Schaum\'s HW problem: I have the answer but can\'t figure outhow to get it. Q: W

Question

Schaum's HW problem: I have the answer but can't figure outhow to get it. Q: What should be the resistance of a heating coil whichwill be used to raise the temperature of 500 g of water from 28 degrees C to the boiling point in 2.0minutes, assuming that 25 % of the heat is lost? The heateroperates on a 110-V line. Answer should be 7.2. Q: What should be the resistance of a heating coil whichwill be used to raise the temperature of 500 g of water from 28 degrees C to the boiling point in 2.0minutes, assuming that 25 % of the heat is lost? The heateroperates on a 110-V line. Answer should be 7.2.

Explanation / Answer

the resistance of a heating coil R 25 % of the heat is lost => 75% is useful 0.75*U2/R*2*60=0.5*4180*(100-28) specific heat capacity of water =4180 J/kgK U=110 V =>R=7.2

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