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Electromagnetic Charge
Atoms contain positively-charged protons in their nuclei, while smaller, negatively-charged electrons orbit around them in a cloud. Adding or removing protons to an atom is nearly impossible, except in high-energy nuclear fusion and fission. Instead, atoms and materials change net charge by losing and gaining electrons.
In a solid material, many atoms join together in an inflexible, lattice-like structure. Nearby atoms share and trade electrons easily, due to their fluid, fast-moving nature. A material may acquire a net negative charge by gaining electrons from another material. The extra electrons float freely throughout the lattice, belonging to no atom in particular but affecting the overall behavior of the material.
Obtaining a Negative Charge
A common way to create negative charge in a material is through friction. Rubbing two different insulators together causes static electricity, or an imbalance of electrons. Some materials, such as rabbit fur, glass and hair, tend to lose electrons in these interactions. Others, such as rubber, wood and cotton, tend to gain electrons.
You may transfer a negative charge to metals and other conductors by bringing them in contact with, or very close to other charged materials. The friction method works poorly with conductors, because electrons flow freely and do not stick in place, as with insulators.
Repulsion
Electromagnetic charges repel each other when they are similar, and attract with they are opposite. Thus, two negatively-charged materials will experience a force pushing them apart. The repulsion and attraction of charged materials is what causes common effects of static electricity. A charged material can bend a stream of water, cause plastic pellets to jump or make your hair stick up.
A neutral, conducting material may behave like it is negatively charged when brought close to an object with a net positive charge. The negative electrons are attracted to the positively-charged object, and in a conductor they are free to move close to the charged side. This creates a negative charge on the near side of the conductor, while the absent electrons leave a positive charge on the other side. Thus, a charged balloon may create an effect even on a neutrally-charged stream of water.
Discharge
Eventually, atoms tend to return to a neutral state that contains an equal number of protons and electrons. Rubbing two objects together may scrape off electrons temporarily, but simply touching the objects together can restore the electrons to their original places. You may also discharge extra electrons by touching conductors, such as a metal doorknob, or large neutral objects which will diffuse the extra electrons.