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?