ELECTRICITY

1
ELECTRICITY

• Part 1 Electrostatics
• Part 2 Current Circuits

2
Electricity

• DO NOWMost of the energy we use on a daily
  basis is in the form of electromagnetic energy.
• What energy transformations must occur in order
  for it to be useful?
• What does this energy allow us to do with
  electrical appliances/devices?

3
Electricity

• The electromagnetic energy we use must be
  converted from other forms at power plants
  (fossil fuel, nuclear, geothermal, hydroelectric,
  biomatter, wind).
• Electrical energy allows us to do WORK with
  electrical appliances and devices.
• But on what object are we doing work?!?
• What force is exerted through what distance?!?

4
Electric Charge

• Electric Charge a property of matter that
  creates a force between objects
• Charge can be positive () or negative (-)
• Like charges REPEL
• Opposite charges ATTRACT
• What objects (particles) do we know that have a
  charge?

5
Electric Charge

• An objects charge depends on imbalance of
  protons () and electrons (-)
• More protons than electrons ? positive
• More electrons than protons ? negative
• Units of charge coulombs (C)
• Protons and electrons have exactly the same
  amount of charge 1.6 x 10-19 C
• Differ in sign ( or -)

6
Charging an Object

• Objects become charged if they have an imbalance
  of protons and electrons.
• Can an object gain or lose protons?(Think Can
  protons MOVE?)
• Can an object gain or lose electrons?(Think Can
  electrons MOVE?)
• Law of Conservation of Chargethe net charge in
  an isolated system remains constant

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Charging an Object

• Conductors materials that transfer and
  redistribute charge easily
• Examples ???
• Insulators materials that do not transfer charge
  easily retain charge within a localized region
• Examples ???
• Semiconductors materials that behave as either
  insulators or conductors, depending on
  temperature
• Examples ???

8
Triboelectric Series

• ONLY ELECTRONS CAN MOVE!!!
• Valence electrons, specifically
• How do we know if a material gains or loses
  electrons?
• Direction of electron movement depends on
  materials involved.
• A triboelectric series allows us to determine the
  resulting charges of two objects being rubbed
  together.
• If two materials are rubbed together, the one
  higher on the list should give up electrons and
  become positively charged.

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Triboelectric Series

• Air
• Human Hands, Skin
• Rabbit Fur
• Glass
• Human Hair
• Nylon
• Wool
• Lead
• Cat Fur
• Silk
• Aluminum

• Paper
• Cotton
• Steel
• Wood
• Lucite
• Rubber Balloon
• Hard Rubber
• Copper
• Gold, Platinum
• Polyester
• Teflon

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Charging by Friction

• Electrons are literally rubbed off one object
  onto another.
• Demonstration Balloon Hair
• Hair positive Balloon negative
• Demonstration Fur Rubber Rod
• Fur positive Rubber Rod negative

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Charging by Conduction

• Conduction involves transfer of charge from one
  object to another by direct contact.
• Walk across the carpet in socks and touch the
  doorknob ZAP!
• Charge is transferred and you experience a shock.
• Demonstration Touch the negatively-charged
  rubber rod to the pith ball.
• Some electrons move from rod to pith ball.
• Pith ball repels rod (both slightly negative).
• Static Discharge movement of charge from one
  object to another by conduction. Charge can
  JUMP! (Lightning)

12
Charging by Induction

• A temporary charge can be induced in a neutral
  object by bringing a charged object close to it.
• Electrons - attracted to a positively charged
  object.
• Electrons - repelled from a negatively charged
  object.
• Demonstration
• Bring negatively charged rubber rod near pith
  ball.
• Pith ball repels.
• Demonstration
• Bring negatively charged balloon near wall.
• Electrons in wall repel.
• Ball sticks to wall.

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Charging by Induction
14
Charging by Induction
15
How Can We Detect Charge?

• Electroscope a device used to detect charge.
• When a charge is present, the straw rotates.
• Degree of rotation or separation indicates
  strength of charge
• More rotation, more charge
• Less rotation, less charge
• Detects both positive and negative charge, but
  cannot tell the difference.

16
Electric Force

• Electrostatic Force force of attraction or
  repulsion between objects due to charge
• Depends on CHARGE and DISTANCE
• Increase charge ? force increases
• Increase distance ? force decreases
• Forces can act over a distance (no physical
  contact) through a FIELD
• Electric Field region around a charged object in
  which other charged objects experience an
  electric force

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Coulombs Law

• F electrostatic force
• k constant (just a )
• q1 charge of object 1
• q2 charge of object 2
• d distance

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Effects of Coulombs Law

• If the charge of one object doubles
• Force doubles (x2)
• If the charge of both objects double
• Force quadruples (x4)
• If the distance between the charges doubles
• Force is quartered (divided by 4)

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Electric Fields

• An electric field is the region around a charge
  in which the electrostatic force is felt by other
  charges.
• Electric field lines are drawn to show the
  magnitude and direction of the force on a small
  positively-charged test charge.
• By convention, electric field lines always extend
  outward from a positively-charged object and
  inward toward a negatively-charged object.

20
Electric Fields

• of field lines drawn is proportional to amount
  of charge creating the field
• 2x charge ? 2x field lines
• Electric field is the strongest where the field
  lines are most dense (closest together).
• Electric field lines never cross each other.
• At locations where electric field lines meet the
  surface of an object, the lines are perpendicular
  to the surface.

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Electric Field Lines
22
Electric Field Lines
23
Electricity, Part 2Electric Current and Circuits

• Most examples and applications of electricity are
  not electrostatic. In order for electric charges
  to do useful work, they need to move through a
  circuit.
• In circuit analysis, we need to understand the
  relationship between Voltage, Current, and
  Resistance
• This relationship is called Ohms Law

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Voltage

• Charges dont move through a circuit on their
  own. They need to be pushed.
• Voltage
• the driving force responsible for pushing charges
  through a circuit
• It is the electrical potential energy per charge
  of the charges moving through the circuit
• Scientifically called Potential Difference
• Analogy pressure in a hose
• Units volts (V)

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Potential Difference

• In a circuit, it is the change in voltage between
  two points that is important.
• This is called a potential difference.
• For a battery, there is a potential difference
  between the two terminals of the battery
• high potential () low potential (-)
• Positive charge naturally flows from positive
  terminal to negative terminal through a circuit.
• As current flows through devices, the voltage
  decreases and is converted into useful energy.

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Current

• Current the rate at which the electric charges
  move through a conductor or circuit
• How much charge is flowing per second
• Analogy flow of water in a hose
• Units coulombs per second (C/s) or amperes (A)
• Direct Current charges move in only one
  direction
• Batteries
• Alternating Current charges move back and forth,
  switching between two directions
• Household circuits

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Electrical Resistance

• Electric current is slowed by friction
• Different factors affect resistance
• Material
• conductors have low resistance
• insulators have high resistance
• Length, cross-sectional area, temperature
• Units of Resistance ohms (O)
• Current Resistance Analogies
• Water through a pipe or hose
• Students through the breezeway doors

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Electrical Quantities
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Ohms Law

• Georg Simon Ohm, a German physicist, investigated
  different wires in circuits. Examined the effect
  resistance had on the current.
• Discovered that current is directly proportional
  to the voltage and inversely proportional to the
  resistance. Does this make sense?
• Mathematical relationship is called Ohms Law
• V I x R

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Ohms Law Example Problem

• The headlights of a typical car are powered by a
  12 V battery. What is the resistance of the
  headlights if they draw 3.0 A of current when
  turned on?

• Given
• V 12 V
• I 3.0 A
• Unknown
• R ?
• Equation
• V I x R
• Plug Chug
• 12 V 3.0 A x R
• R 12 V / 3.0 A
• R 4 O
• Answer with Units
• R 4 O

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Electric Power

• When charges move through a circuit, they lose
  energy.
• The energy is transformed into useful work or
  lost as heat.
• The rate at which this energy does useful work is
  electric power.
• Electric power is measured in Watts (W) and is
  calculated by
• power current x voltageP I x V

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Electric Power Example Problem

• When a hair dryer is plugged into a 120 V outlet,
  it has a 9.1 A current in it. What is the hair
  dryers power rating?

• Given
• V 120 V
• I 9.1 A
• Unknown
• P ?
• Equation
• P I x V
• Plug Chug
• P 120 V x 9.1 A
• P 1092 W
• Answer with Units
• P 1092 W

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Electric Circuits

• Circuit complete path through which current can
  travel
• Components
• Voltage Source (battery)
• Wires
• Load or Resistance (resistors, light bulbs)
• Switch
• Fuse or circuit breaker (Whats the difference?)
• Schematic Diagram circuit drawing using standard
  symbols for each component
• A circuit must be closed for charge to flow

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Simple Series Circuit

• Multiple resistances are connected in the same
  path
• If one resistor is removed, the circuit is opened
  and current stops flowing
• Equivalent resistance of circuit is the sum of
  all individual resistance
• Re R1 R2 R3
• Current through each resistor is EQUAL
• Voltage drop across each resistor is proportional
  to resistance.
• Using Ohms Law ?V1 I x R1 (voltage drop)
• Sum of voltage drops must equal source voltage.

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Simple Parallel Circuit

• Multiple resistances are connected each on its
  own path
• If one resistor is removed, current still flows
  through the other paths and the other components
  still work
• Equivalent resistance of circuit is determined
• 1/Re 1/R1 1/R2 1/R3 (be careful, this
  is tricky)
• Voltage drop across each resistor is the SAME and
  equal to the source voltage
• Current splits and goes through all the different
  paths.
• Current through each resistor (path) depends on
  resistance. More current takes the path of least
  resistance.
• Using Ohms Law I1 V / R1
• Current in each path must add up to total current
  for circuit.