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Unformatted text preview: 11-1 USING ENERGY11Q11.1.Reason:The friction between your hands increases the kinetic energies in molecules of your hands, and this exhibits itself as an increase in thermal energy of your hands. The temperature of your hands goes up. Assess:Thermal energy is related to molecular kinetic energies. Q11.2.Reason:The water entering the hydroelectric plant converts its gravitational potential energy to kinetic energy as it runs through the plant. This turns the turbines in the plant, transferring its kinetic energy to the rotational kinetic energy of the turbines. The turbines convert the mechanical kinetic energy of the rotation into electrical energy, which is then converted back to mechanical energy in the pump, which transfers mechanical energy into the kinetic energy and potential energy of the water being pumped through the fountain. Assess:With each energy transformation, energy is lost to thermal energy. Q11.3.Reason:Sitting down to breakfast, you ingest food which is partly used at the time and partly converted to stored energy. When you get up and walk to your bike you convert that energy to the mechanical energy of walking. The energy is further converted to mechanical energy when you ride your bike. Q11.4.Reason:Riding a bike, you do not change your height. As you run you constantly propel yourself off the ground. Extra energy is required for this. Q11.5.Reason:The chicken’s efficiency in using the corn’s energy is not 100%, so some would be lost if you feed the corn to the chicken. Eat the chicken first, and eat the corn later.Assess:Your own body isn’t 100% efficient in using the corn either, but at least you wouldn’t lose twice.Q11.6.Reason:We know from the previous chapter that kinetic energy increases as the square of the velocity. As velocity increases, kinetic energy increases. As the engine turns faster, the rate at which friction creates waste thermal energy also increases. Also, as the speed of the car increases the drag force on the car increases. All of this requires a faster rate of energy consumption of the energy available as chemical energy in the fuel. Q11.7.Reason:Refer to Equation 11.12, the first law of thermodynamics. (a)The space shuttle gets hotter, and its thermal energy increases, but it isn’t heated in the sense of being in thermal contact with a hotter object. Instead, it gets hot by the other method of changing the thermal energy: work. As the shuttle slams into air molecules in the upper atmosphere, work is done on them by the shuttle and work is done on the shuttle by them. The atoms in the shuttle jiggle more vigorously. (b)Since energy must be conserved but the shuttle is slowing down, that kinetic energy decrease must be balanced by an increase in another form of energy. In this case the shuttle’s initial kinetic energy is decreased while the thermal energy of the shuttle is increased....
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