Introduction to Electrostatics Coulombs Law and Methods of

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, q 1 and q 2, a center-tocenter distance r apart, is given by – where k = 9 109 N m 2/C 2

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 Nm 2/C 2 G = 6. 67 10 -11 Nm 2/kg 2 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 1011 N

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 F 12? What is the magnitude of F 13? What is the net force acting on q 1?