Announcements

1

• Announcements
• My office hours are today from 2 3 pm
• or by appointment
• WebAssign HW Set 4 due this Friday
• Problems cover material from Chapters 17
• Exam 1 820 1010 pm Wednesday, February 16
• Covers Ch. 15-18
• 20 questions
• Room assignments
• QUESTIONS? PLEASE ASK!

2
From last time

• Current
• Drift speed
• Resistance/
• Ohms Law
• Resistivity

L
3
Resistivity

• The resistance of an ohmic conductor is
  proportional to its length, L, and inversely
  proportional to its cross-sectional area, A
• ? is the constant of proportionality and is
  called the resistivity of the material

L
4
Example Problem

• 17.12 Suppose you wish to fabricate a uniform
  wire out of 1 g of copper. If the wire is to
  have a resistance R 0.50 W, and if all of the
  copper wire is to be used, what will be the (a)
  length and (b) diameter of the wire?

5
Temperature Variation of Resistivity/Resistance

• For most metals, resistivity increases
  approximately linearly with temperature over a
  limited temperature range
• ?resistivity at T ?oresistivity at
  To(typically 20 C)
• ? is the temperature coefficient of resistivity
• and resistance is proportional to the
  resistivity, so

6
Example Problem

• 17.30 A toaster rated at 1050 W operates on a 120
  V household circuit and uses a 4.00 m length of
  Nichrome wire as its heating element. The
  operating temperature of this element is 320C.
  What is the cross-sectional area of the wire?

7
Electrical Energy in a Circuit

• Energy DQ?V provided by the battery (A?B) is lost
  in the resistor(C?D)
• Recall PE q?V
• The rate at which the energy is lost is
• Two other ways to write this

8
Example Problem

• 17.40 A certain toaster has a heating element
  made of Nichrome resistance wire. When the
  toaster (at 20C) is first connected to 120 V
  source, the initial current is 1.80 A, but the
  current decreases when the element heats up.
  When the toaster reaches it final temperature,
  the current is 1.53 A. (a) Find the power the
  toaster produces at its final temperature. (b)
  What is the final temperature?

9
Superconductors

• Remarkable materials!
• R ? 0 below a critical temperature, TC
• above TC, behave as normal metals, but suddenly
  drops to zero at TC

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Solution to 17.12
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Solution to 17.30
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Solution to 17.40