Final answer:
Replacing an air core with an iron core in an electromagnet significantly increases the magnetic field strength due to the high magnetic permeability and domain alignment in iron.
Step-by-step explanation:
When the air core of an electromagnet is replaced with an iron core, compared to the strength of the magnetic field in the air core, the strength of the magnetic field in the iron core is significantly increased. Iron is a ferromagnetic material, which means it has a high magnetic permeability and can enhance the magnetic field within an electromagnet by allowing the alignment of magnetic domains in the material. This alignment results in a stronger overall magnetic field, as the additive effect of the aligned domains increases the total magnetic field produced by the electromagnet compared to an air core.