introduction to electrostatics
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Introduction to Electrostatics
Coulomb’s Law and Methods of
Charging
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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
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Rutherford model of the
atom
• Dense positive nucleus with positive protons and neutral neutrons
• “Cloud” of negatively charged electrons surrounding nucleus
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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
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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
r
qqkFE
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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
r
qqkFE 2
21
r
mmGFG
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Conductors
• Conductors are materials that allow the electrons in the outer parts of the electron “cloud” to move freely from atom to atom
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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
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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.
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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.
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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
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Methods of charging• Charging by conduction (direct contact)
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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.”
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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
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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
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• What method is used here to charge the electroscope?
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What is the magnitude of F12? What is the magnitude of F13?
What is the net force acting on q1?
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