“It is said that an iron piece behaves as a magnet when an external magnetic field is applied. Then otherwise why doesn’t it behave as a magnet even though the earth’s magnetic field is acting on it.”
Every molecule of iron possess magnetic behavior. But in the absence of an external strong magnetic field, the individual molecular magnets form closed loops or they are arranged in a random fashion in such a way that the net magnetic effect is zero.
Since the magnetic dipole moments of all the molecular magnets inside the iron add up to zero, the iron rod does not respond to earth’s magnetic field like a magnet.
Further, the strength of earth’s magnetic field is not so large to magnetize a piece of iron. The coercivity of iron is also very less (it can easily lose any acquired magnetism)
However, steel buried in North South direction is found to acquire some magnetism.