11.2-11.3 Electric Power Distribution, Generators and Motors

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11.2-11.3 Electric Power Distribution, Generators
and Motors
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• New ideas for today
• Magnetic induction
• Lenzs law
• Transformers and power transmission
• Motors and Generators

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Why such high voltage?
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Transformers!
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Observations about Power Distribution

• Household power is AC (alternating current)
• Power comes in voltages like 120V 240V
• Power is transmitted at high voltage
• Power transformers are everywhere

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Power Consumption in wires

• Reminder
• power consumption current voltage drop
• voltage resistance current
• power consumption resistance current2
• So what?
• Wires waste power as heat
• Doubling current quadruples wasted power
• Better not transmit high current!

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AC bulb on cord
AC alternating current
Current switches direction 60 times per second
(in N. America)
DC direct current
AC
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Power Transmission

• Power delivered to a city is
• power delivered current voltage drop
• Power wasted in transmission wires is
• power wasted resistance current2
• For efficient power transmission
• Use low-resistance wires (thick, short copper)
• Use low current and high voltage drop
• Can accomplish this with AC (alternating current)
  power transmission.

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Power lines
160-800 kV
7000 V
120 / 240 V
neutral
hot
ground
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Voltage Hierarchy

• High voltage is dangerous
• High current is wasteful
• Use the following scheme
• low voltage circuits in neighborhoods (120/240 V)
• medium voltage circuits in cities (7000 V)
• high voltage circuits across the countryside
• (155,000-765,000 V)
• Use transformers to change voltage

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Click me
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Electromagnetism II

• Magnetic fields created by
• Fundamental particles (dipoles) ? electrons,
  protons, neutrons
• Moving electric charges (current)
• Electric fields created by
• Charges

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Electromagnetism II

• Magnetic fields created by
• Fundamental particles (dipoles) ? electrons,
  protons, neutrons
• Moving electric charges (current)
• Electric fields created by
• Charges
• Changing magnetic fields (induction)

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Electromagnetic Induction
EM cannon

• Changing magnetic field ? electric field
• Electric field in conductor ? current
• Current ? magnetic field
• Induced magnetic field opposes the original
  magnetic field change (Lenzs law)

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Lenzs Law
Magnetic brake
Pipe and magnet
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Transformer
Transformer

• Alternating current in one circuit induces an
  alternating current in a second circuit
• Transfers power between the two circuits
• Doesnt transfer charge between the two circuits

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Click me
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Current and Voltage

• Power arriving in the primary circuit must equal
  power leaving the secondary circuit
• Power current voltage
• A transformer can change the voltage and current
  while keeping the power unchanged!

Secondary turns
Secondary voltage
Primary voltage
Primary turns
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Step Up Transformer

• More turns in secondary circuit so charge is
  pushed a longer distance
• Larger voltage rise
• A smaller current at high voltage flows in the
  secondary circuit

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Step Down Transformer

• Fewer turns in secondary circuit so charge is
  pushed a shorter distance
• Smaller voltage rise
• A larger current at low voltage flows in
  thesecondary circuit

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Transformers can be dangerous
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Clicker question
You decide to use a transformer to increase the
voltage from a battery, and hook it up in the
circuit shown below. When you close the switch,
1.5
the voltage across the lightbulb is
(A) bigger than 1.5 V (B) smaller than 1.5 V (C)
zero
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inductive charging
B
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Electric Generators and Motors

• A generator provides electric power
• A generator requires a mechanical power
• A motor provides mechanical power
• A motor requires electric power

Alternator
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Click me
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Electric Generator
Coil and magnet
Induction flashlight

• Rotating magnet
• makes changing magnetic field
• induces AC current in the loop

Generator
Converts mechanical power into electrical power
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Electric Motor

• Input AC power
• AC current makes changing magnetic field
• causes magnet to turn

Converts electrical power into mechanical power
A motor is a generator run backwards !
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For next class Read Section 13.1
See you next class!