Electrostatics

(aka

“Static Electricity”)

What is Electrostatics?

• Electrostatics is

the study of the

behavior of

stationary

charged objects.

It’s all about the electrons (in constant,

random motion around the nucleus) and

protons (stuck in the nucleus)

Negative

Electron

Nucleus:

positive

protons &

neutral

neutrons

The Atom & its particles…• Electrons are negatively charged particles

moving around the nucleus (in orbitals)

• electron = e-

• Protons are positively charged particles

located inside the nucleus (w/the neutral

neutrons)

• proton = p+

• Electrons are able to be stripped off an

atom (friction: fur & pvc pipe)

• Protons are NOT able to leave an atom

because they’re stuck in the nucleus!

Types of Materials

1. Conductor: a material that transfers charge easily (ex. Metals: gold, silver, copper).

2. Insulator: a material that does not transfer charge easily (ex. Plastics, glass, cork)

3. Semiconductors: somewhere between 1 & 2 (ex. Silicon, carbon, germanium).

4. Superconductors: some metals become perfect conductors below certain temperatures

5. Train Conductor: a person who drives a train.

Law of Conservation

of Electrical Charge• Electrons may be

transferred from one

object to another, but

they will never disappear

or appear from nowhere.

• Electron abundant

= negative charge

• Electron deficient =

positive charge

Electrically charged objects...• Exert a force on each other

• Unlike charges attract

• Like charges repel

There are

four methods

to charge an

object:

1. Charging by Friction

• using friction to remove electrons from one

object and placing them on the other object.

Result: two objects with opposite charge• Walk across carpeted floor with wool socks and you can build up an excess

of charge (either on you or the carpet) and therefore, become charged.

• Fur becomes positively charged. Why?

• Rod becomes negatively charged. Why?

• Can use friction to remove electrons from fur and deposit onto rod.

2. Charging by Contact (or

Conduction)

• The process of giving

one object a net

electric charge by

placing it in contact

w/another object that is

already charged is

known as charging by

contact.

• Result: two objects with

same charge

3. Temporarily Charging by Induction

• An object may become

polarized (opposite ends)

while a charged object is

brought near. But, the

overall charge on this piece

of plastic (for example) is

neutral.

• Result: two objects with

opposite charge temporarily

• When the charged rod goes

away, the electrons

rearrange and disperse

evenly. Object is still

neutral.

4. Permanently Charging by Induction

• The process of giving one object a net electric

charge without touching the object to a second

charged object is called charging by induction.

Permanently Charging

by InductionPolarization

5. Credit Card Charging:

• You may use Visa, Master Card, or

American Express

• Result: Debt from high interest rates!

Check out these animations!

• http://phet.colorado.edu/simulations/sims.p

hp?sim=John_Travoltage

• http://phet.colorado.edu/simulations/sims.p

hp?sim=Balloons_and_Static_Electricity

Van de Graaf

Generator

Electric Fields• What is an electric field?

• A region of space where a charge exerts a force on other charged objects. This field extends outward and permeates all of space.

• Direction of Electric Field = Direction of Force the field produces on a positive charge in the field.

• In comparison: What is a gravitational field?

• How to measure an electric field: Using a SMALL POSITIVE test charge, move it around a charged object (that is creating an Electric Field) and the magnitude and direction of force acting on the small positive test charge is measured.

• The strength of the field is measured based on the force on the test charge.

• F = The force felt by (acting on) the test charge, q, due to the point charge’s electric field. (N)

• q = the charge of the test charge. (C)

• E = Electric Field Strength. (N/C) (Direction is based on imagining a small positive charge in the field.)

• The Electric Field (E) at a given spot can exert a variety of forces, depending on the magnitude of the test charge placed there.

• E = F/q = F/q = F/q• E is constant for that particular point charge at a

given location (the Point Charge is the charge whose field is being tested… the point charge is creating the Electric Field. )

q

FE

qon

....

q

FE

Compare

• Compare the electric field equation to the one we

used to measure Earth’s gravitational field

strength:

• So, Electric Fields are measured in N/C and are

a measure of the force felt by a small positive

test charge. (The test charge must be small in

charge so that we can ignore its electric field.)

m

Fa

q

FE

Electric fields: Man holding point charge, girl

holding test charge.

Must click on 2nd link!

• http://phet.colorado.edu/simulations/sims.p

hp?sim=Electric_Field_Hockey

http://phet.colorado.edu/simulations/sims.ph

p?sim=Charges_and_Fields

http://phet.colorado.edu/simulations/sims.ph

p?sim=Electric_Field_of_Dreams