular coil, the area $A = π r^{2}$ , where $r$ is the radius.

M = N I (π r^{2})

Step 2: Determine the proportionality of magnetic moment to radius○ Expand

The problem states that a 2 amp current is flowing through both coils, meaning the current $I$ is the same for both. Assuming the number of turns $N$ is also the same (as it's not specified to be different), and $π$ is a constant, the magnetic moment $M$ is directly proportional to the square of the radius $r$ .

M \propto r^{2}

💡 Teacher's Secret Hint

Always identify constant and variable parameters in ratio problems.

Step 3: Calculate the ratio of magnetic moments○ Expand

Given the radii ratio $r_{1} : r_{2} = 1 : 2$ . The ratio of their magnetic moments $M_{1} : M_{2}$ will be:

\frac{M_{1}}{M_{2}} = \frac{r_{1}^{2}}{r_{2}^{2}} = {(\frac{r_{1}}{r_{2}})}^{2}

Substituting the given ratio:

\frac{M_{1}}{M_{2}} = {(\frac{1}{2})}^{2} = \frac{1}{4}

Thus, the ratio of their respective magnetic moments is $1 : 4$ .

💡 Teacher's Secret Hint

Pay attention to whether the proportionality is linear, square, or inverse.

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Physics Question 12 – NEET-UG 2025

Clue 1 of 3: Magnetic Moment Definition

🧠 Full Solution Path