## Pages

### Superconductors and Superconductivity

Phenomenon of superconductivity was first observed by Kamerlingh Onnes in Leiden in 1911 ,3 years after he first liquified helium gas. He observed that electrical resistivity of such as mercury, tin, lead completely disappear i.e., suddenly dropped to zero in a small range of temperature at a critical temperature TC which is the characterstics of the material. This heppens when the specimen is cooled down to sufficiently low temperature about few degrees of Kelvin. At critical temperature TC specimen undergoes a phase transition from normal electrical resistivity state to superconducting state. Magnetic properties exhibited by superconductors are as interesting as its electrical properties. Now imagine what happened when a specimen known to exhibit superconductivity is placed in a magnetic field and s then cooled through the transition temperature for superconductivity. In this case magnetic field originally present in the specimen is ejected out from the specimen and this is nothing but the Meissner Effect. Information i had given here in this page is nothing but a mere definition of superconductivity phenomenon. It is a very vast topic and has a lot in it. For more information on superconductors you can visit

### Ampere's Law

For more physics study material and notes visit physicscatalyst.com

• Ampere's law in magnetism is similar to the Gauss's law in electrostatics.
• With the help of this law we can find the magnetic fiels caused by symmetric current configurations.
• Statement:-"The line integral B.dl around any closed path equals μ0I ,where I is the net steady curent passing through any surface bounded by a closed path. Mathematically
B.dl = μ0I
where integral is evaluated along a closed path.
• Without going into the details of how to prove the law I will give you some handy things to remember while solving problems using Ampere's law.
1. If B is everywhere tangent to the path of integration and has same magnitude B at every point on the amperian loop then the line integral becomes equal to B times the circumfrance of the path.
2. If B is everywhere perpandicular to the amperian loop or over some portion of the loop then that portion of the loop does not make any contribution to the line integral because B.dl = Bdlcos90 = 0
3. In the integral ∫B.dl B is the the total magnetic field at each point in the path i.e., field at any point P on the loop is due to both current sources inside and outside the Amperian loop . However the integral ∫B.dl is always zero for currents outside the Amperian loop.
4. Things should be keept in mind before choosing the Amperian loop that points at which field is to be determined must always lie on the Amperian loop and the path or the loop must have enough symmetry so that line integral can easily be evaluated.