Electrostatics Electric Charge, Field, Potential, and Potential Energy 1. Electrostatics a) Electric Charge

a) Electric Charge

If an atom has the same amount of + and - charge Neutral (no net charge) If it’s missing electrons net + charge If it has extra electrons net - charge

Electric Charge is represented by the symbols Q or q and is a property of matter. The smallest charge found isolated in nature is the charge on one electron (we’ll write it e-), which is 1.6 x 10-19 coulombs.

Subatomic particles, such as quarks, have smaller charges—but those charged particles do not exist as isolated charges (as far as we know). This might represent a proton—made up of two “up” quarks (each with a charge of +2/3) and one “down” quark (with a charge of –1/3).

The well-known “Millikan Oil Drop Experiment” determined the amount of charge on one electron. FE FG

A proton (p+) has exactly the same charge as an electron, 1 A proton (p+) has exactly the same charge as an electron, 1.6 x 10-19 C, even though a proton has 1837 times the mass of an electron. A positron (e+), on the other hand, has the same mass as an electron and the same magnitude of charge.

The positron is the antiparticle of the electron The positron is the antiparticle of the electron. Most particles in the Universe have antiparticles. Under the right conditions, particle and antiparticle collisions will cause “annihilations” . (But matter and energy are conserved.)

Smallest possible amount of charge: 1 extra electron: q = -1.60 x 10-19 C 1 missing electron: q = +1.60 x 10-19 C = e = elementary charge … For any charge q: q = ne , where n = 1, 2, 3, etc… Charge is quantized

As you know, like charges (positive-positive or negative-negative) exert forces of repulsion on each other, and unlike charges (positive-negative) exert forces of attraction on each other.

Conductors and Insulators Conductors: are typically metal. In metals, valence electrons are not involved in the interatomic bonds that hold the metal object together. These electrons are able to move around within the object. Insulator: a substance that does not allow electrons to move. Typically non-metals with electrons bound to nucleus and not free to move around. Most things are in between perfect conductor/ insulator (semiconductors)

b) Methods of Charging

Separating Charges A

- - - - - - - - - - + + + + glass (rub) silk plastic (rub) fur Web Links: Static Duster © Laura Fellman

- - - - - - Grounding Object is discharged or “grounded” + The earth is a huge reservoir of positive and negative charge + - © Laura Fellman

1) Charging by Conduction

2) Charging by Induction Electrically neutral When finger is removed When charged rod is removed Net positive charge ( on outer surface)

Q. A person scuffing her feet on a wool rug on a dry day accumulates a net charge of -42 mC. How many excess electrons does she get, By how much does her mass increase?

Van de Graaff The sphere gives the girl a large negative charge. Each strand of hair is trying to: Get away from the charged sphere. Get away from the ground. Get near the ceiling. Get away from the other strands of hair. Get near the wall outlet. Like charges attached to the hair strands repel, causing them to get away from each other.

Lightning Physics

Electrostatics Electric Charge, Field, Potential, and Potential Energy More information at http://ap-physics.david-s.org