19_Ch 17 College Physics ProblemCH17 Current and Resistance

19_Ch 17 College Physics ProblemCH17 Current and Resistance...

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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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