1. When the magnetic flux in the area surrounded by the closed conductor loop changes, the induced potential and induced current will appear in the loop.

2, an air parallel plate capacitor, turn off the power supply after charging, then the energy stored in the capacitor is W. Then the distance between the two plates is doubled, and the energy stored in the capacitor is _2W_.

Judge:

1. The electric field intensity near the surface of electrostatic balance conductor is inversely proportional to the surface charge density. (error)

2. The magnetic flux through any closed surface must be zero. (right)

3. Ferromagnetism has strong magnetism. Ferromagnetism should be placed in electromagnets, motors and transformers. (error)

Any magnetic field has energy, and the energy of the magnetic field exists in the whole space where the magnetic field is located. (right)