Electrostatics
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Electrostatics. The study of electric charges. What do you know about atoms?. All matter is composed of atoms We call different kinds of atoms elements Atoms form compounds Compounds have different properties, including different electrical properties. Structure of the Atom.

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Electrostatics

Electrostatics

The study of electric charges


What do you know about atoms
What do you know about atoms?

  • All matter is composed of atoms

  • We call different kinds of atoms elements

  • Atoms form compounds

  • Compounds have different properties, including different electrical properties


Structure of the atom
Structure of the Atom

  • The term “atom” comes from “atomos” meaning indivisible

  • Now we know atoms have a nucleus comprised of smaller parts including protons and neutrons

  • Electrons are negatively charged

  • The nucleus is positively charged (protons are positively charged, neutrons are neutral)

  • Electrons travel around the nucleus in the electron cloud


Electric charge
Electric Charge

  • The make-up of an element will determine how it is electrically charged (will it gain or lose electrons?)

  • If an atom has the same number of electrons and protons it is considered electrically neutral

  • An atom with unequal numbers of protons and electrons can be considered “electrified” or electrically charged


Ions

  • An ion is an atom or molecule that has unequal numbers of protons and electrons

  • An atom with more electrons than protons is negatively charged

  • An atom with more protons than electrons is positively charged

  • Remember, only electrons can move . . . Protons are tightly bound within the nucleus


Like charges
Like Charges

  • Why do the tapes have the same charge?

  • Objects charged in the same way acquire the same charge (the tape pieces placed on the desk)

  • Like charges repel


Unlike charges
Unlike Charges

  • Objects acquire different charges when charged differently (the ruler and the piece of wool)

  • This was demonstrated with the tape . . . The _______ repels, the ___________attracts

  • Unlike charges attract


Review true or false
Review: True or False

  • _______: The wool in our lab activity became negatively charged. (The wool loses electrons)

  • _______: The ruler in our lab activity became negatively charged.

  • _______: The gravitational force is stronger than the electrical force.

  • _______: Electrical forces can be either attractive or repulsive.

  • _______: Friction is the only way to charge a neutral object.


Static charge and fuel fires
Static Charge and Fuel Fires

  • http://vimeo.com/peijournal/static


Electric force
Electric Force

  • An electric force is due to electric charges

  • Electric force is a non-contact force . . . Objects can experience the force without coming into contact

  • Opposite charges attract

  • Like charges repel

  • How did we see a force at work in the lab?


Comparing electric and gravitational forces
Comparing Electric and Gravitational Forces

  • The electrical force is stronger than the gravitational force (We saw this when the paper jumped up from the table to the ruler).

  • Electrical charges collected on an object can be lost to surroundings

  • Gravitational forces can only be detected when observing large bodies like the Earth or sun

  • The electrical force can be either attractive or repulsive

  • The gravitational force is only attractive


Separation of charge
Separation of Charge

  • The wool and the ruler became charged because electrons are transferred from the wool to the plastic

  • The extra electrons on the ruler result in a net negative charge

  • The missing charges from the wool result in a net positive charge

  • The net charge between the objects remains the same

  • Objects become charged due to the transfer of charges



Conductors
Conductors

  • Conductors are materials that allow electrons to flow freely

  • Conductive materials will allow charge to distribute across the entire surface of the material

  • This occurs because of the repulsive nature of electrons

  • Conductors can transfer charge to other objects

  • Examples include metals(electrons can be removed easily), salts, water, humans


Insulators
Insulators

  • Insulators are materials in which charge will not move easily

  • The charge does not distribute over the object but rather stays in one location

  • Examples include glass, dry wood, plastics, dry air


Charge as a quantity
Charge as a quantity

  • The charge of an atom can be measured

  • This charge is measured in Coulombs (C)

  • Each electron holds a negative charge of -1.6 x 10-19 Coulombs

  • Each proton holds a positive charge of 1.6 x 10-19 C.


Measuring the electric force
Measuring the Electric Force

  • Electric force is a vector quantity

  • The closer two charged objects are to one another the larger the electric force

  • The further away two charged objects are from one another the smaller the electric force


Coulomb s law
Coulomb’s Law

  • States that the that the electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects

  • Assume that any collection of charges is located in the center of an object


Calculating force
Calculating Force

F = Force

k = constant, based on the medium the object is immersed in (air)

For air, the value of k = 9.0 x 109 N·m2/C2

Q = charge of each object

d = distance separating the objects


Example 1
Example 1

Johnny’s soccer ball has a positive charge of +6.0 x 10-6 C. Rachel’s soccer bag, with a charge of +3.0 x 10 –6 C, is 3.0 m away.

a. Calculate the force on the soccer ball.

b. Calculate the force if Rachel’s bag had a negative charge.


What do you know
What do you know?

Q1 = 6.0 x 10-6 C

Q2 = +3.0 x 10 –6C

d = 3.0 m

k = 9.0 x 109 N·m2/C2

F =


Polarization
Polarization

  • The process of separating charges into opposites within one object

  • One side of an object is positive and the other side is negative

  • One charged object is required to induce electron movement or electron rearrangement

  • Polarization occurs when the molecules within an object reorient based on a nearby charged object (balloon and the wall; paper and the ruler)


Review
Review

  • There are two types of electric charges

  • Charges exert forces on other charges at a distance

  • The force is stronger when the charges are closer together

  • Like charges repel, unlike charges attract


How can we determine charge on objects
How can we determine charge on objects?

  • An electroscope consists of a metal knob connected by a metal stem to two thin lightweight pieces of metal foil, called leaves

  • There are three different ways to charge objects

    • Friction

    • Conduction

    • Induction


Charging by friction
Charging by Friction

  • Friction occurs when two objects are rubbed together

  • Electrons transfer from one object to another

  • Both objects are left with an imbalance of charge, one positive and one is negative


Charging by conduction
Charging by Conduction

  • A charged object comes in contact with a neutral object

  • If a negatively charged object touches another object, the excess electrons transfer to the neutral object

  • The neutral object becomes charged, and is no longer neutral

  • If a positively charged object touches a neutral object, it takes electrons from the neutral object

  • The positively charged object becomes neutral, and the neutral object becomes positively charged (it has lost electrons)


Charging by induction
Charging by Induction

  • Process of charging an object without touching it (bringing one charged object near an neural object)

  • Situation: Two identical neutral spheres are touching. A negatively charged rod is brought near one sphere. Because of the repulsive nature of the electrons, the electrons run to the second sphere. Sphere 1 becomes positively charged, sphere 2 becomes negatively charged. (See Firgure 20-8 on page 548)

  • Grounding occurs when a single object contacts Earth to eliminate excess charge


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