Molecular Physics | Thermodynamics Answers

Compare the partial differential equation of Wave and Heat; state their similari ties and differences.

The compressor of a heat pump that is used to maintain the temperature of a space at 298 K consumes 1600 J/s power. The heat pump absorbs heat from hot water flowing into the evaporator at the rate of 3.9 kg/min. The temperature of water at the evaporator inlet and outlet are 333 K and 40°C, respectively. The refrigerant used in the heat pump is R134a, it enters the evaporate with a temperature of 285 K with 15% quality and exits as a saturated vapour. Find (a) refrigerant mass flow rate (b) rate of heat supplied to the space (c) the COP (d) the the minimum power input to the compressor for the same rate of heat supply.

For offices to feel comfortable, we like the relative humidity indoors to be about 50% and the temperature to be about 20◦C. An office is normally ventilated: out- side air flows through the office, where it is heated by a radiator. At the same time the office loses heat through the walls. We assume that the air properties in the office are homogeneous throughout the office.
a) How much water (in grams) would the air contain in a small one-person office of 20 cubic metres?
b) Suppose we measure a 30% relative humidity in this office. Calculate whether we can bring the relative humidity in the office up to the comfortable level by closing the air vents and putting a cup of water on the radiator?

Which of the following heat engines has the highest thermal efficiency? The lowest?
i) An engine that in one cycle absorbs 5000 J of heat and rejects 4500 J of heat; ii) An engine that in one cycle absorbs 25000 J of heat and does 2000 J of work; iii) An engine that in one cycle does 400 J of work and rejects 2800 J of heat

To make ice, a freezer that is a reverse Carnot engine extracts 42 kJ as heat at -15°C during each cycle, with coefficient of performance 5.7. The room temperature is 30.3°C. How much energy per cycle is delivered as heat to the room and work per cycle is required to run the freezer?

A laterally insulated bar of length 10cm and constant cross-sectional area lem, of density 10.6gm/cm, thermal conductivity 1.04cal/(cm Sec °C), and specific heat of 0.056cal/gmºC), has initial temperature 100°C. Then at some instance, say t=0, the temperature at x = L is suddenly changed to 0°C and kept at 0°C, where as the temperature at x = 0 is kept at 100°C, find the temperature at the middle of the bar

at t = 1,2,3, 10, 50 sec.

A house has an electric heating system that consists of a 150-W fan and an electric resistance heating element placed in a duct. Air flows steadily through the duct at a rate of 0.5 kg/s and experiences a temperature rise of 5°C. The rate of heat loss from the air in the duct is estimated to be 250 W. Determine the power rating of the electric resistance heating element in kW?

A 500 m3 living room is to be heated by an electric heater. It is expected that the heater be able to increase the air temperature in the room from 5C to 25C in 1 hour. Assuming no heat losses from the sealed room and an atmospheric pressure of 100kPa, (a) determine the required power in kW of the resistance heater? (b) The total cost of the electrical energy consumed during this 1-hour period. Take the unit cost of electricity to be TL0.80/kWh ?

1kg of a fluid expands reversibly according to a linear law from 5.2bar to 2.4 bar. The initial and final volumes are 0.005m3 and 0.03m3. The fluid is then cooled reversibly at constant pressure, and finally compressed reversibly according to a law pv= constant back to the initial conditions of 5.2bar and 0.005m3. Calculate the work done in each process and the net work of the cycle. Sketch the cycle on a p-v diagram.