Physics
In superconductors, the energy gap is due to

Electron-electron interaction
Electron-photon intercation
Phonon-phonon interaction
Electron-phonon interaction

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Physics
Two identical solid pieces, one of gold and other of silver, when immersed completely in water exhibit equal weights. When weighed in air (given that density of gold is greater than that of silver)

Weighing will depend on their masses
The gold piece will weigh more
The silver piece will weigh more
Both silver and gold pieces weigh equal

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