2 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.
3 Exothermic/ Endothermic SURROUNDINGS/ENVIRONMENTSYSTEMHEAT LEAVES SYSTEM EXOTHERMICHEAT ENTERS SYSTEM ENDOTHERMIC
4 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 -.
5 Work on a gas, cont’d.areadVolume change is d x area
6 Concept CheckGas 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 ifa. it expands at constant pressure to twice its initial volume?b. it is compressed at constant pressure to one-quarter of its initial volume?
8 Concept CheckA gas is enclosed in a container fitted with a piston. The applied pressure is maintained at kPa as the piston moves inward, which changes the volume of the gas from x 10-4 m3 to x m3. How much work is done? Is the work done on or by the gas? Explain your answer.
9 Concept CheckJ; work is done on the gas because the work and volume change are negative.
10 Concept CheckA 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 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?
12 Concept CheckSteam moves into the cylinder of a steam engine at a constant pressure and does 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?
14 CalorimetryQp = 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.
15 Bomb Calorimetry For constant volume, (bomb calorimeter) -P V =0, so U = Q - W, but W = 0, So U = QvQv = c T,c= heat capacity, the energy required to change the temperature of the bomb 1Cp 339
16 1ST LAW OF THERMODYNAMICS WWHEN ENERGY IS TRANSFERRED TO HEATEENERGY OR VICE-VERSA, THE HEAT ENERGYIIS EXACTLY EQUAL TO THE AMOUNT OFTTRANSFORMED ENERGY.E ENERGY IS CONSERVED FOR ANY SYSTEM ANDITS ENVIRNOMENT.
17 Cont’d ** U = Q - W or Q = U + W** Q = AMOUNT OF ADDED HEAT W = EXTERNAL WORK DONE. U = CHANGE IN INTERNAL ENERGY
18 Sign for energy flow Sign indicates direction of flow exothermic = -Q, endothermic = +Qwork done on the system = -Wsystem does work on surrounding = +W
19 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.
20 Specific Processes for 1st Law See Table 2 for isovolumetric, isothermal, adiabatic, and isolated system.Isovolumetric: W=0, Q=U = mcvtIsothermal: U=0, Q = WAdiabatic: Q= 0 =U+W or U = -WIsolated: Q = W= 0, U = 0
21 Concept CheckHeat 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?
23 Concept CheckThe 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?
25 Concept CheckA 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?
29 Concept CheckA 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?
31 “Heat Engine” Hot Temp Reservoir QH Work Out QH=W + Qc Qc Cold Temp Reservoir
32 Cyclic ProcessesThe change in internal energy of a system is zero in a cyclic process. Unet= 0 and Qnet = WnetWnet = Qh – Qc, where Qh > Qc(hot, cold)
33 “Refrigerator” Hot Temp Reservoir QH Work In QH=W + Qc Qc Cold Temp Reservoir
34 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.”
35 Efficiency There are no 100 % efficient engines. Efficiency = net work done by engineenergy added to engine as heateff = W net = Qh – Qc = 1 – QcQh Qh Qh
36 Concept CheckIf a steam engine takes in x 104 kJ from the boiler and gives up x 104 kJ in exhaust during one cycle, what is the engine's efficiency?
40 Concept CheckA 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?
46 Concept CheckIf 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?