Unit 7, Chapter 19

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Unit 7, Chapter 19
CPO Science Foundations of Physics
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Unit 7 Electricity and Magnetism
Chapter 19 Electricity

• 19.1 Electric Circuits
• 19.2 Current and Voltage
• 19.3 Electrical Resistance and Ohms Law

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Chapter 19 Objectives

• Describe the difference between current and
  voltage.
• Describe the connection between voltage, current,
  energy, and power.
• Describe the function of a battery in a circuit.
• Calculate the current in a circuit using Ohms
  law.
• Draw and interpret a circuit diagram with wires,
  battery, bulb, and switch.
• Measure current, voltage, and resistance with a
  multimeter.
• Give examples and applications of conductors,
  insulators, and semiconductors.

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Chapter 19 Vocabulary Terms

• electricity
• electric current
• voltage
• resistance
• Ohms law
• battery
• open circuit
• closed circuit
• short circuit

• switch
• circuit diagram
• electrical conductivity
• potentiometer
• wire
• volt
• electrical symbols
• amperes (amps)

• multimeter
• ohm
• resistor
• ammeter
• electrical insulator
• semiconductor
• conductor
• electric circuit

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19.1 Electric Circuits

• Key Question
• What is an electric circuit?

Students read Section 19.1 AFTER Investigation
19.1
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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.

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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.

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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.

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19.2 Current and Voltage

• Key Question
• How does current move through a circuit?

Students read Section 19.2 AFTER Investigation
19.2
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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.

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19.2 Current and voltage

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

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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.

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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.

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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.

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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.

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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).
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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.

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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.

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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?

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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.

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19.2 Current is a flow of charge
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19.3 Electrical Resistance and Ohms Law

• Key Question
• How are voltage, current, and resistance related?

Students read Section 19.3 AFTER Investigation
19.3
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19.3 Electrical resistance

• Resistance measures how difficult it is for
  current to flow.

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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.

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19.3 Measuring resistance

• Set the meter to measure resistance (W).
• Set the black and red leads on opposite ends of
  the objects.

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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.

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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.

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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.

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19.3 The resistance of electrical devices

• The resistance of electrical devices ranges from
  very small (0.001 O) to very large (10106 O).
• 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.

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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.

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19.3 Electrical Conductivity

• The electrical conductivity describes a
  materials ability to pass electric current.

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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.

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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).

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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.

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Application Hybrid Gas/Electric Cars