Chapter 10 Thermodynamics Honors Physics
Some background information. Remember the Law of Conservation of Energy. Remember the difference between Potential and Kinetic Energies. Remember the difference between temperature and heat. Remember Work.
Exothermic/Endothermic SURROUNDINGS/ ENVIRONMENT SYSTEM HEAT LEAVES SYSTEM EXOTHERMIC HEAT ENTERS SYSTEM ENDOTHERMIC
Work by a gas. WORK BY A GAS = P V see p 337 (P = External Pressure) Expanding gas, the work done by the gas on the piston is +. Compressing gas, the work done by the gas on the piston is -.
Work on a gas, cont’d. area d Volume change is d x area
Concept Check Gas in a container is at a pressure of 1.6 x 105 Pa and a volume of 4.0 m3. What is the work done by the gas if 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?
Concept Check a. 6.4 x 105 J b. -4.8 x 105 J
Concept Check 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 x 10-4 m3 to 2.523 x 10-4 m3. How much work is done? Is the work done on or by the gas? Explain your answer.
Concept Check -164.5 J; work is done on the gas because the work and volume change are negative.
Concept Check A balloon is inflated with helium at a constant pressure that is 4.3 x 105 Pa in excess of atmospheric pressure. If the balloon inflates from a volume of 1.8 x 10-4 m3 to 9.5 x 10-4 m3, how much work is done on the surrounding air by the helium-filled balloon during this expansion?
Concept Check 3.3 x 102 J
Concept Check 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?
Concept Check 2.0 x 105 Pa
Calorimetry Qp = m c t at constant pressure, the change in energy of a solution is equal to the mass of the solution times the specific heat capacity of the solution times the change in temperature.
Bomb Calorimetry For constant volume, (bomb calorimeter) -P V =0, so U = Q - W, but W = 0, So U = Qv Qv = c T, c= heat capacity, the energy required to change the temperature of the bomb 1C p 339
1ST LAW OF THERMODYNAMICS WWHEN ENERGY IS TRANSFERRED TO HEAT EENERGY OR VICE-VERSA, THE HEAT ENERGY IIS EXACTLY EQUAL TO THE AMOUNT OF TTRANSFORMED ENERGY. E ENERGY IS CONSERVED FOR ANY SYSTEM AND ITS ENVIRNOMENT.
Cont’d ** U = Q - W or Q = U + W** Q = AMOUNT OF ADDED HEAT W = EXTERNAL WORK DONE. U = CHANGE IN INTERNAL ENERGY
Sign for energy flow Sign indicates direction of flow exothermic = -Q, endothermic = +Q work done on the system = -W system does work on surrounding = +W
Process Terms Isovolumetric means that the volume doesn’t change. Isothermal means that the temperature is constant. Adiabatic means that energy isn’t transferred as heat. Usually occurs quickly. Isobaric means that the pressure is constant. Isolated has no interactions with surroundings/environment.
Specific Processes for 1st Law See Table 2 for isovolumetric, isothermal, adiabatic, and isolated system. Isovolumetric: W=0, Q=U = mcvt Isothermal: U=0, Q = W Adiabatic: Q= 0 =U+W or U = -W Isolated: Q = W= 0, U = 0
Concept Check 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?
Concept Check 33 J
Concept Check 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?
Concept Check -143 J; removed as heat
Concept Check 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 x 103 J. However, the pressure cooker is not well insulated, and as a result, 2.0 x 103 J of energy is transferred to the surrounding air. How much energy is transferred from the range element to the pressure cooker as heat?
Concept Check 1.00 x 104 J
Concept Check 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?
Concept Check 0 J; 344 J done by gas
Concept Check A steam engine's boiler completely converts 155 kg of water to steam. This process involves the transfer of 3.50 x 108 J as heat. If steam escaping through a safety valve does 1.76 x 108 J of work expanding against the outside atmosphere, what is the net change in the internal energy of the water-steam system?
Concept Check 1.74 x 108 J
“Heat Engine” QH QH=W + Qc Hot TempReservoir Work Out Qc Cold TempReservoir
Cyclic Processes • The change in internal energy of a system is zero in a cyclic process. • Unet= 0 and Qnet = Wnet • Wnet = Qh – Qc, where Qh > Qc (hot, cold)
“Refrigerator” QH QH=W + Qc Hot TempReservoir Work In Qc Cold TempReservoir
2nd Law of Thermodynamics No cyclic process that converts heat entirely into work is possible. Without “help” heat flows from higher heat to lower heat. Systems go in the direction that increases the disorder of a system. “The universe tends to be lazy and messy.”
Efficiency There are no 100 % efficient engines. Efficiency = net work done by engine energy added to engine as heat eff = W net = Qh – Qc = 1 – Qc Qh Qh Qh
Concept Check If a steam engine takes in 2.254 x 104 kJ from the boiler and gives up 1.915 x 104 kJ in exhaust during one cycle, what is the engine's efficiency?
Concept Check 0.1504
Concept Check 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?
Concept Check 0.59
Concept Check A steam engine absorbs 1.98 x 105 J and expels 1.49 x 105 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?
Concept Check a. 0.247 b. 4.9 x 104 J
Concept Check 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?
Concept Check 210 J
Concept Check 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?
Concept Check 755 J
Concept Check If the energy removed from an engine as heat during one cycle is 6.0 x 102 J, how much energy must be added to the engine during one cycle in order for it to operate at 31 percent efficiency?
Concept Check 8.7 x 102 J
Entropy Entropy (S) is a measure of the disorder of a system. Higher the entropy, the less energy that is available to do work Systems with maximum entropy are favored.