Electricity PowerPoint Presentation Resistance

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					CPO Science
Foundations of Physics




  Unit 7, Chapter 19
   Unit 7: Electricity and Magnetism
            Chapter 19 Electricity


 19.1 Electric Circuits
 19.2 Current and Voltage
 19.3 Electrical Resistance and Ohm’s Law
              Chapter 19 Objectives
1. Describe the difference between current and voltage.
2. Describe the connection between voltage, current,
   energy, and power.
3. Describe the function of a battery in a circuit.
4. Calculate the current in a circuit using Ohm’s law.
5. Draw and interpret a circuit diagram with wires,
   battery, bulb, and switch.
6. Measure current, voltage, and resistance with a
   multimeter.
7. Give examples and applications of conductors,
   insulators, and semiconductors.
           Chapter 19 Vocabulary Terms
   electricity         switch                multimeter
   electric current    circuit diagram       ohm
   voltage             electrical            resistor
   resistance           conductivity          ammeter
   Ohm’s law           potentiometer         electrical
   battery             wire                   insulator
   open circuit        volt                  semiconductor
   closed circuit      electrical symbols    conductor
   short circuit       amperes (amps)        electric circuit
              19.1 Electric Circuits
Key Question:
What is an electric circuit?




 *Students read Section 19.1 AFTER Investigation 19.1
              19.1 Electric Circuits
 Electricity refers to the
  presence of electric current
  in wires, motors, light bulbs,
  and other devices.
 Electric current is similar to
  a current of water, but
  electric current flows in solid
  metal wires so it is not
  visible.
 Electric current can carry a
  lot of power.
             19.1 Electric Circuits
 An electric circuit is something that provides a
  complete path through which electricity travels.
 Wires in electric circuits are similar in some ways to
  pipes and hoses that carry water.
             19.1 Electric Circuits
 When drawing a circuit diagram, symbols are used to
  represent each part of the circuit.
 These electrical symbols are quicker and easier to draw
  than realistic pictures of the components.
          19.2 Current and Voltage

Key Question:
How does current move
 through a circuit?




 *Students read Section 19.2
 AFTER Investigation 19.2
          19.2 Current and voltage

 Electric current is measured in
  units called amperes, or amps
  (A) for short.
 One amp is a flow of a certain
  quantity of electricity in one
  second.
 The amount of electric current
  entering a circuit always equals
  the amount exiting the circuit.
            19.2 Current and voltage

 Conventional current was
  proposed by Ben Franklin in the
  1700’s.
 Scientists later discovered that
  the particles that carry electricity
  in a wire actually travel from
  negative to positive.
 Today, we still use Franklin’s
  definition.
                   19.2 Voltage

 Voltage is a measure of electric
  potential energy, just like height
  is a measure of gravitational
  potential energy.
 Voltage is measured in volts (V).
 A voltage difference of 1 volt
  means 1 amp of current does 1
  joule of work in 1 second.
               19.2 Voltage

 Since 1 joule per second
  is a watt (power), you
  can interpret voltage as
  measuring the available
  electrical power per amp
  of current that flows.
                     19.2 Voltage
 The positive end of a 1.5 volt
  battery is 1.5 volts higher than
  the negative end.
 If you connect batteries
  positive-to-negative, each
  battery adds 1.5 volts to the
  total.
 Three batteries make 4.5 volts.
 Each unit of current coming
  out of the positive end of the
  three-battery stack has 4.5
  joules of energy.
19.2 Measuring voltage of a cell
                Set the meter to DC volts.
                Touch the red (+) lead of
                 the meter to the (+) battery
                 terminal.
                Touch the black (-) lead of
                 the meter to the (-) battery
                 terminal.
                Adjust the meter dial as
                 necessary.
     19.2 Measuring voltage in a circuit

     Measure the voltage
      across the battery
      exactly as before.
     DO NOT DISCONNECT
      THE CIRCUIT.

NOTE: Since voltage is measured from
one point to another, we usually assign the
negative terminal of a battery to be zero
volts (0 V).
          19.2 Current and voltage
 A battery uses chemical
  energy to create a voltage
  difference between its two
  terminals.
 In a battery, chemical
  reactions provide the
  energy to pump the current
  from low voltage to high
  voltage.
 A fully charged battery adds
  energy proportional to its
  voltage.
        19.2 What does a battery do?
 A battery uses chemical energy to move charges.
 If you connect a circuit with a battery the charges
  flow out of the battery carrying energy.
  19.2 How do these batteries differ?




 Some are smaller and don't store as much energy.
 Other batteries made with Ni and Cd can be recharged.
 Which battery above has the greatest voltage capacity?
          19.2 Measuring Current

 In practical electricity,
  we still label current
  flowing from plus to
  minus or HIGH voltage
  to LOW voltage.
 Current can't be
  measured unless the
  charges flow through
  the meter.
19.2 Current is a flow of charge
19.3 Electrical Resistance and Ohm’s
                 Law
Key Question:
How are voltage, current, and resistance related?




*Students read Section 19.3 AFTER Investigation 19.3
      19.3 Electrical resistance




 Resistance measures how difficult it is for
 current to flow.
           19.3 Electrical Resistance
 The total amount of electrical resistance in a circuit
  determines the amount of current that in the circuit for
  a given voltage.
 The more resistance the circuit has, the less current
  that flows.
       19.3 Measuring resistance

 Set the meter to
  measure resistance (W).
 Set the black and red
  leads on opposite ends
  of the objects.
                 19.3 The ohm

 Resistance is
  measured in ohms
  (W).
 One ohm is the
 resistance when a
 voltage of 1 volt is
 applied with a
 current of 1 amp.
                  19.3 Ohm's law
 German physicist Georg Ohm
  experimented with circuits to
  find an exact mathematical
  relationship between voltage,
  current and resistance.
 Ohm's law can be used to
  predict any one of the three
  variable if given the other
  two.
           19.3 Calculate current




 A light bulb with a resistance of 2 ohms is
  connected in a circuit that has a single 1.5-volt
  battery.
 Calculate the current that flows in the circuit.
 Assume the wires have zero resistance.
19.3 The resistance of electrical devices
 The resistance of
  electrical devices ranges
  from very small (0.001 Ω)
  to very large (10×106 Ω).
 Each device is designed
  with a resistance that
  allows the right amount
  of current to flow when
  connected to the voltage
  the device was designed
  for.
19.3 Changing resistance

            The resistance of many
             materials, including those
             in light bulbs, increases
             as temperature increases.
            A graph of current versus
             voltage for a light bulb
             shows a curve.
            A device with constant
             resistance would show a
             straight line on this
             graph.
         19.3 Electrical Conductivity
 The electrical conductivity describes a material’s
  ability to pass electric current.
19.3 Conductors and insulators
              A material such as copper is
               called a conductor because it
               can conduct, or carry, electric
               current.
              Materials that insulate against
               (or block) the flow of current
               are classified as electrical
               insulators.
              Some materials are neither
               conductors nor insulators.
              These materials are named
               semiconductors.
                     19.3 Resistors
 Electrical components called
  resistors can be used to
  control current.
 Resistors have striped color
  codes to record their "values"
  (writing on them is difficult).
              19.3 Potentiometers
 Potentiometers are a type of "variable" resistor that
  can change from low to high.
 They are wired so that as you turn the knob, it
  changes the distance the current has to flow.
Application: Hybrid Gas/Electric Cars

				
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