P27_070.PDF

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Chapter 27 - 27.70
70. (Fourth problem of Cluster )
From Eq. 27-23, we obtain the resistance at temperature T :
R = V 2
P
= 12 2
10
=14 . 4Ω .
Thus, the ratio R/R 0 with R 0 representing the resistance at 300 K is 7 . 2, which we take to equal the
ratio of resistivities (ignoring any thermal expansion of the filament). Eq. 27-17, then, leads to
ρ
ρ 0 =7 . 2=1+ α ( T
−
300) .
Using Table 27-1( α =4 . 5
×
10 − 3 / K) we find T =1 . 7
×
10 3 K.
 

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