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78
CHAPTER 17
17.16
We assume that your hair dryer will use about 400 W of power for 10 minutes each day
of the year. The estimate of the total energy used each year is
()( )
()
min
1 hr
0.400 kW
10
365 d
24 kWh
d6
0
m
i
n
Et
=∆=
=
P
If your cost for electrical energy is approximately ten cents per kilowatt-hour, the cost of
using the hair dryer for a year is on the order of
$
24 kWh
0.10
$2.4
or
~$1
kWh
cost
E
rate
=×
=
=
17.17
The resistance is
9.11 V
0.253
36.0 A
V
R
I
∆
==
=
Ω
, so the resistivity of the metal is
( )
( )
2
23
8
4
0.253
2.00
10
m
1.59
10
m
45
0
.
0
m
Rd
RA
LL
ππ
ρ
−
−
⋅Ω
×
⋅
=
=×Ω
⋅
Thus, the metal is seen to be
silver .
17.18
With different orientations of the block, three different values of the ratio

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