3. When does the iron core in an electromagnet become magnetic?

When electricity flows through it.

Explanation:

The electromagnet exhibits magnetic properties only as soon as current flows through the nucleus. Namely, the electric current in the space around the electrical conductor through which it passes creates a magnetic field. Unlike a permanent magnet, which has a permanent property of magnetism, the electromagnet is a temporary magnet because the magnetic field also disappears when the current ceases.

if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask

their crystal patterns have been disrupted by stress