Lesson 2: Power and Heat
Concepts and Solved Exercises on Heat and Power
1) A heat source was used to heat 1.0 L of water (1000 g) from room temperature (20 °C) to boiling point (100 °C) in a time interval of 1 min 40 s with 100% efficiency. Given that the specific heat of water is 1.0 cal/g °C, what is the power of this source?
2) A heat source of constant power supplies 50 cal/min to a 100 g sample of a substance. The graph below shows the temperature as a function of the heating time of this body. What is the specific heat of the material of this substance?
3) A good electric shower, when switched to the "winter" position, dissipates a power of 6.4 kW, supplying this energy to the water passing through it at a flow rate of 50 grams per second. If the water entering the shower has a temperature of 23 °C, what is its temperature at the outlet?
Data: specific heat of water 1.0 cal/g °C; 1 cal = 4 J.
4) In a solar water heater, energy from the sun is absorbed by water circulating through pipes in a collector on the roof. Solar radiation penetrates the collector through a transparent cover and heats the water in the pipes; this water is then pumped to a storage tank. Assume that the efficiency of the entire system is 20% (i.e., 80% of the incident solar energy is lost). What collector area is needed to raise the temperature of 200 L of water in the tank from 20 °C to 40 °C in 1 hour when the intensity of incident sunlight is 700 W/m²?
5) To prepare a cup of instant coffee, a small electric immersion heater is used to heat 100 g of water. The label says it's a "200-watt" heater (that's the rate of conversion of electrical energy into thermal energy). Calculate the time required to heat water from 23.0°C to 100°C, neglecting heat loss.