Electrostatics

The History of Electrostatics

• Electricity has been studied since the Greeks• Benjamin Franklin made 1st major

scientific contributions about electricity.

Benjamin Franklin• Ben proved that lightning is “static

electricity”

• Imagined electricity as type of invisible fluid present in all matter.

• When matter contained too little, it was “negatively” charged.

• When it had an excess, it was “positively” charged

What is an electric charge?

Electric charges exist w/in atoms.

Atom Neutral

• + charge (protons) = -charge (electrons)

Transferring electrons cause an atom to gain/lose charge

• Like charges repel, opposite charges attract.

Subatomic ParticlesType of Particle

Charge (in C) Mass (Kg)

Proton +1.6 x 10 -19 1.67x10-27

Electron -1.6 x 10 -19 9.11x10-31

Neutron 0 1.67x10-27

C => Coulomb

SI measurement for electric charge

Conservation of Charge

• Charge is not created nor destroyed, it is only transferred from one object to another. *

• + and - charges are separated through a transfer of electrons.

*Compare this to The Laws of Conservation of Momentum, Matter, and Mass.

Conductors and Insulators

• Electric insulator• A material in which a charge does not move easily.• Glass, dry wood, most plastics, cloth, and dry air are all good

insulators.

• Electric conductor • A material through which a charge will move easily• Metals are good conductors because at least one electron from each

atom can easily be removed.

3 Ways to Charge an Object

• Friction• Conduction or contact• Induction• The Law of Conservation of Charge states that even after an

object is charged, the combined total charge remains the same.

Charging by Friction

• 2 neutral objects are rubbed together, each can become charged.• Electrons are transferred from the balloon to the hair.

Electricity by Friction

Charging by Conduction or Contact

• Charging a neutral body by touching it with a charged body. • Run a charged comb through your hair, it polarizes the neutral

dandruff and pulls it towards the comb.

Electricity by Conduction

Charging by Induction

• Charging an object without touching it– A charged object is needed to charge an object by induction.– There is never any contact made between the charged object and the object being charged.– Only conductors can be charged by the induction.

Charging by Induction

Lightning

Different Types of Lightning

Coulomb’s Law

Electric Static Force Equation Fe= K q1q2 / r2

• UNITS– Charge (q) measured in coulombs (C)• Proton charge +1.6 x 10 -19

• Electron charge -1.6 x 10 -19

– Distance (r) measured in meters– Force (F) in Newtons– Coulomb’s Constant (K) = -8.99 × 109 N·m2/C2

• Although Force is a vector quantity, Coulomb’s law does NOT give a direction

Similarities Between Coulomb’s Law andNewton’s Law of Universal Gravitation

Similarities and Differences Between Coulomb’s Law and Newton’s Law of Universal

Gravitation

• Both are inverse square laws, the magnitude of the force is dependent on the distance between the two objects

• Newton’s constant is much smaller than Coulomb’s, oftentimes the electrostatic force is much larger than gravitational force.

• Gravitational force affects objects over greater distances; electrostatic force is released before it grows too large.