Diamagnetism

When an external magnetic field is applied to a diamagnetic substance, a weak magnetic moment is induced in the direction opposite the applied field. This causes diamagnetic substances to be weakly repelled by a magnet. Although diamagnetism is present in all matter, its effects are much smaller than those of paramagnetism or ferromagnetism.

We can understand of diamagnetism by considering a classical model of two atomic electrons orbiting the nucleus in opposite directions but with the same speed. The electrons remain in their circular orbits because of the attractive electrostatic force exerted by the positively charged nucleus. Because the magnetic moments of the two electrons are equal in magnitude and opposite in direction, they cancel each other, and the magnetic moment of the atom is zero. When an external magnetic field is applied, the electrons experience an additional magnetic force qv×B. This added magnetic force combines with the electrostatic force to increase the orbital speed of the electron whose magnetic moment is antiparallel to the field and to decrease the speed of the electron whose magnetic moment is parallel to the field. As a result, the two magnetic moments of the electrons no longer cancel, and the substance acquires a net magnetic moment that is opposite the applied field.

A superconductor is a substance in which the electrical resistance is zero below some critical temperature. Certain types of superconductors also exhibit perfect diamagnetism in the superconducting state. As a result, an applied magnetic field is expelled by the superconductor so that the field is zero in its interior. This phenomenon is known as the Meissner effect. If a permanent magnet is brought near a superconductor, the two objects repel each other (from ‘Physics for Scientists and Engineers’).

Exercise 94. Summarize the texts ‘Classification of Magnetic Substances’. Follow these steps.

1. Read the texts again and underline the relevant information in the text.

2. Make notes about the main points. Leave out all unnecessary details.

3. Make simple sentences from the notes and link the sentences with connectors (and, but, because, therefore, however, in addition, due to).

4. Write your first draft. Improve your draft by reducing sentences.

5. Write the final version of your summary. Check the spelling and grammar.

Exercise 95. Fill in the gaps in the text with the words from the box.

diamagnetic positive permanent earth magnetite paramagnetic repel poles electricity ferromagnetic retentivity attract isolate

Permanent magnet

Lodestone (also called ________) is a naturally-occurring permanent magnet mineral. _______ means that the material maintains a magnetic field with no external help. The characteristic of any magnetic material to do so is called ________.

Unlike electric charges, magnetic objects possessed two poles of opposite effect, denoted "north" and "south" after their self-orientation to the _______. As Peregrinus found, it was impossible to _______ one of these poles by itself by cutting a piece of lodestone in half: each resulting piece possessed its own pair of _______. Like electric charges, there were only two types of poles to be found: north and south (by analogy, _______ and negative). Just as with electric charges, same poles _______ one another, while opposite poles _______. This force, like that caused by static ________, could even pass through objects such as paper and wood with little effect upon strength.

________ materials are easily magnetized. __________ materials are magnetized with more difficulty. ________ materials actually tend to repel external magnetic fields by magnetizing in the opposite direction.

Speaking

Exercise 96. Quiz “Electromagnetism”. Divide into two teams.

Team A – look at the information below.

Team B – look at exercise 100.