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Curie-Weiss law says that magnetic susceptibility is exactly proportional to the ratio of the Curie constant to temperature change. It is an essential rule in electromagnetic theory that states that the magnetic susceptibility of a paramagnetic is greater than the Curie temperature point of a ferromagnetic. A magnet's magnetic moment is a quantity that governs its torque in an external magnetic field. A bar magnet, an electric current loop, a molecule, and an electron, for example, all have magnetic moments. This topic is part of class 12’s Chapter-5 named ‘Magnetism and Matter’.
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Key Takeaways: Magnetism, ferromagnetism, curie-weiss law, magnet, curie temperature, electron, bar magnet, electric current loop, molecule, Electromagnetic theory
What is Curie-Weiss Law?
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One of the most significant rules in electromagnetism is the Curie-Weiss law, which states that magnetic susceptibility is greater than the Curie temperature point of a ferromagnet in the paramagnetic zone. A magnet's magnetic moment is a quantity that governs its torque in an external magnetic field. A magnetic moment can be found in a bar magnet, an electric current loop, a molecule, and an electron.
Formula
The Curie Weiss Law formula is shown below:
X = C / T - Tc
The C in this case represents the material-specific Curie.
The absolute temperature is denoted by T.
The Curie temperature is denoted by Tc.
The magnetic moments of magnetic materials are affected by their external fields. It is the connection between the absolute temperature and the magnetic field's substance. Magnetite and nickel are two materials that have comparable characteristics. Curie temperature refers to the temperature of ferromagnetic compounds in a paramagnetic field.
Read More: Magnetism and Matter Important Questions
Limitations
Curie Weiss's law is based on a number of assumptions and incorrect derivations in order to assess susceptibility. Let us consider the believability of the Curie-Weiss formula, which may be stated as follows:
X = (T - Tc) y
T > Tc is true in this case, but if Tc is substituted by the temperature, the result is greater than the Curie temperature, which is untrue.
There is no explanation for why this occurs. Regardless of the constraint, this formula is commonly employed.
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Curie Temperatures
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Here are the Curie Temperatures for a Few Ferromagnetic Substances
| Substance Name | Curie Temperature |
|---|---|
| Iron (Fe) | 1,043K |
| Gadolinium (Gd) | 293K |
| Nickel (Ni) | <631K |
Understanding Ferromagnetism and Weiss Law
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Ferromagnetism is the phenomenon of spontaneous magnetization, in which magnetization arises in a substance in the absence of an applied magnetic field. Some of the most common ferromagnets include Fe, Co, and Ni, as well as a few alloys with ferromagnetism features. It happens when the molecular moments are aligned in the right way.
For ferromagnetism to manifest, a threshold temperature (also known as the ferromagnetic transition temperature) must be reached, which may be as high as 1000K for elements such as Fe, Co, Gd, and others. It happens when there are atomic magnetic dipoles pointing in opposite directions in the absence of an external field.
In Iron, for example, the induced magnetic moment is determined by the spinning of electrons in the nuclei's outer shell. According to Pauli's exclusion principle, no two electrons at the same precise place may have spins pointing in the same direction. It causes an irrational repulsion between the two electrons. Electrons with opposing spins can display attractive interactions with magnetization. As a result of the attraction force present in oppositely spinning electrons, the iron atoms can align with each other. This may be stated mathematically as follows:
The influence of exchange forces yields and effective molecular field Hint in this formula, which relies on the magnitude of magnetization M;
Hint = M... eqn. 2
Where the Weiss Constant is.
The yielding magnetization (M) can alternatively be written as the sum and product of the magnetic susceptibility, p.
χp \s(H + λM) = M ...eqn. 3
The above equation serves as the foundation for the Curie-Weiss Law equation.
Curie-Weiss Law equation
Things to Remember
- One of the most significant rules in electromagnetism is the Curie-Weiss law, which states that magnetic susceptibility is greater than the Curie temperature point of a ferromagnet in the paramagnetic zone.
- Curie temperature refers to the temperature of ferromagnetic compounds in a paramagnetic field.
- Ferromagnetism is the phenomenon of spontaneous magnetization, in which magnetization arises in a substance in the absence of an applied magnetic field.
- χp \s(H + λM) = M
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Previous Year Questions
- Materials suitable for permanent magnet, must have which of the following properties ?
- A bar magnet is oscillating in the earth's magnetic field with a….[NEET 1994]
- A solenoid has core of a material with relative permeability…..[KEAM]
- A metal ring is held horizontally and bar magnet is dropped through...[NEET]
- If a current of 500 mA produces a deflection of…. [KEAM]
- A paramagnetic substance of susceptibility…..[KEAM]
- If the susceptibility of dia, para and ferro magnetic materials are…. [KEAM]
- A charged particle (charge q) is moving in a circle of radius R with uniform..
- At Curie point, a ferromagnetic material becomes….
- At a certain place, horizontal component is…..
- The variation of the intensity of magnetisation (I) with respect to the magnetising field….. [VITEEE 2002]
- A bar magnet is equivalent to ........[KCET 2004]
- Magnetic permeability is maximum for...[BCECE 2003]
- the magnetic moment of bar in the state of magnetic saturation will be….[BHU UET]
- If a bar magnet of magnetic moment M is kept in a uniform magnetic field…. [WBJEE 2016]
- The ferromagnetic substance is converted into paramagnetic substances … [JIPMER 1996]
- The acceleration of the falling magnet is..[NEET]
- A bar magnet is equivalent to ............[KCET 2004]
- The magnetic susceptibility is negative for … [NEET 2016]
- a magnetic induction of strength at its centre is...
Sample Questions
Question 1: What is Ferromagnetism's Weiss Theory? (3 marks)
Answer: According to the Weiss theory of ferromagnetism, adjacent atoms in ferromagnetic materials can generate numerous mutual exchanges in many confined areas known as domains. It can also be subdivided into many points:
Domains aligned in the magnetic field's direction may become more conspicuous than domains oriented in the opposite way.
The domains have the ability to spin and move in the direction of the applied external magnetic field.
Question 2: What Is the Distinction Between Curie and Curie Weiss Temperatures? (3 marks)
Answer: The Curie Temperature (Tc) is the temperature at which the material's susceptibility is blown up:
χ = C / T – Tc and T ~ Tc
The Curie-Weiss temperature, on the other hand, holds for T >> T0 and is near to T0 Tc for first-order transitions.
Question 3: When is Curie-Weiss Law obeyed by iron? (5 marks)
Answer: The Curie-Weiss Law describes how the susceptibility of ferromagnets, such as iron, varies with temperature. χ = CT − Tc. Consider the relationship and recall whether you have ever observed an iron bar repel in a magnetic field.
Complete the following step-by-step response:
- A material's magnetic susceptibility is a measure of its capacity to enhance or converge a magnetic field. Ferromagnetic materials have a value of 100, but paramagnetic and diamagnetic materials have relatively low values.
- Temperature affects a ferromagnetic material's susceptibility. This is due to the fact that at higher temperatures, the domains become more random and contain more thermal energy.
- As a result, when the temperature rises, it becomes more difficult to magnetise a substance. Hence, we anticipate an inverse relationship between susceptibility and temperature.
- This relationship between and the absolute temperature T is predicted by the Curie - Weiss law. The Curie-Weiss law states that: χ = CT − Tc
- To address the question, if the temperature is less than Tc, this rule produces a negative value. This implies that a ferromagnetic substance can oppose the magnetic field in which it is put below a specific temperature.
- In nature, this is never seen. Ferromagnetic materials, such as iron, will never reject a magnetic field.
As a result, the Curie - Weiss rule is only relevant at temperatures larger than the Curie temperature.
Question 4: To which of the following materials is Curie-Weiss law applicable? (2 marks)
a)Piezoelectric
b)Ferroelectric
c)Pyroelectric
d)Anti-ferroelectric
Correct answer is option 'B'. Can you explain this answer?
Answer: The Curie-Weiss law is given as:
χe = εr -1 = C/(T-θ),
where C is the curie constant and is the characteristic temperature, which is normally a few degrees higher than the curie temperature for ferromagnetic materials.
Question 5: What is the valid temperature range to apply ferroelectric Curie-Weiss Law? (3 marks)
Answer: i] In some ferroelectric substances, the temperature dependence of ε can be accurately represented by Curie- Weiss [CW] law up to reasonable accuracy as:
-ε= ε0+C/(T-T0);
C( Curie’s constant) and To(CW temperature), which , in general, differs from Curie’s temperature(Tc)
[ii] ε becomes very high when it reaches T0
[iii] In some ferroelectric substances,Tc= T0 and the phase transition is of the second order while In others , Tc and To differ very largely so that the phase transition is of the first order.
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