PHYS 210 Chapter Heat, Fall 2012

Class Problems Chapter 20 A particular heat en gine has a useful power output of 5.00 kW and an efficiency of 25.0%. The engine expels 8 000 J of exhaust energy in each cycle. Find (a) the energy taken in during each cycle and (b the time interval for each cycle. A refrigerator has a coefficient of performance of 3.00. The ice tray compartment is at –20.0°C, and the room temperature is 22.0°C. The refrigerator can convert 30.0 g of water at 22.0°C to 30.0 g of ice at –20.0°C each minute. What input power is required? Give your answer in watts. A refrigerator has a coefficient of performance equal to 5.00. The refrigerator takes in 120 J of energy f rom a cold reservoir in each cycle. Find (a) the work required in each cycle and (b) the energy expelled to the hot reservoir. At point A in a Carnot cycle, 2.34 mol of a monatomic ideal gas has a pressure of 1 400 kPa, a volume of 10.0 L, and a temperature of 720 K. It expands isothermally to point B, and then expands adiabatically to point C where its volume is 24.0 L. An isothermal compression brings it to point D, where its volume is 15.0 L. An adiabatic process returns the gas to point A. (a) Determine all the unknown pressures, volumes and temperatures as you fill in the following table: P V T A 1 400 kPa 10.0 L 720 K B C 24.0 L D 15.0 L (b) Find the energy added by heat, the work done by the engine, and the change in internal energy for each of the steps AB, BC, CD, and DA. (c) Calculate the efficiency Wnet/Qh. Show that it is equal to 1 – TC/TA, the Carnot efficiency. An ice tray contains 500 g of liquid water at 0°C. Calculate the change in entropy of the water as it freezes slowly and completely at 0°C. The temperature at the surface of the Sun is approximately 5 700 K, and the temperature at the surface of the Earth is approximately 290 K. What entropy change occurs when 1 000 J of energy is transferred by radiation from the Sun to the Earth? A 1.00-mol sample of H2 gas is contained in the left-hand side of the container shown in Figure P22.45, which has equal volumes left and right. The right-hand side is evacuated. When the valve is opened, the gas streams into the right-hand side. What is the final entropy change of the gas? Does the temperature of the gas change? Figure P22.45 A 1.00-mol sample of an ideal monatomic gas, initially at a pressure of 1.00 atm and a volume of 0.025 0 m3, is heated to a final state with a pressure of 2.00 atm and a volume of 0.040 0 m3. Determine the change in entropy of the gas in this process.