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Chapter 05: Magnetism and Matter

Master Magnetism and Matter with NCERT solutions for magnetic materials, Earth's magnetism, magnetic properties, and hysteresis.

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Quick Revision: Electric Charges and Field

  • Magnetic Dipole: Fundamental magnetic entity with North and South poles; cannot be isolated.
  • Magnetic Dipole Moment (m): m = pole strength × length; vector from S to N pole.
  • Torque on Magnetic Dipole: τ = m × B = mB sinθ; aligns dipole with magnetic field.
  • Potential Energy: U = -m·B = -mB cosθ; minimum when aligned with field.
  • Magnetic Field Lines: Continuous closed curves from N to S pole outside magnet.
  • Bar Magnet: Equivalent to solenoid; field similar to electric dipole field.
  • Magnetic Field at Axial Point: B = (μ₀/4π) × (2m/r³); along dipole axis.
  • Magnetic Field at Equatorial Point: B = (μ₀/4π) × (m/r³); opposite to dipole moment.
  • Gauss's Law for Magnetism: ∮B·dA = 0; no magnetic monopoles exist.
  • Earth's Magnetic Field: Described by declination, dip, and horizontal component.
  • Magnetic Declination: Angle between geographic and magnetic meridian.
  • Magnetic Inclination/Dip: Angle between Earth's field and horizontal plane.
  • Magnetic Elements: B_H, B_V, δ, θ completely specify Earth's magnetic field.
  • Magnetic Materials: Diamagnetic, paramagnetic, ferromagnetic based on susceptibility.
  • Diamagnetic Materials: Weakly repelled; χ small and negative (∼10⁻⁵).
  • Paramagnetic Materials: Weakly attracted; χ small and positive (∼10⁻³ to 10⁻⁵).
  • Ferromagnetic Materials: Strongly attracted; χ large and positive (∼10³ to 10⁵).
  • Magnetic Susceptibility (χ): M = χH; measures how material responds to field.
  • Relative Permeability (μ_r): μ_r = 1 + χ; ratio of permeability to μ₀.
  • Hysteresis: Lag of magnetization behind magnetizing field; energy loss as heat.
  • Curie Temperature: Critical temperature above which ferromagnetic becomes paramagnetic.
  • Permanent Magnets: Made from steel (high retentivity, high coercivity).
  • Electromagnets: Made from soft iron (high permeability, low retentivity).

Chapter Summary:

Magnetism and Matter delves into the fascinating world of magnetic materials and their interactions with magnetic fields, extending beyond the basic principles of current-induced magnetism. The chapter beautifully connects the behavior of magnetic dipoles with their electric counterparts, revealing the underlying symmetry in physical laws while highlighting the crucial differences - most notably, the absence of magnetic monopoles.

We explore the Earth itself as a giant magnet, understanding how navigators have used magnetic compasses for centuries by accounting for declination and dip angles. The chapter takes us through the intriguing properties of different magnetic materials - from the subtle repulsion of diamagnetic substances to the powerful attraction of ferromagnetic materials that enable modern technologies.

The concept of magnetic susceptibility helps us quantify how materials respond to external magnetic fields, while hysteresis curves reveal the memory-like behavior of ferromagnetic materials. Practical applications abound, from the design of permanent magnets that retain their magnetism to electromagnets that can be switched on and off at will. The temperature-dependent behavior of magnetic materials, characterized by Curie temperature, shows how heat can fundamentally alter magnetic properties.

This chapter transforms our understanding of magnetism from abstract field lines to tangible material properties that shape everything from compass navigation to data storage in modern computers, beautifully illustrating how matter and magnetism intertwine in our physical world.