Class 12 NEET Physics Magnetism and Matter Questions

Concept:Torque on a magnetic dipole in an external magnetic field is given by the cross product of magnetic moment and magnetic field.Explanation:1. When a bar magnet (magnetic dipole) is placed in an external magnetic field, it experiences a torque that tends to align it with the field.2. The torque is defined as:$$\vec{\tau} = \vec{M} \times \vec{B}$$where $\vec{M}$ is the magnetic moment and $\vec{B}$ is the magnetic field induction.3. This is a vector cross product, not a dot product:

• Dot product ($\cdot$) gives a scalar (energy, not torque)
• Cross product ($\times$) gives a vector (torque)

4. The magnitude of torque is $\tau = MB\sin\theta$, where $\theta$ is the angle between $\vec{M}$ and $\vec{B}$.5. The direction of torque follows the right-hand rule: it acts perpendicular to both $\vec{M}$ and $\vec{B}$.6. Note: $\vec{M} \times \vec{B} = -\vec{B} \times \vec{M}$ (anti-commutative property), so options A and D are equivalent in magnitude but opposite in sign convention. However, the standard physics convention uses $\vec{\tau} = \vec{M} \times \vec{B}$.Answer:A. $\vec{M} \times \vec{B}$(Option D is mathematically equivalent but uses the non-standard form; Option A is the correct standard expression.)