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General Physics 1
E1 Basic Electrostatics
Electrostatics
• Complete the tutorial on electrostatics (both sides of handout)
• Tape activity • www.sos.siena.edu/~rfinn/
phys140s09/tape.pdf
Electrostatics
• electric charges that are not moving
• Definition of new symbols• e- = electron• p = proton• n = neutron• e+ = positron
Nature of charge
• Amber is a natural substance that can be naturally charged
• Few naturally occurring objects can be electrically charged
• Static electricity is easily shown with plastic, rubber, and synthetic fibers
• Electrical charges behave like positive and negative numbers
Electric Force
• Similar form as gravitational force (weird, huh?), except• positive and negative charges
• Like charges repel; opposite charge attract
€
Fe =k q1q2r2
€
Fg =Gm1m2r2
A proton is located just north of an electron. The net force that the electron feels due to the proton is in which direction?
1. North2. South3. West4. East
Coulomb’s Law - magnitude
• k = 9.0 x 109 N m2/C2 • Coulomb’s contant
• q1, q2 = charges• measured in Coulombs (C)• 1 C is a lot of charge!
• r = distance between q1 and q2
€
Fe =k q1q2r2
Charge Carriers
• The electronThe electron• 1 e- = -1.602 x 10-19 C
• smallest unit of charge, also referred to as e• negatively charged• Charge of 1 e- is -1e• mass = 9.11 x 10-31 kg
• The protonThe proton • 1 p = 1.602 x 10-19 C
• positively charged• Charge of 1 p is +1e• mass = 1.67 x 10-27 kg
• Charge is quantized!
Coulomb’s Law with different constants
(more relevant to future chapters)
€
Fe =1
4πε0
q1q2r2
ε0 =permittivity of free space
Illustrations of Coulomb’s Law
• Comb & Paper• Balloon on wall
Illustrations of Coulomb’s Law
• Electroscope
induction conduction
Charge Conservation
• The total charge of an isolated system is conserved.• Rubbing a balloon on hair
• Triboelectric series• Decreasing tendency to
lose electrons
Conductors and Insulators
• Charges aren’t free to move around in an insulator• Why they may be charged• If charge is left on them due to rubbing
another material, the charge can’t go anywhere
• Examples – amber, plastics, synthetic materials
• Charges are free to move very easily in materials called conductors• Examples – metals, salty water
Conductor versus Insulator
Conductor Insulator
e-’s travel
charge separation within molecules
Charging by Induction
Net Force from Multiple Charges
• An electrostatic force exists between each pair of charges according to Coulomb’s law
• Add components of forces to get net force when adding multiple charges
€
Fe = Fe,12 + Fe,13 + Fe,23 =k q1q2r2
+k q1q3r2
+k q2q3r2
Adding Vectors
Magnitude and direction of Net Force given by:
The Atomic Model
How objects become charged
• A macroscopic object is composed of a huge number of tiny atoms
• Each atom has a tiny nucleus (includes protons and neutrons) surrounded by a cloud of electrons
• Equal number of electrons and protons• Avogadro’s number of protons or neutrons
have a mass of ~ 1 g.• Total mass of electrons is negligible• Most of the volume of an atom is occupied by
the electron clouds
Coulomb’s Law – examples
• Review Example E1.2• Determine direction of force
• Review Example E1.3• Determine direction and magnitude of force
Van de Graff Generator
• Demonstration• In-class• Pie plates• Ion motor
• Extra Credit - see schedule and moodle
Group Problems
• E1S.3• E1S.5• E1S.10
Charges in a line
• What is the net force on the 48 C charge?
Charges in a line - Solution
Q1 Q2 Q3
+x
Charges in a line - Solution
Q1 Q2 Q3
+x
Do signs make sense?!!
Charges in a square
Q1
Q4 Q3
Q2
Find net force on Q1
+y
+x
Charges in a square
Do signs of forces make sense?Does trig make sense?
The x and y components of the net force are given by:
Q1
Q4 Q3
Q2
Charges in a square
Charges in a square