In superconductivity, is resistance =0? If a metal shows superconductivity of electricity at definite temperature, then during superconductivity can we consider resistance of metal = 0? 
 A: Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. When you cool metal to its critical temperature it becomes a superconductor.

In Metal there are Cooper pairs and it doesn't have enough energy to break these pairs. If ΔE is larger than the thermal energy of the lattice, given by kT, where k is Boltzmann's constant and T is the temperature (In Kelvin), the fluid will not be scattered by the lattice. The Cooper pair fluid is thus a superfluid, meaning it can flow without energy dissipation.$$E = kT$$Where:$$k = 1.3806488 × 10^{-23}\,m^2\;kg\;s^{-2}\;K^{-1}$$ Superconductivity means that metal resistance has dropped to exactly 0. So answer is yes
source: Wikipedia http://en.wikipedia.org/wiki/Superconductivity
EDIT: What about battery's internal Resistance? If you added Battery in circuit then you must add battery's internal resistance too (It will be small (10 - 100Ohms) but not zero) V = IR = I(R_superconcuctor(which is zero) + R_battery). But after disconnecting battery, voltage and resistance will be zero but current will still be flowing
EDIT 2: Cooper pair or BCS pair is two electrons (or other fermions) that are bound together at low temperature. Cooper pairing is a quantum effect, the reason for the pairing can be seen from a simplified classical explanation. An electron in a metal normally behaves as a free particle. The electron is repelled from other electrons due to their negative charge, but it also attracts the positive ions that make up the rigid lattice of the metal. This attraction distorts the ion lattice, moving the ions slightly toward the electron, increasing the positive charge density of the lattice in the vicinity. This positive charge can attract other electrons. At long distances this attraction between electrons due to the displaced ions can overcome the electrons' repulsion due to their negative charge, and cause them to pair up. The rigorous quantum mechanical explanation shows that the effect is due to electron–phonon interactions. For more info: https://en.wikipedia.org/wiki/Cooper_pair

