Problem 16 - Entrance Test

Let's analyze the given definitions: - **Paramagnetism**: Substances that are weakly attracted by an external magnetic field. They have unpaired electrons, and the magnetic moments are randomly oriented in the absence of a field. In the presence of a field, they align in the direction of the field, leading to weak magnetism. - **Ferromagnetism**: Substances that are strongly attracted by an external magnetic field and can retain their magnetism even after the field is removed. They have unpaired electrons, and their magnetic moments spontaneously align in the same direction in domains, leading to strong permanent magnetism (e.g., Fe, Co, Ni). - **Antiferromagnetism**: Substances that possess unpaired electrons but have domains with magnetic moments aligned in opposite directions, cancelling each other out. This results in a net zero magnetic moment even in the presence of an external magnetic field (e.g., MnO). - **Ferrimagnetism**: Substances with magnetic moments aligned in opposite directions but in unequal numbers or strengths, resulting in a net magnetic moment. They are weakly attracted by external magnetic fields (e.g., Fe₃O₄, ferrites). The description in the question states: "unpaired electrons but the magnetic moments align randomly, resulting in a net zero magnetic moment in the absence of an external magnetic field." This perfectly matches the definition of **paramagnetism** before an external field is applied. In the absence of an external field, paramagnetic substances do not exhibit a net magnetic moment because the individual magnetic moments are randomly oriented. Ferromagnetic, antiferromagnetic, and ferrimagnetic substances exhibit specific types of alignment (or cancellation) of magnetic moments, even without an external field, or have spontaneous alignment leading to a net moment. Therefore, the correct answer is paramagnetism.