Diamagnetic, Paramagnetic and Ferromagnetic Materials

To answer a comparison question: define the basis of classification; write the sign of chi; state whether mu_r is less than, slightly greater than, or much greater than 1; describe attraction or repulsion; mention magnetisation direction; give examples.

A comparison diagram should show three specimens placed in a magnetic field. Diamagnetic response has induced magnetisation opposite to B, paramagnetic response has weak magnetisation along B, and ferromagnetic response has strong domain alignment along B.

A common wrong answer is saying diamagnetic materials are not affected by magnetic fields. They are affected weakly and develop magnetisation opposite to the applied field.

Diamagnetic, paramagnetic, and ferromagnetic materials are three classes of magnetic materials distinguished by the direction and strength of magnetisation produced when they are placed in an external magnetic field.

Diamagnetic, paramagnetic, and ferromagnetic materials are three classes of magnetic materials distinguished by the direction and strength of magnetisation produced when they are placed in an external magnetic field. Qualitative rules: diamagnetic materials have chi < 0 and mu_r slightly less than 1; paramagnetic materials have chi > 0 and mu_r slightly greater than 1; ferromagnetic materials have large positive chi and mu_r much greater than 1. For school-level linear comparison, mu_r = 1 + chi is often used as a guide, but ferromagnetic materials can show non-linear behaviour and hysteresis. Bismuth and copper are diamagnetic, aluminium and oxygen are paramagnetic, and iron, cobalt, and nickel are ferromagnetic. A soft iron core is used in electromagnets because it becomes strongly magnetised in an applied field.

Diamagnetic materials develop weak magnetisation opposite to the applied magnetic field, so they are weakly repelled. Their susceptibility is small and negative, and relative permeability is slightly less than 1. Paramagnetic materials develop weak magnetisation in the direction of the applied field, so they are weakly attracted. Their susceptibility is small and positive, and relative permeability is slightly greater than 1. Ferromagnetic materials show very strong magnetisation in the direction of the applied field due to domain alignment. Their susceptibility is large and positive, and relative permeability is much greater than 1. Qualitative rules: diamagnetic materials have chi < 0 and mu_r slightly less than 1; paramagnetic materials have chi > 0 and mu_r slightly greater than 1; ferromagnetic materials have large positive chi and mu_r much greater than 1. For school-level linear comparison, mu_r = 1 + chi is often used as a guide, but ferromagnetic materials can show non-linear behaviour and hysteresis. Question: Classify three materials with susceptibilities -1.5 x 10^-5, 2.2 x 10^-5, and very large positive value. Answer: The material with negative susceptibility is diamagnetic. The material with small positive susceptibility is paramagnetic. The material with very large positive susceptibility is ferromagnetic. Interpretation: the sign gives direction of magnetisation, and the magnitude gives strength of response. This concept is frequently asked as a table, reason-based classification, assertion-reason question, or application question on why soft iron is used in electromagnets and why diamagnetic materials are weakly repelled. A common wrong answer is saying diamagnetic materials are not affected by magnetic fields. They are affected weakly and develop magnetisation opposite to the applied field.