Welcome to the Phys 606 Chat Room 89 PM Mar 5

1
Welcome to the Phys 606 Chat Room 8-9 PM Mar 5
You are free to type text in the text line
anytime. If you want to speak, click on the hand
and I will give you the floor. Do this now just
to say hello Then I will review some topics Focus
on elementary concepts - help session None of the
questions in the chat room are graded.
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Syllabus
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Agenda

• Two questions on Discussion Board
• Last Time (did not finish)
• Lecture 15 Flashlights
• Lecture 16 Power Distribution
• Lecture 17 Power Distribution II
• Lecture 18 Tape Recording
• Lecture 19 Review

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Which of the following fields can repel or push a
stationary electron?

• Electric fields, but not gravitational fields and
  magnetic fields
• Gravitational fields but not electric or magnetic
  fields
• Electric, magnetic, and gravitational fields
• Magnetic and gravitational fields but not
  electric fields
• Neither electric, magnetic, nor gravitational
  fields
• What is the point here?

5
If you hold a magnet over a piece of metal, do
you feel attraction or repulsion when you try to
pull it away from the surface.
6
Lecture 15 and 16Flashlights and Electric Power
Distribution

• Ohms Law
• Power law
• Transformers

7
Segment of a wireconnecting a battery

• Ohms Law VRI
• Suppose I2 Amps, and there is 5 volt voltage
  drop along the wire, what is R?
• R V/I (5 - 0)/2 2.5 Ohms

For a long wire with lots of current. Then you
can loose lots of energy due to collisions of
the electrons (current) with the
atoms(resistance).
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Power Law

• Power VI
• Suppose Resistance is not known
• P 52 10 Volt Amps 10
  Joules/sec
• 10 Watts

• Power RII RI2
• Note loose energy/power as the square of the
  current.
• P 2.52210 Joules/sec
• 10 Watts

9
Flashlight Circuit
What causes the bulb to light up?
Ignore
12 V
Battery
100 Ohms
-
Bulb
wire

wire
VRI IV/R12/100 0.12 Amps
What is the current flowing in the wire?
10
Electric Power Distribution
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Electromagnet
Thin iron tube
Solid iron core
12
Step-up Transformer NS 100 NP , VS gt VP and IS
lt IP

• Current in wire produces magnetic field in iron
  core
• Changing current in wire produces changing
  magnetic field in iron core
• Changing magnetic field produces changing voltage
  and current in secondary coil, which plays the
  radio
• NS 100 NP
• VS VP NS / NP
• IS IP NP / NS

13
Step-up Transformer NS 100 NP , VS gt VP and IS
lt IP

• Current in wire produces magnetic field in iron
  core
• Changing current in wire produces changing
  magnetic field in iron core
• Changing magnetic field produces changing voltage
  and current in secondary coil, which plays the
  radio
• NS 100 NP
• VS VP NS / NP
• IS IP NP / NS

14
Step-down Transformer NS 0.01 NP ,VS lt VP and IS
gt IP

• Current in wire produces magnetic field in iron
  core
• Changing current in wire produces changing
  magnetic field in iron core
• Changing magnetic field produces changing voltage
  and current in secondary coil, which plays the
  radio
• NS 0.01 NP
• VS VP NS / NP
• IS IP NP / NS

15
Recording Tape
16
Permanent magnets

• Tiny particles contain only one domain
• Single domains magnetize by magnetic flipping
• Long, thin particles are hard to flip
• They make excellent tiny permanent magnets
• They are the basis for magnetic tape
• Magnetic tape is covered with such particles

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Domains

• Domains not aligned
• Domains aligned up

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Permanent Magnets

• Made from hard magnetic materials
• Difficult to magnetize with outside field
• Difficult to demagnetize when field is removed
• Fabricated in non-magnetic state
• Then magnetized by an intense, pulsed field
• Demagnetized by heat, shock, and strong fields

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Recording Tape

• Sound current sent through ring-shaped
  electromagnet
• Split in ring develops north and south poles
• Nearby tape region becomes magnetized

20
Playing Back Tape

• Tape moves past gap in ring-shaped electromagnet
• Fluctuating magnetism in ring induces current in
  playback coil

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Recording Details

• Louder sound ? deeper magnetization
• Higher pitch ? closer magnetic reversals
• Stereo ? two separate magnetic tracks/heads
• Noise reduction ? high pitch expansion
• Pitch control ? tape speed control
• Sound degradation ? magnetization damage