# Introduction to Electrostatics Coulomb’s Law and Methods of Charging.

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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-to- center distance r apart, is given by –where k = 9  10 9 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 conservedMass/energy is conserved k = 9  10 9 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  10 9 N Force between two 1 kg masses a distance of 1 m apart is 6.67  10 - 11 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 ?