1 Law of Thermodynamics
Problems
1. An energetic athlete can use up all the energy from a diet of 4000 Cal/day. If he were to use up this energy at a steady rate, how would his rate of energy use compare with the power of a 100 W bulb? (The power of 100 W is the rate at which the bulb converts electrical energy to heat and the energy of visible light.) (Ans. 19x106 times as great)
2.How many grams of butter, which has a usable energy content of 6.0 Cal/g (=6000 cal/g), would be equivalent to the change in gravitational potential energy of a 73.0 kg man who ascends from sea level to the top of Mt. Everest, at elevation 8.848 km? Assume that the average value of g is 9.80 m/s2.3
3. One way to keep the contents of a garage from becoming too cold on a night when a severe subfreezing temperature is forecast is to put a tub of water in the garage. If the mass of the water is 125 kg and its initial temperature is 20°С. (a) how much energy must the water transfer to its surroundings in order to freeze completely and (b) what is the lowest possible temperature of the water and its surroundings until that happens?
A chief, on finding his stove out of water, decides to boil the water for his wife's coffee by shaking it in a thermos flask. Suppose that he uses tap water at 15°C and that the water falls 30 cm each shake, the chief making 30 shakes each minute. Neglecting any loss of thermal energy by the flask, how long must he shake the flask until the water reaches 100°C? (Ans. 2.8 days)
A 1500 kg Buick moving at 90 km/h breaks to stop, at uniform deceleration and without skidding, over a distance of 80 m. At what average rate is mechanical energy transferred to thermal energy in the braking system?
6. In a solar water heater, energy from the Sun is gathered by water that circulates through tubes in a rooftop collector. The solar radiation enters the collector through a transparent cover and warms the water in the tubes; this water is pumped into a holding tank. Assume that the efficiency of the overall system is 20% (that is, 80% of the incident solar energy is lost from the system). What collector area is necessary to raise the temperature of 200 L of water in the tank from 20°C to 40°C in 1.0 h when the intensity of the incident sunlight is 700 W/m2?
7. The specific heat of water is about two times that of ethyl alcohol. Equal masses of alcohol and water are contained in separate beakers and are supplied with the same amount of energy. Compare the temperature increases of the two liquids.
9.A water heater is operated by solar power. If the solar collector has an area of 6.00 m2 and the power delivered by sunlight is 550 W/m2, how long does it take to increase the temperature of 1.00 m of water from 20.0"С to 60.0CC?
Work Done during Volume Change
Problems
10. Two moles of oxygen are in a container with rigid walls. The gas is heated until the pressure doubles. Neglect the thermal expansion of the container. Calculate the work done by the gas. (Ans. 0)
11. A gas under a constant pressure of 3 x 105 Pa and an initial volume of 0.05 m is cooled until its volume becomes 0.04 m3. Calculate the work done by the gas.
12. Three moles of an ideal gas are heated at constant pressure from 27°C to 127°C. Calculate the work done by the gas. (Ans. 2494 J)
13. One mole of an ideal gas has an initial temperature of 27°C. While the temperature is kept constant, the volume is decreased until the pressure doubles. Calculate the work done by the gas.
16. An ideal gas in a cylinder is fixed with a frictionless piston. The piston is moved from a volume at pressure to volume at pressure The temperature remains constant throughout the process. The work clone by the gas is
A
zero, because the gas obeys Boyle's law, hence
В negative, because the volume decrease
С positive, because the piston has been moved
D
negative, because the pressure
\
19.An ideal gas expands from 50 cm to 100 cm at constant pressure 1 000 N/m2. The temperature of the gas before expansion is 27 °C, what is its temperature after expansion? Calculate the work done during expansion.