Electricity

1
Electricity
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Electrostatics

• Electrostatics, or electricity at rest, involves
  electric charges, the forces between them, and
  their behavior in materials.
• An understanding of electricity requires a
  step-by-step approach, for one concept is the
  building block for the next.

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• The enormous attractive and repulsive electrical
  forces between the charges in Earth and the
  charges in your body balance out, leaving the
  relatively weaker force of gravity, which only
  attracts.

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The Atom

• Electrical forces arise from particles in atoms.
• The protons in the nucleus attract the electrons
  and hold them in orbit. Electrons are attracted
  to protons, but electrons repel other electrons.
• The fundamental electrical property to which the
  mutual attractions or repulsions between
  electrons or protons is attributed is called
  charge.
• By convention, electrons are negatively charged
  and protons positively charged.
• Neutrons have no charge, and are neither
  attracted nor repelled by charged particles.

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• Every atom has a positively charged nucleus
  surrounded by negatively charged electrons.
• All electrons are identical.
• The nucleus is composed of protons and neutrons.
• All protons are identical
• all neutrons are identical.
• Atoms usually have as many electrons as protons,
  so the atom has zero net charge.

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• The fundamental rule of all electrical phenomena
  is that like charges repel and opposite charges
  attract

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Electrically Charged Objects

• Matter is made of atoms, and atoms are made of
  electrons and protons.
• An object that has equal numbers of electrons and
  protons has no net electric charge.
• But if there is an imbalance in the numbers, the
  object is then electrically charged.
• An imbalance comes about by adding or removing
  electrons

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Conductors and Insulators

• Materials through which electric charge can flow
  are called conductors.
• Metals are good conductors for the motion of
  electric charges because their electrons are
  loose
• Electrons in other materialsrubber and glass,
  for exampleare tightly bound and remain with
  particular atoms.
• They are not free to wander about to other atoms
  in the material.
• These materials, known as insulators, are poor
  conductors of electricity.

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Charging by Friction and Contact

• We can stroke a cats fur and hear the crackle of
  sparks that are produced.
• We can comb our hair in front of a mirror in a
  dark room and see as well as hear the sparks of
  electricity.
• We can scuff our shoes across a rug and feel the
  tingle as we reach for the doorknob.
• Electrons are being transferred by friction when
  one material rubs against another.

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• If you slide across a seat in an automobile, you
  are in danger of being charged by friction.

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• Electrons can also be transferred from one
  material to another by simply touching.
• When a charged rod is placed in contact with a
  neutral object, some charge will transfer to the
  neutral object.
• This method of charging is called charging by
  contact.
• If the object is a good conductor, the charge
  will spread to all parts of its surface because
  the like charges repel each other

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Charging by Induction

• When a negatively charged rod is held near one
  sphere, electrons in the metal are repelled by
  the rod.
• Excess negative charge has moved to the other
  sphere, leaving the first sphere with an excess
  positive charge.
• The charge on the spheres has been redistributed,
  or induced.

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• Charging by induction occurs during
  thunderstorms.
• The negatively charged bottoms of clouds induce a
  positive charge on the surface of Earth below.
• Most lightning is an electrical discharge between
  oppositely charged parts of clouds.
• The kind of lightning we are most familiar with
  is the electrical discharge between clouds and
  oppositely charged ground below

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Charge Polarization

• Charging by induction is not restricted to
  conductors. Charge polarization can occur in
  insulators that are near a charged object.
• When a charged rod is brought near an insulator,
  there are no free electrons to migrate throughout
  the insulating material.
• Instead, there is a rearrangement of the
  positions of charges within the atoms and
  molecules themselves

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Examples of Charge Polarization

• Polarization explains why electrically neutral
  bits of paper are attracted to a charged object,
  such as a charged comb.
• Molecules are polarized in the paper, with the
  oppositely charged sides of molecules closest to
  the charged object

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• Rub an inflated balloon on your hair and it
  becomes charged.
• Place the balloon against the wall and it sticks.
  
• The charge on the balloon induces an opposite
  surface charge on the wall.
• The charge on the balloon is slightly closer to
  the opposite induced charge than to the charge of
  the same sign

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Current

• Electric current is related to the voltage that
  produces it, and the resistance that opposes it.
• Current is the flow of electric charge
• Electric charge is carried by the electrons
  through a circuit in a solid conductor (Copper
  wire)

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• Voltage produces a flow of charge, or current,
  within a conductor.
• The flow is restrained by the resistance it
  encounters.
• The rate at which energy is transferred by
  electric current is power.
• When the ends of an electric conductor are at
  different electric potentials, charge flows from
  one end to the other

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• Charge flows when there is a potential
  difference, or difference in potential (voltage),
  between the ends of a conductor.
• The flow continues until both ends reach the same
  potential.
• When there is no potential difference, there is
  no longer a flow of charge through the conductor.
  
• To attain a sustained flow of charge in a
  conductor, one end must remain at a higher
  potential than the other.

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Measuring Current

• Electric current is measured in amperes, symbol
  A.
• An ampere is the flow of 1 coulomb of charge per
  second.
• When the flow of charge past any cross section is
  1 coulomb (6.24 billion billion electrons) per
  second, the current is 1 ampere.

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Voltage Sources

• Voltage sources such as batteries and generators
  supply energy that allows charges to move
  steadily.
• Charges do not flow unless there is a potential
  difference.
• Something that provides a potential difference is
  known as a voltage source.
• Batteries and generators are capable of
  maintaining a continuous flow of electrons

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• In a battery, a chemical reaction releases
  electrical energy.
• Generatorssuch as the alternators in
  automobilesconvert mechanical energy to
  electrical energy.
• The electrical potential energy produced is
  available at the terminals of the battery or
  generator.

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• Power utilities use electric generators to
  provide the 120 volts delivered to home outlets.
• The alternating potential difference between the
  two holes in the outlet averages 120 volts.
• When the prongs of a plug are inserted into the
  outlet, an average electric pressure of 120
  volts is placed across the circuit.
• This means that 120 joules of energy is supplied
  to each coulomb of charge that is made to flow in
  the circuit.

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Electrical Resistor Resistance

• A device that converts electrical energy into
  some other from of energy ? light, heat, sound
• The amount of charge that flows in a circuit
  depends on the voltage provided by the voltage
  source.
• The current also depends on the resistance that
  the conductor offers to the flow of chargethe
  electric resistance.
• The resistance of a wire depends on the
  conductivity of the material used in the wire
  (that is, how well it conducts) and also on the
  thickness and length of the wire.

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• Thick wires have less resistance than thin wires.
  
• Longer wires have more resistance than short
  wires.
• Electric resistance also depends on temperature.
  For most conductors, increased temperature means
  increased resistance.
• The resistance of some materials becomes zero at
  very low temperatures, a phenomenon known as
  superconductivity.
• In a superconductor, the electrons flow
  indefinitely.

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Ohms Law

• For a given circuit of constant resistance,
  current and voltage are proportional.
• Twice the current flows through a circuit for
  twice the voltage across the circuit. The greater
  the voltage, the greater the current.
• If the resistance is doubled for a circuit, the
  current will be half what it would be otherwise.

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• The resistance of a typical lamp cord is much
  less than 1 ohm, while a typical light bulb has a
  resistance of about 100 ohms.
• An iron or electric toaster has a resistance of
  15 to 20 ohms.
• The low resistance permits a large current, which
  produces considerable heat.
• Current inside electric devices is regulated by
  circuit elements called resistors

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• Light bulbs, an iron, a toaster are all examples
  of resistors.
• The current inside electric devices such as an
  Ipod or TV are regulated by resistors

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Ohms Law and Electric Shock

• From Ohms law, we can see that current depends
  on the voltage applied, and also on the electric
  resistance of the human body.

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• The damaging effects of electric shock are the
  result of current passing through the body
• Your bodys resistance ranges from about 100 ohms
  if soaked with salt water to about 500,000 ohms
  if your skin is very dry.
• Touch the electrodes of a battery with dry
  fingers and your resistance to the flow of charge
  would be about 100,000 ohms.
• You would not feel 12 volts, and 24 volts would
  just barely tingle.
• With moist skin, however, 24 volts could be quite
  uncomfortable.

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• Many people are killed each year by current from
  common 120-volt electric circuits.
• Touch a faulty 120-volt light fixture while
  standing on the ground and there is a 120-volt
  pressure between you and the ground.
• The soles of your shoes normally provide a very
  large resistance, so the current would probably
  not be enough to do serious harm.
• If you are standing barefoot in a wet bathtub,
  the resistance between you and the ground is very
  small.
• Your overall resistance is so low that the
  120-volt potential difference may produce a
  harmful current through your body.

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• Drops of water that collect around the on/off
  switches of devices such as a hair dryer can
  conduct current to the user.
• Handling a wet hair dryer can be like sticking
  your fingers into a live socket
• One effect of electric shock is to overheat
  tissues in the body or to disrupt normal nerve
  functions.
• It can upset the nerve center that controls
  breathing.

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High Voltage Wires

• You probably have seen birds perched on
  high-voltage wires.
• Every part of the birds body is at the same high
  potential as the wire, and it feels no ill
  effects.
• For the bird to receive a shock, there must be a
  difference in potential between one part of its
  body and another part.
• Most of the current will then pass along the path
  of least electric resistance connecting these two
  points.

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Ground Wires

• Mild shocks occur when the surfaces of appliances
  are at an electric potential different from other
  nearby devices.
• If you touch surfaces of different potentials,
  you become a pathway for current.
• To prevent this, electric appliances are
  connected to a ground wire, through the round
  third prong of a three-wire electric plug.

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• All ground wires in all plugs are connected
  together through the wiring system of the house.
• The two flat prongs are for the current-carrying
  double wire.
• If the live wire accidentally comes in contact
  with the metal surface of an appliance, the
  current will be directed to ground rather than
  shocking you if you handle it.

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AC/DC Current

• Electric current may be AC or DC current
• DC means direct current where charge flows in one
  direction ? battery is DC
• Electrons always move in the same direction from
  the (-) pole toward the () pole

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• Alternating current (AC), as the name implies, is
  electric current that repeatedly reverses
  direction.
• Electrons in the circuit move first in one
  direction and then in the opposite direction.
• They alternate back and forth about relatively
  fixed positions.
• This is accomplished by alternating the polarity
  of voltage at the generator or other voltage
  source.

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Voltage Standards

• Voltage of AC in North America is normally 120
  volts.
• In the early days of electricity, higher voltages
  burned out the filaments of electric light bulbs.
  
• Power plants in the United States prior to 1900
  adopted 110 volts (or 115 or 120 volts) as
  standard.

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• By the time electricity became popular in Europe,
  light bulbs were available that would not burn
  out so fast at higher voltages.
• Power transmission is more efficient at higher
  voltages, so Europe adopted 220 volts as their
  standard.
• The United States stayed with 110 volts (today,
  officially 120 volts) because of the installed
  base of 110-volt equipment.

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• Although lamps in an American home operate on
  110120 volts, electric stoves and other
  appliances operate on 220240 volts.
• Most electric service in the United States is
  three-wire
• one wire at 120 volts positive
• one wire at zero volts (neutral)
• one wire at a negative 120 volts
• The popularity of AC arises from the fact that
  electrical energy in the form of AC can be
  transmitted great distances.
• Easy voltage step-ups result in lower heat losses
  in the wires.
• The primary use of electric current, whether DC
  or AC, is to transfer energy from one place to
  another

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Converting AC to DC

• The current in your home is AC. The current in a
  battery-operated device, such as a laptop
  computer or cell phone, is DC.
• With an AC-DC converter, you can operate a
  battery-run device on AC instead of batteries
• A converter uses a transformer to lower the
  voltage and a diode, an electronic device that
  allows electron flow in only one direction
• To maintain continuous current while smoothing
  the bumps, a capacitor is used.

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• When input to a diode is AC,
• output is pulsating DC.
• Charging and discharging of a capacitor provides
  continuous and smoother current.
• In practice, a pair of diodes is used so there
  are no gaps in current output.

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The Electrons in a Circuit

• When you flip on the light switch on your wall
  and the circuit is completed, the light bulb
  appears to glow immediately.
• Energy is transported through the connecting
  wires at nearly the speed of light.
• The electrons that make up the current, however,
  do not move at this high speed.
• The source of electrons in a circuit is the
  conducting circuit material itself.

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• When you plug a lamp into an AC outlet, energy
  flows from the outlet into the lamp, not
  electrons.
• Most of this electrical energy appears as heat,
  while some of it takes the form of light.
• Power utilities do not sell electrons. They sell
  energy. You supply the electrons.

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• When you are jolted by an AC electric shock, the
  electrons making up the current in your body
  originate in your body.
• Electrons do not come out of the wire and through
  your body and into the ground energy does.
• The energy simply causes free electrons in your
  body to vibrate in unison.
• Small vibrations tingle large vibrations can be
  fatal.

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Electrical Power

• Unless it is in a superconductor, a charge moving
  in a circuit expends energy.
• This may result in heating the circuit or in
  turning a motor.
• Electric power is the rate at which electrical
  energy is converted into another form such as
  mechanical energy, heat, or light.
• Electric power is equal to the product of current
  and voltage.
• electric power current voltage
• 1 watt (1 ampere) (1 volt)

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• The power and voltage on the light bulb read 60
  W 120 V.
• A kilowatt is 1000 watts, and a kilowatt-hour
  represents the amount of energy consumed in 1
  hour at the rate of 1 kilowatt.
• Where electrical energy costs 10 cents per
  kilowatt-hour, a 100-watt light bulb burns for 10
  hours for 10 cents.
• A toaster or iron, which draws more current and
  therefore more power, costs several times as much
  to operate for the same time.

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Electric Meters and Bills

• An electric meter or energy meter is a device
  that measures the amount of electrical energy
  supplied to or produced by a residence, business
  or machine.

Each of the five dials represent one digit of the
present reading.   As you can see, the dials move
both clockwise and counter clockwise.
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Reading your Electric Meter

• When the hand of one of the dials is between
  numbers, always take the smaller number

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• When the power company representative reads your
  meter, they do NOT set it back to zero.
• The dials keep turning until the next time the
  meter is read. 
• By subtracting two consecutive readings, the
  amount of kilowatt hours is determined for the
  month.
• By subtracting yesterday's reading from today's
  reading, you can get a feel for how much energy
  (kilowatt hours) you use each day

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Reading your Electric Bill
Monthly Basic Service Charge of 8.05 Plus An
Energy Charge of    Summer (June 1 through
September 30)        8.66 / kWh for all
kWh    Winter (October 1 through May 31)       
7.93 / kWh for the first 100 kWh        6.77
/ kWh for the next 900 kWh        4.24 / kWh
for all additional kWh A Minimum Monthly Bill of
10.18
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Electric Circuit

• In a flashlight, when the switch is turned on to
  complete an electric circuit, the mobile
  conduction electrons already in the wires and the
  filament begin to drift through the circuit.
• A flashlight consists of a reflector cap, a light
  bulb, batteries, and a barrel-shaped housing with
  a switch.

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• Electrons flow
• from the negative part of the battery through the
  wire
• to the side (or bottom) of the bulb
• through the filament inside the bulb
• out the bottom (or side)
• through the wire to the positive part of the
  battery
• The current then passes through the battery to
  complete the circuit.

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• Unsuccessful ways to light a bulb.
• Successful ways to light a bulb.

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• Any path along which electrons can flow is a
  circuit.
• A gap is usually provided by an electric switch
  that can be opened or closed to either cut off or
  allow electron flow.
• Most circuits have more than one device that
  receives electrical energy.
• These devices are commonly connected in a circuit
  in one of two ways, series or parallel.
• When connected in series, the devices in a
  circuit form a single pathway for electron flow.
• When connected in parallel, the devices in a
  circuit form branches, each of which is a
  separate path for electron flow.

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Series Circuits

• If three lamps are connected in series with a
  battery, they form a series circuit. Charge flows
  through each in turn.
• When the switch is closed, a current exists
  almost immediately in all three lamps.
• In this simple series circuit, a 9-volt battery
  provides 3 volts across each lamp.

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• The main disadvantage of a series circuit is that
  when one device fails, the current in the whole
  circuit stops.
• Some cheap light strings are connected in series.
  When one lamp burns out, you have to replace it
  or no lights work.

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Parallel Circuits

• In contrast to a series circuit, the parallel
  circuit is completed whether all, two, or only
  one lamp is lit.
• A break in any one path does not interrupt the
  flow of charge in the other paths.

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Schematic Diagrams

• Electric circuits are frequently described by
  simple diagrams, called schematic diagrams.
• Resistance is shown by a zigzag line, and ideal
  resistance-free wires are shown with solid
  straight lines.
• A battery is shown by a set of short and long
  parallel lines, the positive terminal with a long
  line and the negative terminal with a short line.

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• These schematic diagrams represent
• a circuit with three lamps in series, and
• a circuit with three lamps in parallel.

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Overloading Parallel Circuits

• Electric current is fed into a home by two wires
  called lines. About 110 to 120 volts are
  impressed on these lines at the power utility.
• These lines are very low in resistance and are
  connected to wall outlets in each room.
• The voltage is applied to appliances and other
  devices that are connected in parallel by plugs
  to these lines.

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• As more devices are connected to the lines, more
  pathways are provided for current.
• The additional pathways lower the combined
  resistance of the circuit. Therefore, a greater
  amount of current occurs in the lines.
• Lines that carry more than a safe amount of
  current are said to be overloaded, and may heat
  sufficiently to melt the insulation and start a
  fire.
• To prevent overloading in circuits, fuses or
  circuit breakers are connected in series along
  the supply line

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• The entire line current must pass through the
  fuse.
• If the fuse is rated at 20 amperes, it will pass
  up to 20 amperes.
• A current above 20 amperes will melt the fuse
  ribbon, which blows out and breaks the circuit.

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• Before a blown fuse is replaced, the cause of
  overloading should be determined and remedied.
• Insulation that separates the wires in a circuit
  can wear away and allow the wires to touch.
• This effectively shortens the path of the
  circuit, and is called a short circuit.
• A short circuit draws a dangerously large current
  because it bypasses the normal circuit resistance.

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• Circuits may also be protected by circuit
  breakers, which use magnets or bimetallic strips
  to open the switch.
• Utility companies use circuit breakers to protect
  their lines all the way back to the generators.
• Circuit breakers are used in modern buildings
  because they do not have to be replaced each time
  the circuit is opened.