9.6 Superconductors (Page 4/11)

University physics volume 2 Page 4 / 11

Applications of superconductors

Superconductors can be used to make superconducting magnets. These magnets are 10 times stronger than the strongest electromagnets. These magnets are currently in use in magnetic resonance imaging (MRI), which produces high-quality images of the body interior without dangerous radiation.

Another interesting application of superconductivity is the SQUID (superconducting quantum interference device). A SQUID is a very sensitive magnetometer used to measure extremely subtle magnetic fields. The operation of the SQUID is based on superconducting loops containing Josephson junctions. A Josephson junction is the result of a theoretical prediction made by B. D. Josephson in an article published in 1962. In the article, Josephson described how a supercurrent can flow between two pieces of superconductor separated by a thin layer of insulator. This phenomenon is now called the Josephson effect. The SQUID consists of a superconducting current loop containing two Josephson junctions, as shown in [link] . When the loop is placed in even a very weak magnetic field, there is an interference effect that depends on the strength of the magnetic field.

Picture shows the schematics of a SQUID. Current enters a loop and split into two pathways. Two Josephson junctions are placed on the opposite sides of loop. Magnetic field goes through the loop perpendicularly to the current flowing through it. — The SQUID (superconducting quantum interference device) uses a superconducting current loop and two Josephson junctions to detect magnetic fields as low as $10^{−14} T$ (Earth’s magnet field is on the order of $0.3 \times 10^{−5} T$ ).

Superconductivity is a fascinating and useful phenomenon. At critical temperatures near the boiling point of liquid nitrogen, superconductivity has special applications in MRIs, particle accelerators, and high-speed trains. Will we reach a state where we can have materials enter the superconducting phase at near room temperatures? It seems a long way off, but if scientists in 1911 were asked if we would reach liquid-nitrogen temperatures with a ceramic, they might have thought it implausible.

Summary

Superconductivity is a phenomenon that occurs in some materials when cooled to very low critical temperatures, resulting in a resistance of exactly zero and the expulsion of all magnetic fields.
Materials that are normally good conductors (such as copper, gold, and silver) do not experience superconductivity.
Superconductivity was first observed in mercury by Heike Kamerlingh Onnes in 1911. In 1986, Dr. Ching Wu Chu of Houston University fabricated a brittle, ceramic compound with a critical temperature close to the temperature of liquid nitrogen.
Superconductivity can be used in the manufacture of superconducting magnets for use in MRIs and high-speed, levitated trains.

Key equations

Average electrical current	$I_{ave} = \frac{Δ Q}{Δ t}$
Definition of an ampere	$1 A = 1 C/s$
Electrical current	$I = \frac{d Q}{d t}$
Drift velocity	$v_{d} = \frac{I}{n q A}$
Current density	$I = \iint_{area} \vec{J} \cdot d \vec{A}$
Resistivity	$ρ = \frac{E}{J}$
Common expression of Ohm’s law	$V = I R$
Resistivity as a function of temperature	$ρ = ρ_{0} [1 + α (T - T_{0})]$
Definition of resistance	$R \equiv \frac{V}{I}$
Resistance of a cylinder of material	$R = ρ \frac{L}{A}$
Temperature dependence of resistance	$R = R_{0} (1 + α Δ T)$
Electric power	$P = I V$
Power dissipated by a resistor	$P = I^{2} R = \frac{V^{2}}{R}$

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Source: OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3

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