Chapter 21: Superconductivity

21.1 Introduction

In this chapter, we will discuss superconductivity, a remarkable phenomenon in which certain materials exhibit zero electrical resistance when cooled below a critical temperature. Superconductivity has important applications in various fields, including medical imaging, transportation, and energy transmission.

21.2 Superconductivity Basics

Superconductivity is a state in which a material exhibits zero electrical resistance, allowing electric current to flow without any energy loss. This phenomenon was first observed in 1911 by Heike Kamerlingh Onnes when he discovered that mercury’s resistance vanished when cooled to a temperature close to absolute zero.

21.3 Critical Temperature and Critical Magnetic Field

For a material to become superconductive, it must be cooled below a specific temperature called the critical temperature (). The critical temperature varies among different materials. Additionally, superconductivity can be destroyed if a magnetic field stronger than the critical magnetic field () is applied.

21.4 Type I and Type II Superconductors

Superconductors are classified into two types:

• Type I Superconductors: These are typically pure metals or simple alloys that exhibit superconductivity at low temperatures. They undergo an abrupt transition from a normal to a superconducting state when cooled below their critical temperature. Type I superconductors can be completely diamagnetic, meaning they expel all magnetic fields when in a superconducting state (Meissner effect).
• Type II Superconductors: These are usually complex alloys or compounds that exhibit superconductivity at higher temperatures than Type I superconductors. They show a gradual transition to the superconducting state, allowing some penetration of magnetic fields into the material in the form of quantized vortices. This property makes Type II superconductors more suitable for applications requiring high magnetic fields.

21.5 BCS Theory of Superconductivity

The BCS (Bardeen, Cooper, and Schrieffer) theory, formulated in 1957, provides a microscopic explanation of superconductivity. According to the BCS theory, electrons in a superconductor form Cooper pairs through interactions with lattice vibrations (phonons), resulting in a condensation of these pairs into a single quantum state that can flow without resistance.

21.6 High-Temperature Superconductors

High-temperature superconductors are materials that exhibit superconductivity at temperatures significantly higher than traditional superconductors, often above the boiling point of liquid nitrogen (77 K). The discovery of high-temperature superconductors, such as copper oxide ceramics, has sparked considerable interest in finding new materials and understanding the underlying mechanisms behind their superconducting properties.

21.7 Applications of Superconductivity

Superconductivity has numerous applications due to its unique properties, including:

• Magnetic Resonance Imaging (MRI): Superconducting magnets generate the strong and stable magnetic fields necessary for MRI machines used in medical diagnostics.
• Maglev Trains: Superconducting magnets enable magnetic levitation (maglev) trains to float above their tracks, reducing friction and enabling high-speed transportation.
• Energy Transmission: Superconducting power lines can transmit electrical energy with minimal losses, potentially increasing the efficiency of power grids.

Chapter Summary

In this chapter, we explored superconductivity, a phenomenon where certain materials exhibit zero electrical resistance when cooled below their critical temperature. We discussed the distinction between Type I and Type II superconductors, the BCS theory, and high-temperature superconductors. Superconductivity has a range of applications in medical imaging, transportation, and energy transmission, making it an important area of study for advancing these fields.

Continue to Chapter 22: Mutual Inductance

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