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Current and Resistance
87
17.44
The energy used was
3
$200
2.50
10 kWh
$0.080 kWh
cost
E
rate
==
=×
The total time the furnace operated was
3
2.50
104 h
24.0 kW
E
×
=
P
t
, and since
January has 31 days, the average time per day was
104 h
3.36 h d
31.0 d
average daily operation
17.45
The energy saved is
and the monetary savings is
()
(
)
3
40 W
11 W
100 h
2.9
10 Wh
2.9 kWh
high
low
Et
∆=
−
⋅=
−
=
×
=
PP
( )
( )
2.9 kWh
$0.080 kWh
$0.23
23 cents
savings
E rate
=∆ ⋅
=
=
=
17.46
The power required to warm the water to 100°C in 4.00 min is
( )
( )
( )( )
2
0.250 kg
4 186 J kg °C
100 C
20°C
3.5
10 W
4.00 min
60 s 1 min
mc
T
Q
tt
⋅°
−
∆
∆
=
∆∆
P
The required resistance (at 100°C) of the heating element is then

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