Electrostatics

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Electrostatics
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Electrostatics

• Electrostatics is electricity at rest.
• Has to do with subatomic particles and their
  interactions with each other.

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Electrostatics

• Atoms have subatomic particles
• Protons
• Neutrons
• Electrons

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Protons

• Are positively charged particles
• Are located in the nucleus of the atom
• Have mass of 1.7 x 10-27 kg

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Neutrons

• Are neutral neither positive or negative.
• Are located in the nucleus of the atom
• Have a mass slightly larger than a proton.
  

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Electrons

• Are negatively charged.
• Are located outside of the nucleus.
• Have a mass of 9.1 x 10-31 kg

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Necessary Assumptions

• All protons are identical to each other proton.
• All neutrons are identical to each other neutron.
• All electrons are identical to each other
  electron.

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Electrical Forces

• Electrical Forces stem from positive and negative
  charges and how they interact with each other.
• Like charges repel
• Opposite charges attract

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Charges

• The SI unit for charge is the Coulomb.
• 1 Coulomb carries the equivalent of 6.24 x 1018
  electrons.
• This is roughly the amount of electricity needed
  to power a 100W lightbulb for about 1 second.

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Charges

• The charge on a proton (qp) is
  1.6 x 10-19 Coulombs
• The charge on an electron (qe) is -1.6 x 10-19
  Coulombs
• There is no charge on any neutron.

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Charges

• Sodespite the difference in mass between protons
  and electrons, they carry equivalent magnitude
  charges.

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Conservation of Charge

• If you add or remove electrons from an atom, you
  create an ion.
• Add an electron, the charge of the atom becomes
  negative, we call this an anion.
• Remove an electron, the charge of the atom
  becomes positive, we call this a cation.

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Ions

• Ions are electrically charged.
• Any object that has unequal numbers of protons
  and electrons is considered to be electrically
  charged.

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Ions

• When ions are created, electrons are neither
  created nor destroyed.
• The electrons merely move from one atom to
  another.
• The charge is not lost, just moved.
• We call this Conservation of Charge.

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Coulombs Law

• Coulombs Law explains the relationship among
  force, charge, and distance of particles.
• It is related to the gravitational force equation.

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Coulombs Law

• Fkq1q2/d2

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Coulombs Law

• Where F is force and is measured in Newtons
• Where k is the proportionality constant. It has a
  value of 9.0 x109 Nm2/C2
• Where q1 is the quantity of charge in particle 1
  and is measured in Coulombs
• Where q2 is the quantity of charge in particle 2
  and is measured in Coulombs
• Where d is distance and is measured in meters.

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Sample problem

• Consider two charged objects. One carries a
  charge of 18µC (0.000018C). When the two are
  separated by a distance of 0.9m, there is a force
  of 2.7 N between them. What is the charge on the
  2nd object?

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Solution

• Fkq1q2/d2

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Solution

• F 2.7 N
• k 9.0 x 109
• q1 1.8 x 10-5
• q2 unknown
• d 0.9

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Solution continued

• Fkq1q2/d2
• 2.7 9.0 x 109 (1.8 x 10-5)(q2) (0.9)2
• q2 13.5 µC

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Insulators

• Objects that do not permit freedom of electron
  movement are considered to be insulators.
• The more tightly bound the electrons are, the
  better the insulator.
• Non-metals are usually good insulators (both of
  heat and electricity.)

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Conductors

• Any object that allows free movement of electrons
  is considered a conductor.
• The easier electrons move, the better the
  conductor.
• Metals are very good conductors

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Conductors

• Good examples of conductors would be copper and
  iron.
• We generally use copper wiring in our homes.

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Semiconductors

• Some substances that are usually good insulators
  can be converted into good conductors.
• We take the pure crystalline form of the
  substance (examples are germanium and silicon)
  and substitute impurities into the matrix.
• It doesnt take much (1 in 10 million) but those
  substances then become good conductors and
  insulators.
• We call these semi-conductors.

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Superconductors

• Variations in temperature can change
  conductivity.
• The closer you go towards absolute zero, the
  better the conductivity of metals.
• The colder the temperature, the better the
  electron flow.
• Theoretically you could get indefinite electron
  flow.
• This is why they are called Superconductors.

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Charging

• By friction rubbing two substances together
  encourages electron flow.
• By contact simply touching two substances
  together transfers electrons.

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Charging

• By induction. There is no actual contact here.
  Charges are separated using the concept of like
  charges repelling and opposite charges attract.
  Once the separation of charges has taken place, a
  physical separation occurs, leaving two
  oppositely charged substances.

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Two neutral objects in contact with each other
share charges back and forth while in contact
with each other.
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A charged object (in this case a negatively
charged object) is introduced into the system.
Like charges are repelled. Opposite charges are
attracted. The two spheres are experiencing a
separation of charges.
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Now the two spheres are physically separated.
They sphere on the right keeps the electrons
which moved over earlier. Each sphere is no
longer neutral. The one on the left lost
electrons, and is now positively charged. The
sphere on the right gained electrons and is now
negatively charged.