electrostatics
TRANSCRIPT
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Electrostatics Electrostatics Electrostatics Electrostatics
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• Charge is the total balance between electrons and protons– Charge is conserved
• Conductors easily allow the flow of electricity, insulators prevent it
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Static Electricity; Electric Charge and Its
Conservation
Charge comes in two types, positive and negative; like charges repel and opposite charges attract
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Static Electricity; Electric Charge and Its Conservation
Electric charge is conserved – the arithmetic sum of the total charge cannot change in any interaction.
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Electric Charge in the Atom
Atom:
Nucleus (small, massive, positive charge)
Electron cloud (large, very low density, negative charge)
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Electric Charge in the Atom
Atom is electrically neutral.
Rubbing charges objects by moving electrons from one to the other.
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Electric Charge in the Atom
Polar molecule: neutral overall, but charge not evenly distributed
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Insulators and Conductors
Conductor:
Charge flows freely
Metals
Insulator:
Almost no charge flows
Most other materials
Some materials are semiconductors.
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Metal objects can be charged by conduction:
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They can also be charged by induction:
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16.4 Induced Charge; the Electroscope
Nonconductors won’t become charged by conduction or induction, but will experience charge separation:
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Induced Charge; the Electroscope
The electroscope can be used for detecting charge:
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Induced Charge; the ElectroscopeThe electroscope can be charged either by conduction or by induction.
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Induced Charge; the Electroscope
The charged electroscope can then be used to determine the sign of an unknown charge.
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Summary• 4 ways to change the charge of an object
– Friction – friction causes some atoms to lose electrons, creating a negative charge
– Induction – The use of an outside charge polarizes the objects
– Conduction – Touching the two objects transfers charge
– Grounding – Charge is moved to a large object
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Coulomb’s Law
Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them.
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Coulomb’s Law
Coulomb’s law:
(16-1)
This equation gives the magnitude of the force.
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Coulomb’s LawThe force is along the line connecting the
charges, and is attractive if the charges are opposite, and repulsive if they are the same.
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Coulomb’s Law
Unit of charge: coulomb, C
The proportionality constant in Coulomb’s law is then:
Charges produced by rubbing are typically around a microcoulomb:
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Coulomb’s Law
Charge on the electron:
Electric charge is quantized in units of the electron charge.
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Coulomb’s Law
The proportionality constant k can also be written in terms of , the permittivity of free space:
(16-2)
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Coulomb’s LawCoulomb’s law strictly applies only to point charges.
Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.
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Solving Problems Involving Coulomb’s Law and Vectors
The net force on a charge is the vector sum of all the forces acting on it.
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• 2 electrons are placed 2 x 10-9m apart. What is the force each electrons applies to the other?– Force must be equal– Concept of net force is the same
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• To the previous problem we add a proton 1 x 10-9 to the left of the electrons. With what force, and in which direction will the middle electron move?