Define Magnetic Susceptibility. Mention its Unit.

Magnetic susceptibility is a measure of how much a material gets magnetised when it is placed in an external magnetic field. It is denoted by the Greek letter χ (chi). It is defined as the ratio of the magnetisation (M) produced in the material to the applied magnetic field strength (H). 

Understanding magnetic susceptibility helps us in designing electromagnets, magnetic storage devices, MRI machines, and many scientific instruments. It also helps in selecting materials for magnetic shielding and sensors.

Mathematically, the formula for Magnetic Susceptibility is:

χ= H/M

Where:

• χ = Magnetic susceptibility
• M = Magnetisation (magnetic moment per unit volume)
• H = Magnetic field strength
  

Unit of Magnetic Susceptibility

Magnetic susceptibility is a dimensionless quantity, meaning it has no unit. It is because both M and H are measured in the same unit (A/m), so they cancel out.

Example:

Imagine placing a piece of iron near a magnet. The iron starts behaving like a magnet itself. This happens because the atoms inside the iron get aligned due to the external magnetic field.

Now, how strongly the iron gets magnetized depends on its magnetic susceptibility.

• If the material gets strongly magnetized, it has high susceptibility.
• If it shows very little magnetism, it has low susceptibility.

Types of Materials Based on Magnetic Susceptibility

Magnetic Susceptibility varies as per the material. Here we have divided the materials based on their magnetic susceptibility.

1. Diamagnetic Materials
   • Magnetic Susceptibility (χₘ): Negative and very small
   • Examples: Bismuth, copper, silver, lead, water, quartz
   • Behavior in a Magnetic Field: Slightly repelled
   • Reason: These materials do not have permanent magnetic dipoles (tiny atomic magnets). When you place them in a magnetic field, they create a weak magnetic field in the opposite direction, which slightly pushes them away from the external field.
2. Paramagnetic Materials
   • Magnetic Susceptibility (χₘ): Positive but small
   • Examples: Aluminum, platinum, manganese, chromium
   • Behavior in a Magnetic Field: Weakly attracted
   • Reason: These materials have unpaired electrons that act like tiny magnets. In the presence of a magnetic field, these dipoles try to align in the same direction as the field. But the alignment is weak and easily disturbed by heat (thermal motion)
3. Ferromagnetic Materials
   • Magnetic Susceptibility (χₘ): Positive and very large
   • Examples: Iron, nickel, cobalt, and some of their alloys
   • Behavior in a Magnetic Field: Strongly attracted
   • Reason: These materials have regions called magnetic domains, where atomic magnets are naturally aligned. When an external magnetic field is applied, these domains grow in the same direction, causing a very strong magnetic effect. Even after the field is removed, some alignment stays, making them permanent magnets.
     

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