Magnetism and Magnetic Materials
Bar magnet as a dipole, earth's magnetism, dia-, para- and ferromagnetism. (Physics › Magnetic Effects of Current and Magnetism, NEET UG syllabus.)
What is Magnetism and Magnetic Materials?
A system of two equal and opposite magnetic poles separated by a small distance, analogous to an electric dipole. A bar magnet is a magnetic dipole.
Key formula / rule: Magnetic moment of a bar magnet
Key points
- Describe a bar magnet as a magnetic dipole and its field lines.
- Explain Earth's magnetism and its elements (declination, dip, horizontal component).
- Differentiate between diamagnetic, paramagnetic, and ferromagnetic materials based on their properties and susceptibility.
- Understand the concept of magnetic intensity, magnetization, and magnetic susceptibility.
Common exam trap
Confusing geographic and magnetic poles of Earth.
Definitions
- Term
Magnetic Dipole
- Meaning
A system of two equal and opposite magnetic poles separated by a small distance, analogous to an electric dipole. A bar magnet is a magnetic dipole.
- Term
Magnetic Moment (M)
- Meaning
A vector quantity representing the strength and orientation of a magnet or magnetic field source. For a bar magnet, it's the product of pole strength and magnetic length.
- Term
Magnetic Field Lines
- Meaning
Imaginary lines representing the direction of the magnetic field. They originate from the North pole and terminate at the South pole outside the magnet, forming continuous closed loops.
- Term
Magnetic Declination
- Meaning
The angle between the geographic meridian and the magnetic meridian at a place.
- Term
Magnetic Dip (or Inclination)
- Meaning
The angle that the total magnetic field of the Earth makes with the horizontal plane at a given location.
- Term
Magnetic Susceptibility (χ)
- Meaning
A dimensionless quantity that indicates how easily a material can be magnetized in an external magnetic field. It is the ratio of magnetization (M) to magnetic intensity (H).
- Term
Relative Permeability (μr)
- Meaning
The ratio of the magnetic permeability of a material (μ) to the permeability of free space (μ₀). It indicates how much a material concentrates magnetic flux.
- Term
Diamagnetism
- Meaning
A form of magnetism where materials are weakly repelled by an externally applied magnetic field. They have negative magnetic susceptibility.
- Term
Paramagnetism
- Meaning
A form of magnetism where materials are weakly attracted by an externally applied magnetic field. They have small positive magnetic susceptibility.
- Term
Ferromagnetism
- Meaning
A strong form of magnetism where materials are strongly attracted to magnetic fields and can retain their magnetization even after the external field is removed. They have large positive magnetic susceptibility and exhibit magnetic domains.
- Term
Curie Temperature
- Meaning
The critical temperature above which a ferromagnetic material loses its ferromagnetic properties and becomes paramagnetic.
Learning objectives
Describe a bar magnet as a magnetic dipole and its field lines.
Explain Earth's magnetism and its elements (declination, dip, horizontal component).
Differentiate between diamagnetic, paramagnetic, and ferromagnetic materials based on their properties and susceptibility.
Understand the concept of magnetic intensity, magnetization, and magnetic susceptibility.
Explain the phenomenon of hysteresis in ferromagnetic materials.
Formulae
- Name
Magnetic moment of a bar magnet
- Note
m is pole strength, 2l is magnetic length.
- Expression
M = m × (2l)
- Name
Torque on a magnetic dipole in a uniform magnetic field
- Note
M is magnetic moment, B is magnetic field.
- Expression
τ = M × B
- Name
Potential energy of a magnetic dipole in a uniform magnetic field
- Note
θ is angle between M and B.
- Expression
U = -M ⋅ B = -MB cosθ
- Name
Relation between Magnetic Field (B), Magnetic Intensity (H), and Magnetization (M)
- Note
μ₀ is permeability of free space.
- Expression
B = μ₀(H + M)
- Name
Magnetic Susceptibility
- Note
Dimensionless quantity.
- Expression
χ = M/H
- Name
Relative Permeability
- Note
Dimensionless quantity.
- Expression
μr = 1 + χ
- Name
Relation between B, H, and μr
- Note
μ is absolute permeability of the medium.
- Expression
B = μ₀μr H = μH
- Name
Horizontal component of Earth's magnetic field
- Note
BE is total Earth's magnetic field, δ is angle of dip.
- Expression
BH = BE cosδ
- Name
Vertical component of Earth's magnetic field
- Note
δ is angle of dip.
- Expression
BV = BE sinδ
- Name
Relation between BH, BV, and δ
- Note
δ is angle of dip.
- Expression
tanδ = BV / BH
- Name
Curie's Law for Paramagnetism
- Note
T is absolute temperature.
- Expression
χ ∝ 1/T
Prerequisites
Basic understanding of electric fields and dipoles.
Knowledge of current electricity and magnetic effects of current (Biot-Savart Law, Ampere's Circuital Law).
Concept of force on a current-carrying conductor in a magnetic field.
Common mistakes
Confusing geographic and magnetic poles of Earth.
Misunderstanding the direction of magnetic field lines (always closed loops).
Mixing up diamagnetic (repelled) with paramagnetic (attracted).
Forgetting that magnetic dip is zero at the magnetic equator and 90° at magnetic poles.
Not knowing the relationship between B, H, and M (B = μ₀(H + M)).
Incorrectly applying Curie's Law (χ ∝ 1/T for paramagnets, but more complex for ferromagnets).
Keywords
Bar magnet
Magnetic dipole
Earth's magnetism
Magnetic declination
Magnetic dip
Diamagnetism
Paramagnetism
Ferromagnetism
Magnetic susceptibility
Relative permeability
Curie temperature
Magnetic domains
Hysteresis
Practice preview
Which of the following statements about magnetic field lines is incorrect?…
easy
For a paramagnetic material, the magnetic susceptibility (chi) is:…
medium
If the angle of dip at a place is 30 degrees and the horizontal component of Earth's magnetic field is 0.5 G, what is the total intensity of Earth's magnetic field at that place? (Given cos 30 degrees = 0.866)…
medium