Question 1186725
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It takes 4200J to raise 1kg of water by 1°C

a) To boil, you must raise the temperature to 100°C  (1 atmosphere of pressure assumed).   That's a change of 100-20 = 80°C

1.5l has a mass of 1500g or 1.5kg

80*1.5*4200 = 504000J

Same calculation including unit analysis:
80°C*1.5kg*4200J/kg/°C = 504000J


b) It takes  2260J/g of heat to turn water at 100°C into steam.
One half of the water in this case is 0.5*1.5kg = 0.75kg

2260J/g = 2260000J/kg * 0.75kg = 1695000J


c) One watt is one joule per second  (1W = 1J/s)  Watts are units for POWER (work per unit time) while Joules are units of ENERGY or WORK.

1000W ==>  1000J/s * t  work can be done in t seconds

504000J = 1000J/s * t ==>  t = 504sec ( =  8min 24sec )