In atoms:

* Electrons are fundamental particles: They're not made of anything smaller. They exist within the atom, orbiting the nucleus.

* The nucleus is the source of protons: Protons are positively charged particles in the nucleus. To maintain electrical neutrality, atoms have an equal number of protons and electrons.

In everyday materials:

* Metals are good conductors: They have loosely bound electrons that can easily move throughout the material. This makes metals excellent conductors of electricity.

* Semiconductors: These materials have properties between those of metals and insulators. They can conduct electricity under certain conditions.

* Insulators: These materials have tightly bound electrons that are difficult to remove. They don't conduct electricity well.

In electrical circuits:

* Power sources: Batteries and generators provide the energy to move electrons through a circuit.

* Current flow: When a voltage is applied to a circuit, it creates an electric field that pushes electrons through the conductor.

In other contexts:

* Thermionic emission: Heating a metal can cause electrons to gain enough energy to escape from the surface.

* Photoelectric effect: Light can knock electrons loose from the surface of certain materials.

* Radioactive decay: Some radioactive isotopes release electrons (beta particles) as they decay.

To summarize:

Electrons are fundamental particles, and their source depends on the specific situation. Whether you're looking at an atom, a piece of metal, an electrical circuit, or a radioactive decay process, the source of electrons can be explained by the underlying principles of physics and chemistry.