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Physics Chapter 10
Transcript of Physics Chapter 10
a. it expands at constant pressure to twice its initial volume?
b. it is compressed at constant pressure to one-quarter of its initial volume? 2. A gas is enclosed in a container fitted with a piston. The applied pressure is maintained at 599.5 kPa as the piston moves inward, which changes the volume of the gas from 5.317 × 10^−4 m^3 to 2.523 × 10^−4 m^3. How much work is done? Is the work done on or by the gas? Explain your answer. 3. A balloon is inflated with helium at a constant pressure that is 4.3 × 10^5 Pa in excess of atmospheric pressure. If the balloon inflates from a volume of 1.8 × 10^−4 m^3 to 9.5 × 10^−4 m^3, how much work is done on the surrounding air by the helium-filled balloon during this expansion? 4. Steam moves into the cylinder of a steam engine at a constant pressure and does 0.84 J of work on a piston. The diameter of the piston is 1.6 cm, and the piston travels 2.1 cm.What is the pressure of the steam? Physics Chapter 10 Thermodynamic Processes heat (Q) internal energy (U) work (W) energy is transferred as both heat and work one type of energy transfer is dominant other type is negligible real process can be approximated
with an ideal process No work is done in a constant-volume process isovolumetric process a thermodynamic process that
takes place at constant volume so that no work is done on or by the system when a gas undergoes a change in temperature but no change in volume, no work is done on or by the system Internal energy is constant in a constant-temperature process isothermal process a thermodynamic process that takes place at constant temperature Energy is not transferred as heat in an adiabatic process adiabatic process a thermodynamic process during which no energy is transferred to or from the system as heat Energy Conservation The principle of energy conservation that takes into account a system’s internal energy as well as work and heat is called the first law of thermodynamics IThe first law of thermodynamics can be expressed mathematically The total change in the internal energy is the difference between the final internal energy value (Uf) and the initial internal energy value (Ui) A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transferred as heat? Has energy been added to or removed from the refrigerant as heat? 1. Heat is added to a system, and the system does 26 J of work. If the internal energy increases by 7 J, how much heat was added to the system? 2. The internal energy of the gas in a gasoline engine’s cylinder decreases by 195 J. If 52.0 J of work is done by the gas, how much energy is transferred as heat? Is this energy added to or removed from the gas? 3. A 2.0 kg quantity of water is held at constant volume in a pressure cooker and heated by a range element. The system’s internal energy increases by 8.0 × 10^3 J.However, the pressure cooker is not well insulated, and as a result, 2.0 × 10^3 J of energy is transferred to the surrounding air.How much energy is transferred from the range element to the pressure cooker as heat? 4. The internal energy of a gas decreases by 344 J. If the process is adiabatic, how much energy is transferred as heat? How much work is done on or by the gas? 5. A steam engine’s boiler completely converts 155 kg of water to steam. This process involves the transfer of 3.50 × 10^8 J as heat. If steam escaping through a safety valve does 1.76 × 10^8 J of work expanding against the outside atmosphere, what is the net change in the internal energy of the water-steam system? Cyclic Process Cyclic Process a thermodynamic process in which a system returns to the same conditions under which it started In a cyclic process, the system’s properties at the end of the process are identical to the system’s properties before the process took place. In a cyclic process, the system’s properties at the end of the process are identical to the system’s properties before the process took place. all energy is transferred as work and heat no net change in the system’s internal energy is like isothermal process but... Heat engines use heat to do work A heat engine is a device that uses heat to do mechanical work Efficiency of Heat Engines A heat engine cannot transfer all energy as heat to do work (1) the substance absorbs energy as heat from a high-temperature reservoir (2) work is done by the engine (3) energy is expelled as heat to a lowertemperature reservoir all heat engines operating in a cycle must expel some energy to a lower-temperature reservoir does not follow from the first law of thermodynamics The second law of thermodynamics No cyclic process that converts heat entirely into work is possible. W can never be equal to Qh in a cyclic process some energy must always be transferred as heat to the system’s surroundings (Qc > 0). Efficiency measures how well an engine operates efficiency is a measure of the useful energy taken out of a process relative to the total energy that is put into the process For a heat engine, the efficiency is the ratio of work done by the engine to the energy added to the system as heat during one cycle. Note that efficiency is a unitless quantity that can be calculated using only the magnitudes for the energies added to and taken away from the engine. 100 percent efficiency (eff = 1) only if there is no energy transferred away from the engine as heat (Qc = 0) The efficiency equation gives only a maximum value for an engine’s efficiency. Problems Find the efficiency of a gasoline engine that,during one cycle, receives 204 J of energy from combustion and loses 153 J as heat to the exhaust. Problems 1. If a steam engine takes in 2.254 × 10^4 kJ from the boiler and gives up 1.915 × 10^4 kJ in exhaust during one cycle, what is the engine’s efficiency? 2. A test model for an experimental gasoline engine does 45 J of work in one cycle and gives up 31 J as heat.What is the engine’s efficiency? 3. A steam engine absorbs 1.98 × 10^5 J and expels 1.49 × 10^5 J in each cycle. Assume that all of the remaining energy is used to do work.
a. What is the engine’s efficiency?
b. How much work is done in each cycle? 4. If a gasoline engine has an efficiency of 21 percent and loses 780 J to the cooling system and exhaust during each cycle, how much work is done by the engine? 5. A certain diesel engine performs 372 J of work in each cycle with an efficiency of 33.0 percent. How much energy is transferred from the engine to the exhaust and cooling system as heat? 6. If the energy removed from an engine as heat during one cycle is 6.0 × 10^2 J, how much energy must be added to the engine during one cycle in order for it to operate at 31 percent efficiency? Entropy Entropy a measure of the randomness or
disorder of a system In thermodynamics, a system left to itself tends to go from A state with a very ordered set of energies (one that has only a small probability of being randomly formed) to one in which there is less order (or that has a high probability of being randomly formed) greater entropy = greater disorder once a system has reached a state of greatest disorder, it will tend to remain in that state and have maximum entropy Greater disorder means there is less energy to do work Because of the connection between a system’s entropy, its ability to do work... ...and the direction of energy transfer... ...the second law of thermodynamics can also be expressed in terms of entropy change Applies to entire universe The second law of thermodynamics The entropy of the universe increases in all natural processes