ElectrostaticsElectric Charge, Field, Potential,

and Potential Energy

1. Electrostaticsa) 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 Chargeis 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.

FG

FE

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. 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

For any charge q:

q = ne , where n = 1, 2, 3, etc…

…

Charge is quantized

= e = elementary

charge

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.

Most things are in between perfect conductor/ insulator (semiconductors)

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.

b) Methods of Charging

A

Separating Charges

silk

glass

(rub)

- --

(rub)

- --

+ + + +

fur

plastic- - - -

Web Links: Static Duster© Laura Fellman

Grounding

- - - -

The earth is a huge reservoir of positive and negative charge

+ +

+

+

++

+

+

+-

-

--

-

--

--

-

-

---

- Object is discharged or “grounded”

© Laura Fellman

When finger is removed

When charged rod is removed

Electrically neutral

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?

lectronse 10 x 622.2C 10 x 602.1

electron 1 C 10 x 42 14

196

kg 10 x 389.2e 1

kg 10 x .119e 10 x 622.2 16

3114

The sphere gives the girl a large negative charge. Each strand of hair is trying to:

1) Get away from the charged sphere.2) Get away from the ground.3) Get near the ceiling.4) Get away from the other strands of hair.5) Get near the wall outlet.

Like charges attached to the hair strands repel, causing them to get away from each other.

ElectrostaticsElectric Charge, Field, Potential,

and Potential Energy

More information athttp://ap-physics.david-s.org