ELECTRICITY

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ELECTRICITY
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ELECTRICITY

• When studying electricity it is essential to
  understand that electrically charged bodies exert
  a force on each other. Thus, electricity and
  electrical currents must obey the basic laws of
  physics.
• Electricity and magnetism are closely related.
  The force generated by magnets or by charged
  particles is called electromagnetism.
• Electromagnetism, like gravitation, is an
  inherent property of matter. In fact, the
  electromagnetic force is one of the four
  fundamental forces of nature, the other three
  being gravity, the strong nuclear force, and the
  weak nuclear force.

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ELECTRICAL CHARGES AND THE ATOM

• Benjamin Franklin (1700s) was the first to state
  that there are two types of electrical charges,
  positive and negative. However, he thought that
  electric current was the movement of positive
  charges.
• Today we know that most electric current is due
  to the movement of electrons. Electrons are
  negatively charge particles.
• During the late 1800s and early 1900s, scientific
  research gave us an understanding of the nature
  of matter and the nature of electrical charge.
  Understanding the nature of the atom was basic to
  our understanding of electrical charge and
  electricity. A model of the atom emerged in the
  early 1900s.

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MODEL OF THE ATOM
Electron Orbits
Protons Red. Positive Charge.
Neutrons Green. Neutral Charge.
Nucleus
Electrons Tan. Negative Charge.
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ELECTRICITY AND ATOMIC STRUCTURE

• All ordinary matter is composed of fundamental
  partcles called atoms, the smallest particles of
  elements.
• In atoms, electrons are traveling about the
  nucleus so fast that they can be thought of as a
  diffuse cloud of negative electricity.
• Almost all the mass of the atom is contained in
  the nucleus.
• All neutral atoms contain the same number of
  protons and electrons.
• The electrical charges on protons and electrons
  are opposite (protons are positive, electrons are
  negative). But a proton and electron have the
  same magnitude of charge.
• The electrons are attracted to the positive
  nucleus of the atom by electrical forces.
• However, it is possible to remove electrons from
  an atom, or to add electrons to an atom.
• When electrons are removed from or added to an
  atom, the atom becomes charged. Charged atoms
  are referred to as ions.
• Objects may also be charged by adding excess
  electrons to their surface, like the balloon.

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ELECTROSTATIC FORCESOR STATIC ELECTRICITY

• When you rub a balloon against your hair or a
  cotton rag the balloon takes on a charge. The
  balloon may even stick to a wall because of its
  static electrical charge.
• Remember that there are two types of charges,
  positive and negative. The balloon has captured
  excess electrons on its surface by the rubbing
  action. Electrons have actually been removed
  from the atoms of your hair or the cotton rag and
  are on the surface of the balloon.
• Remember that like electrical charges repulse
  each other and unlike electrical charges attract
  each other.
• We have charged the balloon with a negative
  charge. Why did it stick to the wall? The atoms
  in the wall have a neutral charge.

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THE BALLOON EXAMPLE

• The balloon was charged because excess electrons
  were rubbed off of your hair or the cotton rag.
  Your hair or the rag took on a positive charge
  and the balloon took on a negative charge.
• The wall is neutral. However, the negative
  charge on the balloon induces a positive charge
  on the wall. Balloons are composed of rubber,
  which is not a good conductor of electrical
  charge. In fact rubber is an electrical
  insulator. Therefore the electrons stay in the
  same place on the balloon and do not evenly
  distribute themselves on the surface of the
  balloon. Thus the side of the balloon with the
  excess of electrons is attracted to the wall by
  induction.
• Objects may get charged in three ways 1)
  conduction charges move from one object to
  another 2) induction temporary charges on
  objects due to a redistribution of the charges in
  an object typically caused by placing the
  object in an electrical field (electrical forces
  can act a distance) and 3) friction charges
  can be removed from one object and added to
  another object (like the balloon example).

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WHY THE BALLOON STICKS TO THE WALL

• When the balloon is brought near the wall, the
  wall is polarized ( a charge is induced in the
  wall). The wall's negative charges move away from
  the balloon and the wall's positive charges move
  towards the balloon. So although the net charge
  on the wall is still zero, the wall will behave
  as if it is positively charged. Since the balloon
  is negatively charged and the wall is acting as
  if it is positively charged, the balloon is
  attracted to the wall. The force of attraction is
  so strong, in fact, that the balloon 'sticks' to
  the wall.
• Remember the gravitational force between the
  earth and the balloon is also trying to pull the
  balloon towards the floor. However, the
  electromagnetic force is billions and billions of
  times stronger than the gravitational force.

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AN EXAMPLE OF STATIC ELECTRICITY

• What causes the girls hair to react this way when
  she holds on to the Van de Graf machine?

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MEASURING THE AMOUNT OF ELECTRICAL CHARGE

• In the early 1900s, Robert Millikan and
  co-workers, in the famous oil drop experiment,
  determined the electrical charge of a single
  electron. Since the electron cannot be split,
  this is the smallest possible charge. In fact,
  Millikan determined that all electrical charges
  are multiples of this number. The magnitude of
  this fundamental charge is 1.60 x 10-19 Coulombs.
  The coulomb is the SI unit for measuring charge
  and one coulomb of charge is equivalent to the
  charge on 6.24 x 1018 electrons. Remember, a
  proton would also have this charge, but the
  charge would be positive rather than negative.

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Millikans 1909 Oil Drop Experiment
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MEASURING STATIC ELECTRICAL CHARGE

• The charge on an object can be related to the
  following equation
• q ne, where q is the total charge on an
  object, n is the number of electrons (added to or
  removed from an object), and e is the charge on
  an electron.
• PROBLEM How many excess electrons would be on
  the surface of a Van de Graaf machine
  (electrostatic converter) that has acquired a
  negative charge of 0.5 µC?

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MEASURING THE FORCE OF STATIC ELECTRICAL CHARGES

• There is an interesting comparison of the
  equations for magnitude of gravitational force
  and static electrical force. Both are inverse
  square laws.
• Recall Newtons gravitational equation.
• A similar equation exists for the electrostatic
  force between two charges

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COULOMBS LAW
COULOMBS LAW
Where k 9.00 x 109 Nm2/C2
Problem Compare the electrostatic force between
the electron and proton in a hydrogen atom to the
gravitational force between the electron and
proton in a hydrogen atom. Which force is
greater? How much greater?
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ELECTRIC FORCE FIELDS

• A force field is a region of space in which
  forces can be detected. An example would be a
  gravitational force field around the earth. In a
  similar way, every point near an electric charge
  is under the influence of that charge, so that an
  electric field exists in the region near the
  charge.
• It is convenient to be able to draw some sort of
  picture as a visual representation of a force
  field. We draw electrical lines of force to
  visualize an electrical force field around a
  charge or charges.
• An electrical line of force indicates the path
  along which a charged particle will travel if it
  is placed at any point on the line of force. By
  convention, it is always assumed that a test
  particle (to test the direction of the field) is
  always a positive particle.

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ELECTRIC FIELDS
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ELECTRIC FIELDS
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ELECTRIC FIELDS
ELECTRIC FIELDS
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ELECTRIC FIELDS
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ELECTRIC FIELDS
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ELECTRONS IN MOTION

• The movement of charged particles from one place
  to another is referred to as an electric current.
• Most of the time electric current is due to the
  flow of electrons in a particular direction
  within a conducting material (like copper wire).
• Electrons move in a particular direction when
  they are subjected to an electric field emplaced
  in the conducting material.
• When electrons move because of an electric field,
  they move relatively slow as compared to the
  movement of the field.
• The electric field moves nearly at the speed of
  light, but electrons in a conductor have a net
  forward speed of only about 0.02 cm/s.

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ELECTRONS IN MOTION

• If electrons move in just one direction through a
  conductor we refer to the current as direct
  current or DC. Battery circuits are usually DC.
• Electricity from power plants push electrons a
  fraction of a cm one way then pulls them back
  in the opposite direction. This is because power
  plants produce an alternating electric field
  thus an alternating electric current. An
  alternating current is referred to as AC
  electricity.
• In a household circuit, each electron makes 60
  such alternating motions, or cycles, every
  second. We say the circuit operates at 60 cycles
  per second or 60 Hertz.
• An important point here is that it is the motion
  of the electrons, not the electrons themselves,
  that does the work in a circuit (like make a
  light bulb glow).
• For example, the collisions of electrons with
  atoms in the filament of a light bulb imparts
  energy to the atoms and the temperature of the
  filament rises eventually getting hot enough to
  give off light.

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IONS IN MOTION

• Electric currents can also flow through liquids
  and gases that do not have free mobile single
  electrons. Here moving ions are the current.
• For example, an electric current can move through
  a salt water solution, where Sodium and Chloride
  ions transfer the charge.
• Electroplating takes advantage of ions moving
  through a solution to carry electrical charge
  under the influence of an electric field.
• However, as we have said earlier, it is usually
  electrons in motion that gives rise to most
  electrical current.

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ELECTRIC POTENTIAL DIFFERENCE OR VOLTAGE

• Charged particles in an electric field have
  energy by their position thus electrical
  potential energy. Thus, there is a potential
  difference between the low potential position and
  the high potential position.
• Electrical potential difference is also referred
  to as voltage. The unit of measurement of
  voltage is the volt.
• The change in electrical potential energy, or the
  voltage difference between two points in an
  electric field is defined as the work performed
  per unit charge to cause the change in potential
  energy.
• V W/q or V ?E/q
• The unit of voltage is the joule/coulomb, which
  is called a volt.
• Thus, voltage is the work that is required to
  separate the charges divided by the magnitude of
  the charge moved.

q 1C


W 1 J
Then V 1J/C 1 volt
W Fd
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VOLTAGE (CONT.)

• Thus, the battery has chemical potential energy.
  Chemistry inside the battery has caused a charge
  separation. Therefore the battery has the
  potential energy to do work. For example, if the
  terminals of the battery (say a 12 volt battery)
  are connected to the starter motor of a car,
  potential energy is converted into kinetic energy
  of motion of charged particle (electrons) and
  work is performed by turning the starter motor.
• Typically in a car battery, the positive terminal
  has a voltage of 12 volts and the negative
  terminal has a potential of zero volts. So when
  1 coulomb of charge moves from the positive
  terminal to the negative terminal, 12 joules of
  work is done.
• A volt is a potential, like an apple falling from
  a tree. As a potential there must be a reference
  point. For the apple on the tree, the reference
  point is the ground and the apple has so much
  potential energy relative to the ground.
• In a household circuit, the most convenient
  reference for measuring voltage is the earth
  itself. Thus the statement that the hot wire
  of house circuit has a potential of 120 volts
  means that if a convenient path is found, 1
  coulomb of charge moving from the hot wire to
  the earth can perform 120 J of work.

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ELECTRIC CURRENT

• Any concerted motion of electric charges.
  Therefore, electric current is the rate of flow
  of electrical charges (usually electrons).
• Current charge per time
• I q/t , I is current in amperes, q is charge in
  coulombs, and t is time in seconds.
• Thus, the unit of charge is the ampere. One
  ampere is the rate of flow of one coulomb per
  second.
• 1 A 1C/s 6.24 x 1018 electrons/second through
  a cross-section of conducting material.

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ELECTRICAL RESISTANCE

• When a current passes through a resistor its
  electrical energy is diminished (i.e. work is
  done). Some energy is also converted to thermal
  energy loss (heat).
• There is always a voltage drop across a resistor
  because either work is being done or heat is
  generated or both.
• The magnitude of the voltage drop across a
  resistor is given by the simple relationship
  known as Ohms Law.

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OHMS LAW

• The voltage drop across a resistor equals current
  times resistance.
• V IR or I V/R or R V/I

V volts
Slope resistance
I (amps)
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EXAMPLE PROBLEM ON OHM'S LAW The Basic Circuit

• Question An emf source of 6.0V is connected to a
  purely resistive lamp and a current of 2.0
  amperes flows. All the wires are resistance-free.
  What is the resistance of the lamp?
• Hints
• Where in the circuit does the gain in potential
  energy occur?
• Where in the circuit does the loss of potential
  energy occur?
• What is Ohm's Law?
• Solution The gain of potential energy occurs as a
  charge passes through the battery, that is, it
  gains a potential of  6.0V. No energy is lost to
  the wires, since they are assumed to be
  resistance-free. By conservation of energy, the
  potential that was gained (i.e.  V6.0V) must be
  lost in the resistor. So, by Ohm's Law
• V I R
• RV/I
• R 3.0  ohms.

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RESISTANCE (CONT.)

• The unit of resistance is volts/ampere or
• v/A. 1v/A is called an Ohm.
• The resistance in a typical lamp cord or
  househould extension cord is less than 1 ohm, but
  the resistance of a 60 watt lamp bulb is about
  240 ohms.
• Resistance is dependent on several factors.
• 1) Different substances have different
  conductivity. The electrons are less tightly
  bond in some substances. Iron wire is about 7
  times more resistive than copper wire.
• 2) Length of conductor resistance increases
  directly proportional to the length of the
  conductor.
• 3) Resistance is inversely proportional to the
  cross-sectional area. A conductor with only 1
  sq. cm has twice the resistance of a conductor of
  2 sq. cm cross-sectional area.
• Resistance increases with temperature for most
  substances.
• Some materials become superconductive at very low
  temperatures.

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ELECTRICAL POWER

• As we already know,
• power work done per time
• or power energy supplied per time
• P W/t or P ?E/t
• Voltage work done per charge
• V W/q, so W Vq, therefore electrical power
  Vq/t or P Vq/t VI
• Thus, P VI (or Electrical Power supplied by a
  voltage source (like a battery or generator) is
  equal to Voltage times Current.)

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ELECTRICAL ENERGY

• The power company supplies electrical energy to
  be used by its customers to do electrical work.
• The power company measures electrical energy in
  killowatt hours rather than joules. Since P
  W/t or P ?E/t, then
• ? E Pt, the power company measures power in
  kilowatts and time in hours. So customers are
  billed in killowatt hours (a unit for measuring
  electrical energy and electrical work).

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EXAMPLE PROBLEM

• Assume that the local power company charges
  0.10/Kwh. How much would it cost you to leave a
  100.W bulb on for a whole month (30 days) while
  you are on vacation.

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CIRCUITS

• Resistance in ciruits.
• Series Circuits
• Parallel Circuits