Diamagnetism — Revision Notes

⚡ 30-Second Revision

Origin — Induced magnetic moment from electron orbital motion (Lenz's Law).
Permanent Dipoles — No permanent atomic magnetic dipoles.
Behavior — Weakly repelled by external magnetic fields.
Movement — From stronger to weaker field regions.
Magnetic Susceptibility ($chi$) — Small and negative (e.g., $-10^{-5}$ to $-10^{-9}$).
Relative Permeability ($mu_r$) — Slightly less than 1 ($mu_r < 1$).
Temperature Dependence — Largely independent of temperature.
Examples — Water, Copper, Bismuth, Gold, Noble gases.
Formula — $\mu_r = 1 + \chi$

2-Minute Revision

Diamagnetism is a universal magnetic property arising from the orbital motion of electrons in atoms. When an external magnetic field is applied, it induces a change in electron motion, generating a magnetic moment that, by Lenz's Law, *opposes* the applied field.

This opposition leads to several key characteristics: diamagnetic materials are weakly repelled by magnets and tend to move from stronger to weaker field regions. Their magnetic susceptibility ($chi$) is small and negative, typically in the range of $-10^{-5}$ to $-10^{-9}$.

Consequently, their relative permeability ($mu_r = 1 + chi$) is slightly less than 1. Crucially, diamagnetic properties are largely independent of temperature, distinguishing them from paramagnetic and ferromagnetic materials.

Common examples include water, copper, bismuth, and noble gases. Remember, while all materials exhibit diamagnetism, it's only dominant in substances where permanent atomic magnetic moments are absent or cancel out.

5-Minute Revision

Diamagnetism is the most fundamental type of magnetism, present in all materials, though often masked by stronger effects. Its origin lies in the orbital motion of electrons around the nucleus. In diamagnetic atoms, the individual orbital magnetic moments typically cancel out, leaving no net permanent magnetic moment.

However, when an external magnetic field ($vec{B}_{\text{ext}}$) is applied, it exerts a Lorentz force on the orbiting electrons, altering their angular velocities. According to Lenz's Law, this change induces an additional magnetic moment ($vec{M}_{\text{ind}}$) that is always directed *opposite* to $vec{B}_{\text{ext}}$.

This opposition leads to the characteristic properties of diamagnetic materials:

1
Repulsion — They are weakly repelled by external magnetic fields. In a non-uniform field, they move from stronger to weaker field regions.
2
Magnetic Susceptibility ($chi$) — It is small, negative, and typically ranges from $-10^{-5}$ to $-10^{-9}$. A negative $chi$ signifies opposition to the applied field.
3
Relative Permeability ($mu_r$) — Since $mu_r = 1 + chi$, and $chi$ is negative and small, $mu_r$ for diamagnets is slightly less than 1 (e.g., 0.9999).
4
Temperature Independence — The induced effect is largely unaffected by thermal agitation, so diamagnetic properties are independent of temperature.
5
Temporary Magnetization — The induced magnetic moment exists only as long as the external field is present.

Examples: Water, Copper, Bismuth, Gold, Silver, Nitrogen, Noble gases, most organic compounds. Superconductors are perfect diamagnets (Meissner effect).

NEET Focus: Distinguish diamagnetism from paramagnetism and ferromagnetism based on these properties, especially the sign of $chi$, value of $mu_r$, and temperature dependence. Be prepared for questions on identifying materials or predicting their behavior in magnetic fields.

Prelims Revision Notes

Diamagnetism: NEET Quick Facts

1. Definition & Origin:

Universal Property: — Present in all materials, but often masked.
Origin: — Induced magnetic moment due to the orbital motion of electrons.
Key Principle: — Governed by Lenz's Law. External magnetic field induces a change in electron orbital motion, creating a moment that *opposes* the applied field.
No Permanent Dipoles: — Atoms/molecules have no net permanent magnetic moment (electron spins are paired, orbital moments cancel).

2. Key Properties:

Behavior in External Field: — Weakly repelled by magnets.
Movement in Non-uniform Field: — Tends to move from regions of stronger magnetic field to regions of weaker magnetic field.
**Magnetic Susceptibility ($chi$):**

* Small and negative. * Typical range: $chi approx -10^{-5}$ to $-10^{-9}$. * Indicates induced magnetization opposes the field.

**Relative Permeability ($mu_r$):**

* Slightly less than 1 ($mu_r < 1$). * Formula: $mu_r = 1 + chi$. * Implies magnetic field lines are slightly expelled from the material.

Temperature Dependence: — Independent of temperature. Thermal energy does not affect the induced orbital changes significantly.
Temporary Magnetization: — Induced moment disappears when the external field is removed.

3. Common Examples:

Metals: — Copper (Cu), Gold (Au), Silver (Ag), Bismuth (Bi), Zinc (Zn).
Non-metals/Compounds: — Water (H$_2$O), Nitrogen (N$_2$), Hydrogen (H$_2$), Noble gases (He, Ne, Ar), Diamond.
Superconductors: — Perfect diamagnets (Meissner effect).

4. Distinguishing Features (from Paramagnetism):

Diamagnetism: — Repulsion, $chi < 0$, $mu_r < 1$, Temperature Independent.
Paramagnetism: — Attraction, $chi > 0$, $mu_r > 1$, Temperature Dependent (Curie's Law: $chi propto 1/T$).

5. Experimental Observations:

U-tube Experiment: — Diamagnetic liquid level falls in the arm placed in a strong magnetic field.
Rod Orientation: — A diamagnetic rod aligns perpendicular to a uniform magnetic field.

Vyyuha Quick Recall

Don't Interact Really Nice, Temperature Ignored, Susceptibility Negative.

Don't Interact: Diamagnetism is an Induced effect.
Really Nice: Repelled, moves from Near (strong) to far (weak) field.
Temperature Ignored: Temperature Independent.
Susceptibility Negative: Susceptibility is Negative ($chi < 0$).

(Also remember $mu_r < 1$ and examples like Water, Copper, Bismuth - 'WCB' for 'Weakly Cold Bismuth').