11.2-11.3 Electric Power Distribution, Generators and Motors.
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Transcript of 11.2-11.3 Electric Power Distribution, Generators and Motors.
11.2-11.3 Electric Power Distribution, 11.2-11.3 Electric Power Distribution, Generators and Motors Generators and Motors
New ideas for todayNew ideas for today
•Magnetic induction•Lenz’s law•Transformers and power transmission•Motors and Generators
Observations about Power Observations about Power DistributionDistribution Household power is AC (alternating current) Power comes in voltages like 120V & 240V Power is transmitted at “high voltage” Power transformers are everywhere
Power Consumption in wiresPower 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!
AC = alternating currentAC = alternating current
Current switches direction 60 times per second (in N. America)
DC= “direct current” AC
AC bulb on cordAC bulb on cord
Power TransmissionPower 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.
Voltage HierarchyVoltage 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
Electromagnetism IIElectromagnetism II
Magnetic fields created by Fundamental particles (dipoles) ― electrons,
protons, neutrons… Moving electric charges (current)
Electric fields created by Charges
Electromagnetism IIElectromagnetism II
Magnetic fields created by Fundamental particles (dipoles) ― electrons,
protons, neutrons… Moving electric charges (current)
Electric fields created by Charges Changing magnetic fields (induction)
Electromagnetic InductionElectromagnetic Induction Changing magnetic field electric field Electric field in conductor current Current magnetic field Induced magnetic field opposes the original
magnetic field change (Lenz’s law)
EM cannonEM cannon
TransformerTransformer Alternating current in one circuit induces an alternating
current in a second circuit Transfers power between the two circuits Doesn’t transfer charge between the two circuits
TransformerTransformer
Current and VoltageCurrent 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 voltage =Primary voltageSecondary turns
Primary turns
Step Up TransformerStep 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
Step Down TransformerStep 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 the
secondary circuit
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
Clicker questionClicker question
Electric Generators and MotorsElectric 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
Electric GeneratorElectric GeneratorRotating magnet makes changing magnetic field induces AC current in the loop
Converts mechanical power into electrical power
Coil and magnetCoil and magnet
Induction flashlightInduction flashlight
GeneratorGenerator
Electric MotorElectric Motor Input AC power AC current makes changing magnetic field causes magnet to turn
Converts electrical power into mechanical power
A motor is a A motor is a generator run generator run backwards !backwards !