How Things Work - University of Illinois at Urbana–Champaign
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Transcript How Things Work - University of Illinois at Urbana–Champaign
11.2-11.3 Electric Power Distribution,
Generators and Motors
New ideas for today
•Magnetic induction
•Lenz’s law
•Transformers and power transmission
•Motors and Generators
Why such high voltage?
Transformers!
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
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!
AC
DC
vs.
Edison
Tesla
Westinghouse
AC = alternating current
AC bulb on cord
Current switches direction 60 times per
second (in N. America)
DC= “direct current”
AC
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.
Power lines
160-800 kV
7000 V
120 / 240 V
neutral
ground
hot
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
Click me
Electromagnetism II
Magnetic fields created by
Fundamental particles (dipoles) ― electrons,
protons, neutrons…
Moving electric charges (current)
Electric fields created by
Charges
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)
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 (Lenz’s law)
Lenz’s Law
Magnetic brake
Pipe and magnet
Transformer
Transformer
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
Click me
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 voltage = Primary voltage
Secondary turns
Primary turns
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
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 the
secondary circuit
Transformers can be dangerous…
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
inductive
charging
B
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
Click me
Electric Generator
Rotating magnet
Coil and magnet
Induction flashlight
Generator
makes changing magnetic field
induces AC current in the loop
Converts
mechanical power
into
electrical power
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 !
For next class: Read Section 13.1
See you next class!