ELECTRICITY

1
ELECTRICITY

• Part 1 Electrostatics
• Part 2 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

7
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

10
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

11
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.

13
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

17
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)

19
Electric Fields

• An electric field is the region around a charge
  in which the electrostatic force is felt by other
  charges.
• Diagram electric fields using field lines.
• By convention, electric field lines always extend
  from a positively-charged object to a
  negatively-charged object, from a
  positively-charged object to infinity, or from
  infinity to a negatively-charged object.
• Electric field lines are most dense around
  objects with the greatest amount of charge.
• 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.

20
Electric Field Lines
21
Electric Field Lines
22
Electric Fields

• Force experienced by a positive test charge at
  any point in space is in the direction tangent to
  the line of force at that point.
• Electric Field (intensity or strength)
• Electric force experienced by a positive test
  charge divided by that charge
• E F/q0
• Measured in N/C (or V/m)

23
Electric Field for a Point Charge

• Derived from Coulombs Law
• E kq / r2
• Not uniform inversely proportional to square of
  the distance
• By contrast, the electric field between two
  parallel plates (capacitor) IS uniform.

24
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.
• In order to move an electric charge, we need to
  apply a force.

25
Electrical Potential Energy

• What is needed in order to move a charge against
  the influence of an electric field?
• An external force
• Work
• If work is done on the charged particle, what
  happens to its energy?
• It increases!!! (Law of Conservation of Energy)
• What can do work on the charged particle to
  decrease its energy?
• The field!!!

26
Electrical Potential Energy

• Since this work changes a charges position in
  the electric field, the type of energy it gains
  is potential, specifically electrical potential
  energy (PEE)
• Work done against the field always increases PEE
• Work done by the field always decreases PEE

27
Electrical Potential Energy

• Draw field diagram for an electric field
  surrounding a positive charge (proton)
• Draw field diagram for an electric field
  surrounding a negative charge (electron)
• For a small, positive test charge where would it
  have a large PE? A small PE?
• Would the PE be numerically the same for a
  slightly larger (or smaller) test charge?

28
Electrical Potential

• Electrical Potential Energy depends on the size
  of the test charge.
• If we want to get a picture for the energy
  independent of the test charge, we need to look
  at electrical potential
• Electrical Potential (V) potential energy per
  unit charge
• V PEE / q0
• Units joules per coulomb (J/C) or volts (V)
• Electric Potential is commonly known as voltage

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Potential Difference

• Electric potential itself is not useful, only the
  change in electric potential provides us with
  useful information.
• Potential Difference (?V) change in electrical
  potential energy per unit charge
• ?V Vf Vi
• The potential difference between any two points A
  B equals the work done against the field in
  moving a positive test charge from A to B with no
  acceleration
• ?V WAB / q0 and ?V ?PEE / q0
• The work done is independent of the path taken

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Potential Difference

• For a proton or an electron (qe) a change in
  potential of 1 V, produces a change in PEE of 1.6
  x 10-19 J
• While very small in size, many atomic phenomena
  involve energies of this order of magnitude. A
  reasonable unit is needed in order to report this
  energy.
• Electron Volt (eV) amount of energy
  corresponding to an electron falling through a
  potential difference of one volt
• 1 eV 1.6 x 10-19 J

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Voltage

• So how is this useful?
• Batteries have two terminals, one of high
  potential () one of low potential (-).
• A potential difference!!!! (i.e. VOLTAGE)
• Positive charge naturally flows from pos.
  terminal to neg. terminal by means of an external
  circuit.
• Potential energy of charges decreases and is
  converted into useful energy.

32
Current

• Current the rate at which the electric charges
  move through a conductor or circuit
• How much charge is flowing per second
• 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

33
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

34
Electrical Quantities
35
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

37
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

38
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
• A circuit must be closed for charge to flow

40
Simple Series Circuit

• Multiple resistances are connected all along the
  same path
• 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. ?V1 I x R1
• Sum of voltage drops must equal voltage supplied
  by voltage source.

41
Simple Parallel Circuit

• Multiple resistances are connected each on its
  own path
• Equivalent resistance of circuit is determined
• 1/Re 1/R1 1/R2 1/R3
• Voltage drop across each resistor is EQUAL
• Current through each resistor (path) is
  proportional to resistance. I1 V / R1

42
Compound Circuits

• Apply rules of series and parallel circuits.
• Resistances in series or parallel can be combined
  to make an equivalent resistance (this may need
  to be done multiple times)
• Voltage drops around a circuit must equal voltage
  source
• Total current entering a node must equal current
  leaving a node
• Ohms Law (V I x R) always applies!!!
• For individual resistors and the circuit as a
  whole!