Definitions [5]
Define Curie temperature.
The temperature above which a ferromagnetic substance becomes paramagnetic is called curie temperature.
Magnetic field lines are imaginary continuous curves drawn in a magnetic field such that the tangent at any point on the curve gives the direction of the net magnetic field \[\vec B\] at that point.
Substances which when placed in a magnetic field are feebly magnetised in the direction of the magnetising field are called paramagnetic substances.
Substances which when placed in a magnetising field are strongly magnetised in the direction of the magnetising field are called ferromagnetic substances.
Substances which when placed in a magnetic field are feebly magnetised in a direction opposite to that of the magnetising field are called diamagnetic substances.
Formulae [6]
m = IA (or m = NIA)
\[B=\frac{\mu_0}{4\pi}\cdot\frac{2m}{d^3}\]
\[B=\frac{\mu_0}{4\pi}\cdot\frac{m}{d^3}\]
\[\tau=MB\sin\theta\]
Vector form: \[\vec{\tau}=\vec{M}\times\vec{B}\]
\[B=\mu_0nI\]
n = turns per unit length
\[B=\frac{\mu_0NI}{2\pi r}\]
Theorems and Laws [3]
For ferromagnetic substances above the Curie temperature, the magnetic susceptibility is inversely proportional to (T − TC), where TC is the Curie temperature. Mathematically,
On heating beyond the Curie temperature (TC(iron) = 770 °C), ferromagnetic substances get converted into paramagnetic materials.
The magnetic susceptibility of a paramagnetic material varies inversely with its absolute temperature. Mathematically,
On cooling, paramagnetic substances get converted to ferromagnetic materials at the Curie temperature.
Curie's Law describes the relationship between the magnetic susceptibility (χ) of a paramagnetic material and its temperature (T). According to Curie's Law, the magnetic susceptibility is directly proportional to the inverse of the absolute temperature
\[\chi=\frac{C}{T}\]where:
(χ) is the magnetic susceptibility.
C is the Curie constant, which is specific to each material.
T is the absolute temperature in kelvin.
Key Points
- A current-carrying loop behaves like a magnetic dipole (bar magnet)
- Polarity Rule
Anticlockwise current → North pole (upper face)
Clockwise current → South pole (lower face)
- Direction given by right-hand thumb rule; for a loop, B at centre and M are parallel.
- Magnetic moment of a straight current-carrying wire = 0.
- Magnetic moment of a toroid = 0.
- Dipole moment direction: S → N (inside magnet field taken N → S).
- A bar magnet behaves like a solenoid
- Both produce similar magnetic field patterns
- Solenoid Relation: M = NIA
- Magnetic field lines are imaginary lines and do not physically exist in space — only the magnetic field itself is real.
- Outside a magnet, field lines always point from the north pole to the south pole.
- Inside a magnet, field lines point from the south pole to the north pole to complete the closed loop.
- Magnetic field lines never intersect each other because a point in space can have only one direction of magnetic field at a time.
- The region where field lines are closely packed has a stronger magnetic field, and the poles of a magnet have the densest field lines.
- Diamagnetic substances are weakly repelled and have negative susceptibility.
- Paramagnetic substances are weakly attracted and obey Curie law.
- Ferromagnetic substances are strongly attracted and contain domains.
- Ferromagnets become paramagnetic above the Curie temperature.
- The comparison table is the most important revision tool for board preparation.
| Quantity | Symbol | Definition | Formula | Unit | Nature |
|---|---|---|---|---|---|
| Magnetising Field (Magnetic Field Intensity) | H | Measure of the external magnetic field applied to a material | \[H=\frac{B}{\mu}\] | A/m | Vector |
| Intensity of Magnetisation | I | Magnetic dipole moment per unit volume | \[I=\frac{M}{V}\] | A/m | Vector |
| Magnetic Susceptibility | \[\chi_{m}\] | Ratio of magnetisation to magnetising field | \[\chi_m=\frac{I}{H}\] | No unit | Scalar |
| Magnetic Permeability | \[\mu\] | Ratio of magnetic field to magnetising field | \[\mu=\frac{B}{H}\] | H/m (or T·m/A) | Scalar |
| Relative Permeability | \[\mu_{r}\] | Ratio of permeability of medium to free space | \[\mu_r=\frac{\mu}{\mu_0}\] | No unit | Scalar |
Concepts [10]
- Current Loop as a Magnetic Dipole
- Magnetic Dipole Moment
- Magnetic Field Due to Magnetic Dipole (Bar Magnet)
- Torque on a Magnetic Dipole (Bar Magnet) in a Uniform Magnetic Field
- Bar Magnet and Solenoid Analogy
- Magnetic Field Lines
- Magnetic Field Due to Solenoid & Toroid
- Magnetic Properties of Materials
- Terms Used in Magnetism
- Curie Temperature
