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QPage 285: Q1: Magnetic Properties of Matter Solutions — HC Verma: Concepts of Physics - Volume 2

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often assume that any induced dipole moment creates a field that opposes the applied one (as in dielectrics), and therefore conclude that the internal magnetic field must also be reduced. In reality, paramagnetic atoms have permanent magnetic moments that align *parallel* to the external field, giving a *positive* magnetic susceptibility; the resulting magnetisation adds to the applied field, so the net field inside is larger. Remember: dielectric polarization always produces a depolarising field, whereas paramagnetic magnetisation reinforces the external field.

When a dielectric is placed in an electric field, it gets polarised. The electric field in a polarised material is less than the applied field. When a paramagnetic substance is kept in a magnetic field, the field in the substance is more than the applied field. Explain the reason of this opposite behaviour.

Step-by-Step Verified Solution

**Answer:** In a dielectric, the induced dipoles align in a direction that creates an internal electric field opposing the applied field, so the net field inside is less than the applied field. In a paramagnetic substance, the atomic magnetic dipoles align parallel to the applied magnetic field, creating an internal magnetic field that adds to the applied field, so the net field inside is greater than the applied field. **Step-by-Step Explanation:** When a dielectric is placed in an electric field, its molecules get polarised. The induced dipole moments are directed along the electric field. However, the field produced by these aligned dipoles inside the material opposes the external field. Hence, the net electric field inside a polarised dielectric is less than the applied field. In a paramagnetic substance, each atom has a permanent magnetic dipole moment. When placed in an external magnetic field, these atomic dipoles tend to align parallel to the field. The magnetic field produced by these aligned dipoles is in the same direction as the applied field. Hence, the net magnetic field inside a paramagnetic substance is greater than the applied field.

All Questions in this Chapter

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