The electric heating element in a 40-gallon water heater is rated at 5kW. You are planning on taking the hottest shower possible, and are therefore hoping your water heater is ready with all 40 gallons at 120ºF. Unfortunately, you realize your roommate has just gotten out of the shower and the temperature inside the water heater is now a measly 90ºF. How long will you have to wait before the water heater is back at 120ºF?
It might help to know that the density of water is 3.79kg per gallon. – 35 minutes
You’re planning on making some hot water for tea. You need about 250 grams of water, and it comes out of the tap at 10ºC. You’re trying to decide whether you should use the microwave or your aluminum kettle (with a mass of 1kg). How much more energy would it take to heat up the water in the kettle, assuming that the aluminum of the kettle also starts at 10ºC? If the stove under the kettle and the microwave both were rated to output 1000W, how much longer would the kettle take to boil the water than the microwave? – 81 seconds
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