Fundamentals of Electricity

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Fundamentals of Electricity

• Circuits 1
• Fall 2005
• Harding University
• Jonathan White

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Outline

• Benjamin Franklin / History of Electronics
• Atoms/Electrons
• Electron shells and orbits
• Valence electrons
• Charge
• Current
• Voltage
• Electrical Ground
• Resistance
• Electric circuits

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History of Electronics

• Began with Benjamin Franklin in 1747
• He determined that electricity was a single
  force, with positive and negative aspects
• Coined over 25 new terms, including armature,
  battery, and conductor
• Famous kite-flying experiment in a thunderstorm
  was performed in 1752, near the end of his work
  in the field
• Ben Franklin arbitrarily assumed that the actual
  carriers of electrical current had a positive
  electrical charge.
• While this assumption was wrong, all his
  experiments still worked, and this assumption is
  still often used today.
• However, the theories of electricity go back a
  lot farther than this.

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History 1

• 600 B.C. Thales of Miletus writes about amber
  becoming charged by rubbing.
• 1600 English scientist William Gilbert coins
  the term electricity from the Greek word for
  amber. Experiments with magnets, coining the
  terms electric force, magnetic pole, and electric
  attraction.
• 1745 Dutch physicist Pieter van Musschenbroek
  invented the Leyden Jar, a device that stored
  static electricity. This was the first
  capacitor.
• 1747 William Watson discharged a Leyden Jar
  through a circuit. This begins the comprehension
  of current and circuits.
• 1800 Alessandro Volta invents the first
  electric battery. He also proved that
  electricity could travel over wires.

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History 2

• 1820 Oersted and Ampere observe that a coil of
  wires acts like a magnet when a current is passed
  through it
• 1821 Faraday invents the first electric motor
• 1826 Ohm states a relationship between
  potential, current, and circuit resistance
• 1873 Maxwell writes equations that described
  the electromagnetic field
• 1876 Edison Electric Light Co. founded
• 1879 First commercial power station opens in
  San Francisco, uses a Brush generator and arc
  lights
• 1883 Transformer invented
• 1886 Alternating current electric system
  developed by William Stanley
• 1897 Electron discovered by J.J. Thompson

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Atoms

• Atom Smallest particle of an element that
  retains all the characteristics of that element.
• Consists of positively charge protons and
  uncharged neutrons in the nucleus, and several
  negatively charge electrons that surround the
  nucleus.
• Electrons occupy specific energy levels, or
  shells around the nucleus

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Electrons

• Each shell is discrete and electrons will try and
  occupy the lowest energy level available to them.
  
• Negative electrons that are close to the nucleus
  are attracted to the positive nucleus and are
  tightly bound.
• Electrons farther out are loosely bound, and
  hence have a higher potential energy
• However, there is a limit to the number of
  electrons that each shell can hold.
• Number of electrons possible 2N2, where N is
  the shell number.
• The outermost shell is known as the valence
  shell, and the electrons in it are called valence
  electrons.
• The factor that becomes important is that those
  elements with only 1 or 2 electrons in their
  outermost shells dont hold on the them very
  strongly. Therefore it requires little energy to
  pull these electrons from their parent atoms and
  move them someplace else.

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The Copper Atom

• 29 electrons that orbit the nucleus in 4 shells.
• When a copper atom gains sufficient thermal
  energy, it can break away from the parent and
  become a free electron.
• This happens at room temperature for copper.

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Categories of Materials

• Conductors readily allow current, large number
  of free electrons, 1 3 valence electrons.
  Whats the best conductor? Exp silver, copper,
  gold, aluminum, iron.
• Semiconductors 4 valence electrons in their
  structures. Exp silicon, germanium.
• Insulators poor conductors of electric current,
  more than 4 valence electrons.

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Electrical Charge

• 2 types, positive and negative. Charge of a
  proton is equal in magnitude to the charge of an
  electron.
• Symbolized by letter Q.
• Static electricity is the presence of a net
  positive or negative charge.
• Like charges repel, unlike charges attract.
• This attraction or repulsion is a force called an
  electromagnetic field.
• This force and gravity are the only forces that
  we humans can experience directly.
• Charge is measured in coulumbs
• One coulumb is the total charge possesed by 6.25
  X 1018 electrons
• Like matter, charges follow an inverse square
  law.
• F1,2 q1 q2/(4pE0 r2)
• Where q1, q2 are measured in coulumbs, r is the
  distance between the charges measured in meters,
  and E0 is a fundamental constant of nature,
  8.885419 x 10-12 Farads/meter
• However, charge is different than matter. Does
  anti-matter have a negative mass? Also, two
  masses always seem to attract each other.

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Charge 2

• The charge on an electron is always the same for
  every electron in the universe.
• If you have one electron and one proton, then the
  two charges cancel each other out and the atom is
  said to have no charge. Also, the electron and
  proton will cancel out each other exactly.
• The charge on an electron is a fundamental
  quantity - a constant of nature.
• Where is charge used?
• When you plug in a light bulb, charge flows from
  the socket, through the connecting wire, and then
  through the bulb filament, heating it up and
  giving off light. The electrons arent
  destroyed, but they do lose energy.
• Your car battery stores energy by storing charge
  on the battery plates. When you start your car,
  charge flows from your battery to the engine,
  providing enough energy for the vehicle to run.

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Current

• Current charge in motion.
• The flowing of charges through something.
• We typically think of charge flowing through a
  wire, but it can also flow through water, air and
  even vacuum.
• You can think of current as water flowing through
  the interior of a pipe, though current actually
  flows though the empty spaces between atoms in a
  wire.
• Current is represented by the mathematical symbol
  i.
• i Q/t, or, current is equal to the number of
  electrons that flow past a point in a given
  amount of time.
• Current is measured in amperes, which is equal to
  coulombs/sec. Amperes is abbreviated with the
  letter A.
• Current is a through variable, meaning that in
  order to measure it, you need the current to go
  through something.

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Current 2

• Charge comes in discrete packets, but it is
  useful to assume that it can take continuous
  values. Then, we can imagine making the time
  frame very small and find the current at an
  instant. Then, i(t) dQ(t)/dt
• It takes energy to make charges flow through
  something. The energy that makes current is
  called voltage.

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Voltage

• Can be thought of as the driving force behind the
  current (though it isnt really a force).
• Voltage is the energy per unit charge.
• Current flows through electrical elements when a
  voltage appears across the terminals of the
  element, similar to when water flows through a
  pipe when a pressure difference appears across
  the pipe.
• Voltage is an across variable. We talk about
  pressure differences and voltage differences.
• Voltage is related to potential energy. Voltage
  is defined as the electrical potential energy
  that a charge has by its position in space.
• If you pull two charges apart, you put potential
  energy into the system
• That potential energy can be converted into other
  forms of energy
• Energy can neither be created or destroyed, only
  transferred

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Voltage 2

• A mass m, h meters above the Earth has potential
  energy mgh. A mass at h0 has 0 potential
  energy. A charge at electrical ground has 0
  potential engergy. Voltage is the potential
  energy per unit charge, or V W/Q.
• If we move a charge from point A to point B, and
  put a given number of joules of work into the
  charge, we will recover exactly the same number
  of joules from the charge if it moves back from
  point B to point A.  If we move the charge
  through any closed path or circuit, there will be
  no net energy input to the system and no net
  energy recovered from the charge.

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Voltage and Batteries

• Batteries are voltage sources.
• Batteries can be thought of as charge pumps.
• They take a charge and though chemical reactions
  pump them up to a certain voltage, or potential
  energy level.
• As the charge flows through the circuit, this
  potential energy can be used by the circuit to do
  work. The charge loses energy as it goes through
  the loads.
• Heat up a filament
• Make a motor turn.
• Energy gained from the battery energy lost by
  the loads.
• Law of conservation of energy

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Ground

• Reference voltage from which all other
  measurements are measured the potential of the
  Earth.
• Defined as having 0 V potential energy with
  respect to the rest of the circuit.
• In physics equations, ground level is used as the
  point of 0 potential energy when lifting a
  weight, another thing electrical systems have in
  common with mechanical systems.
• In wiring for houses, the ground is physically
  connected to the Earth a place of 0 potential
  energy when compared to the rest of the wiring.
• Ground provides a return path for the current
  back to the source because all the ground points
  are electrically the same point and provide a
  zero resistance path

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Resistance

• Opposition to the flow of current.
• When there is current through any material that
  has resistance, heat is produced by the
  collisions of electrons and atoms.
• Can be thought of as partially closed valve in
  our pipe system it restricts the flow of water.

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Circuits

• Consists of a voltage source, a load, and a path
  for current between the source and the load.
• A load is a device on which work is done by the
  current through it.
• Open circuits versus closed circuits.

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Review

• Charge
• Definition Fundamental property of matter based
  on the absence or excess of electrons.
• Symbol Q
• Measured in Coulombs.
• 1C total charge of 6.25 X 1018 electrons
• 2 types, positive and negative

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Review

• Current
• Definition Charge in motion
• Symbol I
• Measured in amperes (A).
• I Q/T, current is the amount of charge that
  passes a point in a given amount of time. Or,
  i(t) dq / dt
• Current is a through variable.
• The water thats flowing through a pipe.

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Review

• Voltage
• Definition Energy per unit charge.
• Symbol V
• V W/Q , voltage is the energy per unit charge.
  Where, W is the energy expressed in joules, Q is
  in Coulombs.
• Voltage is an across variable.
• The pressure that pushes the water through the
  pipe.

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Ground and Resistance

• Ground
• Definition Reference point that is at 0 volts
  with respect to all other points in the circuit.
• Resistance
• Opposition to the flow of electrons.
• The resistance caused by the collision of
  electrons and atoms causes heat to be given off.