GTBL042-12 GTBL042-Callister-v2 August 13, 2007 18:22
512 • Chapter 12 / Electrical PropertiesQUESTIONS AND PROBLEMS
Additional problems and questions for this chapter may be found on both Student and
Instructor Companion Sites atwww.wiley.com/college/callister.Ohm’s Law
Electrical Conductivity
12.1 (a)Compute the electrical conductivity of a
7.0-mm (0.28-in.) diameter cylindrical sil-
icon specimen 57 mm (2.25 in.) long in
which a current of 0.25 A passes in an
axial direction. A voltage of 24 V is mea-
sured across two probes that are sepa-
rated by 45 mm (1.75 in.).
(b)Compute the resistance over the entire
57 mm (2.25 in.) of the specimen.
12.2A plain carbon steel wire 3 mm in diameter
is to offer a resistance of no more than 20
. Using the data in Table 12.1, compute the
maximum wire length.
12.3 (a)Using the data in Table 12.1, compute
the resistance of an aluminum wire 5 mm
(0.20 in.) in diameter and 5 m (200 in.) long.
(b)What would be the current flow if the po-
tential drop across the ends of the wire is 0.04
V?(c)What is the current density?(d)What
is the magnitude of the electric field across
the ends of the wire?Energy Band Structures in Solids
12.4How does the electron structure of an iso-
lated atom differ from that of a solid mate-
rial?Electron Mobility
12.5 (a)Calculate the drift velocity of electrons
in silicon at room temperature and when
the magnitude of the electric field is
500 V/m.
(b)Under these circumstances, how long
does it take an electron to traverse a
25-mm (1-in.) length of crystal?
12.6 (a)Calculate the number of free electrons
per cubic meter for silver, assuming that there
are 1.3 free electrons per silver atom. The
electrical conductivity and density for Ag are
6.8× 107 (-m)−^1 and 10.5 g/cm^3 , respec-
tively.(b)Now compute the electron mobil-
ity for Ag.Electrical Resistivity of Metals
12.7 (a)Using the data in Figure 12.8, determine
the values ofρ 0 andafrom Equation 12.10 for
pure copper. Take the temperatureTto be in
degrees Celsius.(b)Determine the value ofA
in Equation 12.11 for nickel as an impurity in
copper, using the data in Figure 12.8.(c)Us-
ing the results of parts (a) and (b), estimate
the electrical resistivity of copper containing
2.50 at% Ni at 120◦C.
12.8Tin bronze has a composition of 89 wt% Cu
and 11 wt% Sn, and consists of two phases at
room temperature: anαphase, which is cop-
per containing a very small amount of tin in
solid solution, and anphase, which consists
of approximately 37 wt% Sn. Compute the
room temperature conductivity of this alloy
given the following data:Electrical Resistivity Density
Phase (-m)(g/cm^3 )
α 1.88× 10 −^8 8.94
5.32× 10 −^7 8.25Intrinsic Semiconduction
12.9 (a)Using the data presented in Figure 12.16,
determine the number of free electrons
per atom for intrinsic germanium and sil-
icon at room temperature (298 K). The
densities for Ge and Si are 5.32 and 2.33
g/cm^3 , respectively.
(b)Now explain the difference in these free-
electron-per-atom values.
12.10At room temperature the electrical conduc-
tivity of PbS is 25 (-m)−^1 , whereas the elec-
tron and hole mobilities are 0.06 and 0.02
m^2 /V-s, respectively. Compute the intrinsic
carrier concentration for PbS at room tem-
perature.
12.11For each of the following pairs of semicon-
ductors, decide which will have the smaller
band gap energy,Eg, and then cite the reason
for your choice.(a)C (diamond) and Ge,(b)