Electric Charge and Fields Notes

Objects can become electrically charged, as demonstrated by rubbing a plastic comb through hair or a balloon against hair on a dry day.
Excessive moisture can cause charge to leak off.
Rubbing shoes on a wool rug or sliding across a car seat can give your body an electric charge.
There are two kinds of electric charge: positive and negative, named by Benjamin Franklin.
Like charges repel, and unlike charges attract.
By convention, hair rubbed by a balloon becomes positively charged, while the balloon becomes negatively charged.
An object with equal amounts of positive and negative charge has no net charge.
Electrostatic spray painting uses the attraction between unlike charges to reduce paint waste.

Atoms contain positively charged protons and uncharged neutrons in the nucleus, and negatively charged electrons outside the nucleus.
Electrons are easily transferred from one atom to another.
An atom with balanced electrons and protons is neutral; otherwise, it's an ion.
Charge is transferred between unlike materials when they are rubbed together, increasing the contact area.
When a balloon is rubbed against hair, electrons transfer from hair to the balloon, making the balloon negative and the hair positive.
Electric charge is conserved; no charge is created or destroyed. This is a fundamental law of nature.
Cosmetic products use positively charged chitin to stick to negatively charged hair and skin.

Robert Millikan's oil-drop experiment (1909) showed that charge is quantized.
Charge occurs as integer multiples of a fundamental unit of charge, $e$ .
The electron has a charge of $-e$ , and the proton has a charge of $+e$ .
The value of $e$ is approximately $1.602 \times 10^{-19} C$ , where C is the coulomb, the SI unit of electric charge.
A total charge of $-1.0 C$ contains $6.2 \times 10^{18}$ electrons.
In electrostatic experiments, a net charge on the order of $10^{-6} C$ (
$= 1 \mu C$ ) is obtained.

Electrical conductors are materials in which electric charges move freely (e.g., copper, aluminum, silver).
Electrical insulators are materials in which electric charges do not move freely (e.g., glass, rubber, silk, plastic).
Semiconductors have electrical properties between those of insulators and conductors (e.g., silicon, germanium).
Superconductors have zero electrical resistance below a critical temperature.

Charging by contact occurs when objects are rubbed together, like a balloon and hair.
Examples include rubbing a glass rod with silk or a rubber rod with wool.
Insulators and conductors can be charged by contact.
If a copper rod is held without an insulating handle, charges flow through the body to the Earth causing the rod to become neutral again.
If a copper rod is held with an insulating handle, the rod remains charged.

A conductor connected to the Earth is grounded and Earth can accept an unlimited number of electrons.
Charging by induction involves bringing a charged object near a neutral conductor, causing charge redistribution.
If a grounded conducting wire is connected to the sphere some electrons leave the sphere and travel to Earth.
The charge is induced on the sphere.
Induction requires no contact with the object inducing the charge but does require a third object.
A sink is a system that can absorb a large number of charges, such as Earth, without becoming locally charged itself.

Polarization is a process where the centers of positive and negative charge in a neutral atom or molecule shift slightly in the presence of a charged object.
This creates an induced surface charge on the insulator.
Polarized objects have no net charge but can still attract or repel objects.
This explains how a plastic comb attracts small pieces of paper.