Which of the following characteristics is true for ferromagnetic materials?

Explanation:

Ferromagnetic Materials

Ferromagnetic materials are substances that exhibit strong magnetic properties. These materials have unpaired electrons, which cause their atoms to act like tiny magnets. The key feature of ferromagnetic materials is that they can retain their magnetization even after the external magnetic field is removed, a property known as hysteresis.

Examples: Common examples of ferromagnetic materials include iron, cobalt, nickel, and their alloys. These materials are widely used in various applications due to their strong magnetic properties.

Hysteresis: One of the distinctive characteristics of ferromagnetic materials is hysteresis. When the material is exposed to a varying magnetic field, the magnetization of the material does not follow the applied field precisely. Instead, it exhibits a lag, creating a hysteresis loop. This property is crucial for applications like magnetic storage devices, transformers, and electric motors.

Analysis of Other Options:

Option 2: "They are weakly repelled by a magnetic field and have negative magnetic susceptibility." This statement is true for diamagnetic materials, not ferromagnetic materials. Diamagnetic materials develop a weak magnetic moment in opposition to an applied magnetic field, causing them to be repelled.

Option 3: "They show no effect in a magnetic field and are considered neutral." This statement is true for materials that are neither paramagnetic nor diamagnetic, such as some non-magnetic materials. Ferromagnetic materials, however, show a strong attraction to magnetic fields.

Option 4: "They are strongly attracted to a magnetic field but do not exhibit hysteresis." This statement is true for paramagnetic materials, which are attracted to magnetic fields but do not retain their magnetization once the external field is removed. Ferromagnetic materials, in contrast, do exhibit hysteresis.

In conclusion, the correct option is Option 1: "They retain their magnetisation even after the external magnetic field is removed." This is the defining characteristic of ferromagnetic materials, distinguishing them from other types of magnetic materials.