Warm-up

• Like charges __________ and unlike charges __________

Electrostatics

What is electrostatics?

• The study of electrical charges that are held in one place and not in motion– Electro - Electric stuff– Statics - not moving

• Good in theory, not in practice, because charges don’t like to stay still

What is charge?

• Charge is an electrical attraction or repulsion due to absence or excess of electrons

• Objects can be either positive, negative, or neutral in charge– Positive --> absence of electrons– Negative --> excess electrons– Neutral --> equal

How is charge measured?

• Charges are represented in Coulombs (C)– One C is equal to the charge of

6.25 x 1018 electrons

Conductors and Insulators

• Conductors– Easily conduct electricity– Charges easily move– Metals (copper, aluminum, etc)

• Insulators– Don’t conduct electricity– Charges don’t easily move– Glass, dry wood, most plastic, dry air

What about air? (don’t need to write down)

• Dry air is a good insulator– Doesn’t conduct electricity well– About 33 kV/cm

• Moist or wet air conducts better– Electrons can escape through the water in the air

*Old house story (then generator)

Coulomb’s law

• Positive --> like charges --> repel• Negative --> unlike charges --> attract

• Coulomb’s law describes how like or opposite charges attract or repel

F = Force q1= charge 1 q2 = charge 2

d2 = distance squared

What is this k?

• k = 9.0 x 109 Nm2/C2

– Called “Coulomb’s Law Constant”

• K is related to the permittivity of free space– Basically, how easily charge can communicate

though empty areas

• If there is more than one charge acting on an object, the sum of the forces is the net force

Charging stuff

• Something can be charged in two ways:– Conduction

• When two items contact to exchange charges

– Induction• When two items come close but do not touch

and charges move in one object to orient the positive and negative charges

*Demonstrations

Alright…a few whiteboard problems

A positive and a negative charge, each of 1.5 x 10-5 C,

are separated by .15 m.

• Is the force attractive or repulsive?

• What is the force?

Two electrons in an atom are separated by a distance of

1.5 x 10-10 m.

• If each electron has a charge of 1.60 x 10-19 C, what is the force that repels the two electrons?

• If the mass of an electron is 9.11 x 10-31 kg, what is the acceleration of the masses if they were set at rest? (reminder a = F/m)

A positive charge of 3.0 x 10-6 C is pulled on by two negative

charges, A and B.

• A is 0.05m to the north with a -2.0x10-6 C• B is 0.03m to the south with a -4.0x10-6 C

• What is the force from A?• What is the force from B?• What is the total force on the charge?

• A proton and electron have equal and opposite charges of 1.60 x 10-19 C

• In a special atom, there are 2 electrons and one proton separated by a 90o angle with the proton at the vertex.

• If the distance between the proton and electron is 5.3 x 10-11 m, – What is the force of one of the electrons on

the proton?– what is the net force on the proton?