Introduction to Electrostatics

Coulomb’s Law and Methods of

Charging

Rutherford Experiment (1911)

• Positive charges called alpha particles were shot toward a thin gold foil.

• Scattering of these particles revealed that the gold atoms had

– Very small positive nucleus

– Larger diffuse “cloud” of negative charge

Rutherford model of the

atom

• Dense positive nucleus with positive protons and neutral neutrons

• “Cloud” of negatively charged electrons surrounding nucleus

Electrical Charges• Two types of charge – “positive” and

“negative” (coined by B. Franklin in 1750’s)

• Charge is conserved – never created or destroyed

• Like charges repel, unlike charges attract• Charge is measured in coulombs (C) after

Charles-Augustin de Coulomb

Electrical Forces• Like charges repel, unlike charges

attract• Coulomb’s Law

– The force between two charges, q1 and q2, a center-to-center distance r apart, is given by

– where k = 9 109 Nm2/C2

221

rqqkFE

Coulomb’s Law vs. Newton’s Law of Gravity

Like charges repel; unlike charges attract

Masses attract, never repel

Unit of charge is coulomb (C)

Unit of mass is kilogram (kg)

Charge is conserved Mass/energy is conserved

k = 9 109 Nm2/C2 G = 6.67 10-11 Nm2/kg2

Force between two 1 C charges a distance of 1 m apart is 9 109 N

Force between two 1 kg masses a distance of 1 m apart is 6.67 10-11 N

221

rqqkFE 2

21

rmmGFG

Conductors• Conductors

are materials that allow the electrons in the outer parts of the electron “cloud” to move freely from atom to atom

Insulators• Insulators are materials that do not

allow the electrons in the outer parts of the “cloud” to move freely. All electrons must stick with their atom.

• Examples of insulators are:– Glass, paper, plastic, wood, pure water

• Examples of conductors are:– Metals, salt water

When a charged object approaches a conductor:

• The outer electrons from the atoms can move toward a positive object, or away from a negative object. The neutral conductor is attracted to the charged object.

When a charged object approaches an insulator:

• The electrons are trapped, but the electron cloud can distort – it polarizes – to allow the clouds to get slightly closer to a positive object or slightly farther from a negative object. The neutral insulator is attracted to the charged object.

Methods of charging• Charging by friction

• Because of conservation of charge, the amount of negative charge removed from one object exactly equals the amount of positive charge left on that object

Methods of charging• Charging by conduction (direct contact)

Methods of charging• Charging by induction

• In this process, the charged object that acts as a “lure” never touches the object to be charged… Something else touches the object, or the object is “grounded.”

Grounding• The Earth is overall considered

neutral• It acts as a giant ocean of equal

amounts of positive and negative charge

• It can be a large “source” or “sink” for excess charge

Electroscope• The electroscope is a device used for

detecting excess charge• It is composed of a conductor attached

to a movable metal leaf or arm• When a charged object is brought

near, charges in the conductor redistribute themselves and cause the leaves to move

• What method is used here to charge the electroscope?

What is the magnitude of F12? What is the magnitude of F13?

What is the net force acting on q1?