Magnetism And Matter Notes

Introduction

Magnetism is a fundamental force of nature that arises due to the motion of electric charges. The study of magnetism and its interaction with matter is crucial in understanding various physical phenomena and is a significant part of the NEET Physics syllabus. This document will break down the complex ideas of magnetism and matter into smaller, digestible sections, providing clear explanations and examples.

Magnetic Properties of Materials

Types of Magnetic Materials

Magnetic materials can be broadly classified into three categories based on their magnetic properties:

1. Diamagnetic Materials
2. Paramagnetic Materials
3. Ferromagnetic Materials

Diamagnetic Materials

Diamagnetic materials are those that create an induced magnetic field in a direction opposite to an externally applied magnetic field, causing a repulsive effect.

• Characteristics:
  • Weakly repelled by a magnetic field.
  • No permanent magnetic moment.
  • Examples: Bismuth, Copper, Gold.
• Explanation: When a magnetic field is applied, the orbital motion of electrons in atoms of diamagnetic materials changes in such a way that an induced magnetic field is created in the opposite direction.

$$ \chi_m

< 0 $$

Paramagnetic Materials

Paramagnetic materials are those that are weakly attracted by a magnetic field and form an induced magnetic field in the same direction as the applied magnetic field.

• Characteristics:
  • Weakly attracted by a magnetic field.
  • Possess permanent magnetic moments due to unpaired electrons.
  • Examples: Aluminum, Platinum, Oxygen.
• Explanation: In the presence of an external magnetic field, the magnetic moments of the atoms align with the field, causing a net attraction.

$$ \chi_m > 0 $$

Ferromagnetic Materials

Ferromagnetic materials are those that exhibit strong attraction to magnetic fields and can retain their magnetic properties even after the external field is removed.

• Characteristics:
  • Strongly attracted by a magnetic field.
  • Possess permanent magnetic moments that align parallel to each other in regions called domains.
  • Examples: Iron, Nickel, Cobalt.
• Explanation: In ferromagnetic materials, magnetic domains align in the direction of the applied magnetic field, resulting in strong magnetization.

$$ \chi_m \gg 0 $$

Comparison of Magnetic Materials

Property Diamagnetic Paramagnetic Ferromagnetic Magnetic Susceptibility ($\chi_m$) Negative ($\chi_m

< 0$) | Positive ($\chi_m >

0$) | Large Positive ($\chi_m \gg 0$) | | Response to Magnetic Field | Weakly repelled | Weakly attracted | Strongly attracted | | Permanent Magnetic Moment | No | Yes | Yes | | Examples | Bismuth, Copper | Aluminum, Platinum | Iron, Nickel |

Magnetic Field and Magnetic Force

Magnetic Field

A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials.

• Representation:
  • Represented by the symbol $\mathbf{B}$.
  • Measured in teslas (T).
• Magnetic Field Lines:
  • Imaginary lines representing the direction of the magnetic field.
  • Emanate from the north pole and terminate at the south pole of a magnet.

Magnetic Force

The force exerted by a magnetic field on a moving charge or a current-carrying conductor is known as the magnetic force.

Force on a Moving Charge

The force $\mathbf{F}$ on a charge $q$ moving with velocity $\mathbf{v}$ in a magnetic field $\mathbf{B}$ is given by:

$$ \mathbf{F} = q (\mathbf{v} \times \mathbf{B}) $$

• Direction:
  • Perpendicular to both $\mathbf{v}$ and $\mathbf{B}$.
  • Determined by the right-hand rule.

$$ F = qvB = (1.6 \times 10^{-19} , \text{C})(2 \times 10^6 , \text{m/s})(0.5 , \text{T}) = 1.6 \times 10^{-13} , \text{N} $$

Force on a Current-Carrying Conductor

The force $\mathbf{F}$ on a conductor of length $L$ carrying a current $I$ in a magnetic field $\mathbf{B}$ is given by:

$$ \mathbf{F} = I (\mathbf{L} \times \mathbf{B}) $$

• Direction:
  • Perpendicular to both $\mathbf{L}$ and $\mathbf{B}$.
  • Determined by the right-hand rule.

Magnetic Moment and Magnetization

Magnetic Moment

The magnetic moment $\mathbf{m}$ is a vector quantity that represents the magnetic strength and orientation of a magnet or current loop.

• For a current loop:
  • Given by $\mathbf{m} = I \mathbf{A}$, where $I$ is the current and $\mathbf{A}$ is the area vector.
• Unit:
  • Ampere-square meter (A·m²).

Magnetization

Magnetization $\mathbf{M}$ is the vector field that expresses the density of permanent or induced magnetic moments in a magnetic material.

• Definition:
  • $\mathbf{M} = \frac{\mathbf{m}}{V}$, where $\mathbf{m}$ is the magnetic moment and $V$ is the volume.
• Unit:
  • Ampere per meter (A/m).

Relation Between Magnetic Field, Magnetization, and Magnetic Susceptibility

The total magnetic field $\mathbf{B}$ inside a material is given by:

$$ \mathbf{B} = \mu_0 (\mathbf{H} + \mathbf{M}) $$

where $\mathbf{H}$ is the magnetic field intensity and $\mu_0$ is the permeability of free space.

• Magnetic Susceptibility:
  • $\mathbf{M} = \chi_m \mathbf{H}$, where $\chi_m$ is the magnetic susceptibility of the material.

Hysteresis

Hysteresis is the lag between the change in magnetization of a ferromagnetic material and the external magnetic field applied to it.

• Hysteresis Loop:
  • A plot of magnetization ($M$) versus magnetic field ($H$).
  • Shows the history of magnetization, including saturation, remanence, and coercivity.

• Key Points:
  • Saturation: Maximum magnetization achieved.
  • Remanence: Residual magnetization after removing the external field.
  • Coercivity: The reverse field required to reduce the magnetization to zero.

Earth’s Magnetism

The Earth itself acts as a giant magnet with a magnetic field that resembles that of a bar magnet tilted at an angle to its rotational axis.

Components of Earth’s Magnetic Field

• Magnetic Declination: The angle between geographic north and magnetic north.
• Magnetic Inclination (Dip): The angle between the horizontal plane and the magnetic field line.

Causes of Earth’s Magnetism

• Dynamo Effect:
  • The motion of molten iron and nickel in the Earth's outer core generates electric currents, which in turn produce the Earth's magnetic field.

Summary

This study note has covered the essential concepts of magnetism and matter, including the types of magnetic materials, magnetic field and force, magnetic moment and magnetization, hysteresis, and Earth's magnetism. Understanding these concepts is vital for mastering the topic and excelling in the NEET Physics examination.