Final answer:
The subject of this question is Physics and it pertains to the concept of electromagnetic induction. When the switch on the electromagnet is reopened, it causes a change in the magnetic field through the coil, leading to the generation of an electromotive force (emf).
Step-by-step explanation:
The subject of this question is Physics. The question is related to electromagnetism and the concept of electromagnetic induction.
Electromagnetic induction is the process of generating an electromotive force (emf) in a conductor by changing the magnetic field through it. When the switch on the electromagnet is reopened, the magnetic field through the coil changes, which induces an emf in the coil. This phenomenon is explained by Faraday's law of electromagnetic induction, which states that the emf induced in a circuit is proportional to the rate of change of magnetic flux through the circuit. When the switch is reopened, the magnetic flux through the coil changes, resulting in the generation of an emf.