Chapter 5: Electricity and Magnetism Part 1

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Chapter 5 Electricity and Magnetism Part 1

• Alyssa Jean-Mary

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Electricity

• Electricity can not be explained by gravity or
  the kinetic theory of matter
• It is in our everyday lives making light bulbs
  glow, making motors run, making telephones and
  radios bring us sound, making our televisions
  bring us images
• This electricity that is used every day is to
  transport energy and information
• All matter is electrical in nature
• Electric forces binds electrons to a nuclei,
  which is what forms atoms
• They also are the forces that hold solids and
  liquids together

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Positive and Negative Charge

• An experiment
• If a ball is touched with a rubber rod, the ball
  wont move
• If this same ball is touched with a rubber rod
  that has been rubbed against a piece of fur, the
  ball will now move away from the rubber rod
  because the rubber rod transferred its newly
  gained electrical charge to the ball
• If a ball that is touched with a rubber rod that
  has been rubbed against a piece of fur is placed
  next to another ball that has been touched by the
  same thing, the balls will move away from each
  other because they have the same electrical
  charge
• However, if this same ball (the one that is
  touched with a rubber rod that has been rubbed
  against a piece of fur) is placed next to a ball
  that has been touched with a glass rod that has
  been rubbed with a piece of silk, the balls will
  move towards each other because they each have a
  different electrical charge
• There are two different electric charges
• Negative charge - i.e. the charge produced by a
  rubber rod against a piece of fur
• Positive charge i.e. the charge produced by a
  glass rod against a piece of silk
• Something with a negative charge is attracted to
  something with a positive charge because opposite
  charges attract
• This same something with a negative charge is
  repelled by something else with a negative charge
  because like charges repel

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Charge Separation

• When two objects interact with each other and an
  electrical charge is produced, one of the objects
  ends up with a negative charge and the other
  object ends up with a positive charge
• When the rubber rod is rubbed with the piece of
  fur, since the rubber rod gains a negative
  charge, the fur has gained a positive charge
• The same is true for the glass rod that is rubbed
  with the silk since the glass rod gains a
  positive charge, the silk has gained a negative
  charge
• The process of rubbing two objects together is
  not what creates the electrical charge. Every
  uncharged object actually has an equal amount
  of both positive and negative charges within it.
• The bond between these charges in an uncharged
  object can be weak, which is why rubbing could
  release some of the charges, or strong, which
  needs a lot more to release some of the charges
• An uncharged object, who has an equal amount of
  positive and negative charges is called neutral

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What is Charge?

• Every substance is composed of tiny bits of
  matter called atoms
• Every atom, no matter the kind, is made up of
  three different elementary particles
• Protons mass 1.673 x 10-27 kg, charge
  positive
• Electrons mass 9.11 x 10-31 kg, charge
  negative
• Neutrons mass 1.675 x 10-27 kg, charge no
  charge neutral
• Protons and Electrons have the same amount of
  charge they just have opposite signs
• Protons and Neutrons have almost equal mass, and
  their masses are almost 2000 greater than the
  mass of electrons
• Every atom has a nucleus at its center. The
  nucleus contains the protons and the neutrons.
  The electrons are contained outside the nucleus,
  in energy levels or shells.
• In a neutral atom, the amount of electrons equals
  the amount of protons
• What is CHARGE? It is a fundamental property of
  certain elementary particles of which all matter
  is composed. The charge of a particle gives rise
  to electric forces just as an objects mass gives
  rise to gravitational forces.

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The Coulomb

• The unit of electric charge is the Coulomb (C)
• Since the proton has a charge of 1.6 x 10-19C,
  and the electron has a charge of -1.6 x 10-19C,
  all charges, whether they are positive or
  negative, only occur in multiples of 1.6 x 10-19C
• Thus, equation for the basic unit of charge in
  nature (e) is
• e 1.6 x 10-19C
• Since e is such a small quantity, it appears that
  charge is continuous when looked at outside the
  laboratory (i.e. a charge of -1C would equal 6
  billion billion electrons)
• Since atoms are small, 6 billion billion atoms of
  carbon will make a piece of coal, which is almost
  pure carbon, that is only about the size of a pea

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Coulombs Law

• The force between two charged objects depends on
  how close the objects are to each other and on
  how much charge each of the objects has
• If the distance between two objects is increased,
  the force between them is decreased AND if the
  distance between two objects is decreased, the
  force between them is increased i.e. the
  distance and force are inversely proportional
• If the charge on the objects is increased, the
  force between them is also increased AND if the
  charge on the objects is decreased, the force
  between them is also decreased i.e. the charge
  and force are directly proportional
• The equation of Coulombs Law is
• F (KQ1Q2)/R2
• where F is the electric force, Q1 is the charge
  on object 1, Q2 is the charge on object 2, R is
  the distance between the two objects, and K is a
  constant that is called the electric force
  constant and is equal to 9 x 109 Nm2/C2
• This equation shows us that if two charges, each
  with a charge of 1C, are separated by 1m, the
  electric force between them is 9 x 109N, which is
  9 billion Newtons, which is an enormous force
• A Coulomb is a very large unit it is son large
  that even the most highly charged objects will
  not contain more than a small fraction of a
  Coulomb

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Example Calculation of Electric Force

• What is the electric force between two objects if
  one of the objects has a charge of 5.3C and the
  other object has a charge of 3.2C if they are 56m
  apart?
• Answer
• Given 5.3C, 3.2C, 56m
• Looking for electric force
• Equation F (KQ1Q2)/R2
• Solution F ((9 x 109 Nm2/C2)(5.3C)(3.2C))/(56m
  )2 4.87 x 107N

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Force on an Uncharged Object

• An object that is charged will attract small
  uncharged particles towards it because electrons
  can have some freedom of movement without leaving
  their parent atoms or molecules
• When a comb is used in a persons hair, it gains
  a negative charge. If the comb is placed near a
  piece of paper, the paper, which is neutral, will
  be attracted to the comb because all the negative
  charge of the paper will move to the side of the
  paper away from the comb, which leaves all the
  positive charge of the paper on the side close to
  the comb. This results in attraction because the
  comb is negative and the paper near the comb is
  positive. If the comb doesnt actually touch the
  paper, the positive and negative charge will be
  returned to their normal positions. Also, since
  there is only a small amount of charge separation
  between the positive and negative charges on an
  object that comes into contact with a charged
  particle, there is only a small amount of force
  available, which means that only small objects
  will be picked up.

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Matter in Bulk

• Coulombs Law (electric force) vs. Newtons Law
  of Gravity (gravitaitonal force)
• F (KQ1Q2)/R2 vs. F (Gm1m2)/R2
• The two laws resemble each other, but
  gravitational forces are always attractive
  forces, and electric forces can be either
  attractive forces or repulsive forces
• Because matter always attracts other matter
  gravitationally, matter always tends to come
  together into large masses in the universe. Even
  though there are other dispersive influences that
  exist (i.e. other influences that drive matter
  apart), matter must fight against this steady
  attraction. All galaxies, stars, and planets
  where made out of matter that was originally
  spread out all over space that came together
  because of this gravitational attraction.
• To collect such a large amount of electric charge
  of one sign in one place is not as easy to do as
  that. It is hard to separate neutral matter into
  its differently charged particles because
  negative and positive particles attract each
  other so strongly. Also, like charges repel each
  other, so it really is hard to collect a large
  amount of electric charge of the same sign.
• Thus, neutral particles are most stable (i.e. it
  has a minimum potential energy) when all the
  particles make up only one single body and
  electric charges are most stable when positive
  and negative charges pair off to cancel each
  other out. On a universal scale, gravitational
  forces are more significant that electric forces,
  whereas on an atomic scale, electric forces are
  more significant than gravitational forces. On an
  atomic scale, the mass of particles is so small
  to see a gravitational effect, but their charges
  are large enough to see a significant electric
  effect.