2/3/11

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 8:20 – 10:10 pm Wednesday, February 16 •  Covers Ch. 15-18 •  20 questions •  Room assignments:

QUESTIONS? PLEASE ASK!

From last time

n  Current:

n  Drift speed:

n  Resistance/ Ohm’s Law

n  Resistivity

Δ≡ΔavQIt

ΔV = I R

=lRA

ρL

Resistivity n  The resistance of an ohmic conductor is

proportional to its length, L, and inversely proportional to its cross-sectional area, A

n  ρ is the constant of proportionality and is called the resistivity of the material

=lRA

ρL

Example Problem

n  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 Ω, and if all of the copper wire is to be used, what will be the (a) length and (b) diameter of the wire?

2/3/11

2

Temperature Variation of Resistivity/Resistance

n  For most metals, resistivity increases approximately linearly with temperature over a limited temperature range

n  ρ=resistivity at T; ρo=resistivity at To(typically 20° C) n  α is the temperature coefficient of resistivity

n  … and resistance is proportional to the resistivity, so…

)]TT(1[ oo −α+ρ=ρ

)]TT(1[RR oo −α+=

Example Problem

n  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 320°C. What is the cross-sectional area of the wire?

Electrical Energy in a Circuit

n  Energy ΔQΔV provided by the battery (AàB) is lost in the resistor(CàD) n  Recall PE = qΔV

n  The rate at which the energy is lost is

n  Two other ways to write this:

Q V I Vt

Δ℘ = Δ = Δ

Δ

22 VI R

RΔ

℘ = =

Example Problem n  17.40 A certain toaster has a

heating element made of Nichrome resistance wire. When the toaster (at 20°C) 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?

2/3/11

3

Superconductors n  Remarkable materials! n  R à 0 below a critical

temperature, TC n  above TC, behave as

normal metals, but suddenly drops to zero at TC

Solution to 17.12

Solution to 17.30 Solution to 17.40