Electric Circuits

1
Electric Circuits
Why do batteries make flashlights work? Why do
you have to flip a switch? Why are magnets
magnetic? How do the electrons get to your
house? How do you make trains fly?
Proverb We are fishing for minnows from the back
of a whale
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Visible Flow

• Always helpful to have flow
• you can see
• Just like water pumped to a
• high place can do work
• flowing back down
• Charge pumped to a
• high voltage can do work
• flowing back down

3
Electric Circuits Work

• Current through lamp filament requires closed
• path
• Current does work, heats filament, makes light

4
Inside the Flashlight

• Batteries stack end to end between bulb
• and spring contact
• Switch slides to make or to break contact with
  other
• bulb contact

5
The Path of the Electrons

• A conductor is a material through which current
  flows easily.
• An insulator is a material through which current
  does not flow.
• (No material is a perfect insulator or conductor)
• A Light bulb filament is a ????
• How easily do the charges flow??
• What resists their flow??

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What Do You Want from Your Electrons?

• Each collision does work -- absorbs energy
• Lost energy can convert to light (LEDs work this
  way)

or to heat (think incandescent bulb, bathroom
heater) or to work (think mp3 player, vacuum
cleaner, etc) Or to all three (think bulb, vacuum
cleaner)
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Ohms Law for Current and Voltage

• Ohm sez voltage change (charge flow) ?
• (resistance to flow)
• V I ? R V IR
• potential energy change (how many charges
  fall)
• ? (how hard they fall)
• Power Energy / time Joules / second Watts
• P V ? I I ? R ? I I2 ? R, so P
  I2R

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Why 1/4 Step Lights were Dimmer

• Circuit resistance R R R
• so I V/3R less than for a single bulb
• I V/3R and P (V/3R)2(3R) V/3R

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One the Full Step Lamps Were Brighter

• I V/R for each filament so total current from
  battery is 3V/R
• P (V/R)2R
• More current,
• More power
• More light
• (Tank is emptied
• much faster)

R
R
R
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Faradays and Lenzs Law

• Faraday Current flow causes a magnetic field

Lenz Changes in magnetic field induce current
flow
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Electrons Spin Theyre Magnets

• Any rotating charged object creates
• circulating current i.e. a magnet
• Electrons spin and are magnets
• If all the small magnets align,
• it becomes a big magnet

N
-
-
-
-
-
S
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Induced Magnetism at Work

• The same coil and nail trick is used in
• industry
• Can lift many metals by
• induced magnetic polarization
• Can levitate trains by inducing the
• wrong polarization

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Current Induced in Neighboring Wires
A coil of wire creates a magnetic field when the
current flows (Faraday) Some of the lines of
magnetic force can thread other coils If the
first current changes, so does the magnetic
field and a current is induced in the
second coil (Lenz) Transformers use this
induced current to transfer alternating
currents from one coil to the other
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The Transformer

• Coils of wire induce AC currents in remote coils
• Usually use iron bar to concentrate field lines
• Prefer a close loop for field lines because there
  are no charges
• where they can terminate

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Battle Between DCers and ACers

• Business is about money and prejudice too

Edison DC
Tesla AC
Westinghouse AC
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Change the Voltage

• Transformers change voltage
• by coupling different
• length wires
• Long wire driving short wire
• less voltage out (more current)
• Short wire driving long wire
• more voltage out (less current)

17
Power distribution

• High voltage, low current to save heat/waste
• Then
• Low voltage, high current to save lives

18
Magnetic Levitation Trains
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Basic Operation

• Like poles repel each other
• Tricks will be in stability,
• acceleration, and
• BRAKING (!)
• Much of the technology exists
• in hybrid cars now, so
• trains will work soon.
• Ride one at ODU soon

20
Traffic Light Sensors Work This Way, too

• Metal in car induces
• current in coil under
• pavement.

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Take home messages

• Electric circuits are closed loops of conductors
• Ohms Law V IR (and P VI I2R)
• Faradays Law Currents cause magnetic fields
• even tiny currents like spinning electrons
• Lenzs Law Changes in magnetic fields induce
  currents
• Transformers couple AC electric circuits by
  induction
• More loops on the transformer more voltage but
  lower current
• and vice versa