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Meissner Effect.pptx
- 2. MEISSNER EFFECT • When a material makes the transition from the normal to superconducting state, it actively excludes magnetic fields from its interior; this is called the Meissner effect. • The German physicists Walther Meissner and Robert Ochsenfeld discovered this phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples. The samples, in the presence of an applied magnetic field, were cooled below their superconducting transition temperature(Tc), whereupon the samples cancelled nearly all interior magnetic fields.
- 3. • This Meissner effect happens when electric current loops spontaneously appear on the surface of a material that becomes superconducting in the presence of a magnetic field. These currents create a magnetic field, similar to that of an electromagnet. • Superconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism, meaning that the total magnetic field is very close to zero deep inside them (many penetration depths from the surface). • A superconductor with little or no magnetic field within it is said to be in the Meissner state. The Meissner state breaks down when the applied magnetic field is too strong. Superconductors can be divided into two classes according to how this breakdown occurs • In type-I superconductors, superconductivity is abruptly destroyed when the strength of the applied field rises above a critical value Hc. • Type II superconductors have a different magnetic behavior. It has two critical magnetic fields Hc1 and Hc2 below Hc1 it behaves as type I, and above Hc2 it becomes normal.
- 4. LONDON'S EQUATION : The Meissner effect was given a phenomenological explanation by the brothers Fritz and Heinz London, who showed that the electromagnetic free energy in a superconductor is minimized. ∇2 H = λ-2H ,this is known as London's equation where H is the magnetic field and λ is the London's penetration depth. This equation predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at the surface. This exclusion of magnetic field is a manifestation of the superdiamagnetism emerged during the phase transition from conductor to superconductor, for example by reducing the temperature below critical temperature.
- 5. APPLICATIONS OF MEISSNER EFFECT : This effect of superconductivity, is used in magnetic levitation which is the base of modern high-speed bullet trains. In superconducting state (phase), due to expulsion of external magnetic field, the sample of superconducting material levitates above magnet or vise-versa. Modern high-speed bullet trains use the phenomenon of magnetic levitation. Standard test – Proof for a superconductor A separation technique, based on the Meissner effect, to purify and classify superconducting powders was developed.