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