1. What is the two-fluid model of superconductors ?

In the two-fluid model, below T_c, the superconductor is considered to have two types of electrons normal and superconducting. The fraction of normal electrons decreases with temperature and is finally zero at 0K.

2. Although the dc conductivity of a superconductor can be infinite, why is it finite in an ac electric field ?

Since the imaginary part of the conductivity remains finite for non-zero frequency and approaches as the conductivity is finite in an ac electric field.

3. State the two London equations. Based on them, how can Meissner effect be explained ?

                  (1)

                                             (2)

4. For use in superconducting magnets, are superconductors with low or high critical fields desirable ?

Materials with high critical fields are desirable so that they can be used to generate high magnetic fields.

5. A substance has a transition temperature of 100 K and the zero-temperature critical field is 100 Tesla. What field has to be applied at 50 K in order to destroy superconductivity in this material ?

Using the formula

we get Tesla.

6. Why is there a limit to the the quantity of current that can pass through a superconductor beyond which it becomes normal (non-superconducting)?

The current passing through a superconductor will generate a magnetic field on itself which increases with increasing current. When that exceeds the critical field, the material will become normal.