Final answer:
A cloud of electrons around the filament is known as a space charge, and the resulting repulsion that hinders further electron emission is called the space charge effect.
Step-by-step explanation:
A cloud of negative electrons around the filament which repel one another and make it difficult for subsequent electrons to be emitted by the filament is called a space charge. This electrostatic repulsion is called the space charge effect.
Understanding the Concept
The phenomenon of space charge forms due to the presence of negative electrons that emit from a filament or cathode in devices like cathode ray tubes (CRTs) or X-ray tubes. As more electrons accumulate in the region close to the filament, their negative charges repel each other due to the electromagnetic force. This repulsion creates an area of high electron density that can hinder further emission of electrons because like charges repel each other, as stated by Coulomb's Law.
Coulomb's Law describes the force between charged particles: it varies directly with the product of their charges and inversely with the square of the distance between them. When this principle applies to the behavior of electrons around the filament, it predicts the observed repulsive forces that contribute to the space charge effect.