Phys 2101 Gabriela González

Transcription

1 Phys 2101 Gabriela González

2 If volume is constant ( isochoric process), W=0, and Tp -1 is constant. pv=nrt If temperature is constant ( isothermal process), ΔE int =0, and pv is constant. If pressure is constant ( isobaric process), W, Q, and ΔE in are not zero, but TV -1 is constant. Adiabatic processes happen when Q = 0: the system is thermally insulated, or because the process happens very quickly. Temperature, pressure and volume all change, but the quantities pv γ and T V γ-1 are constant, where γ = C P /C V (=4/3=1.67 for monoatomic gases) The curve in a pv diagram is called an adiabat 2!

3 One liter of gas with γ = 1.3 is at 292 K and 1.4 atm pressure. It is suddenly compressed (adiabatically) to half its original volume. Find its final pressure and temperature. Adiabatic process: pv γ and T V γ-1 are constant: p 2 = p 1 (V 1 /V 2 ) γ = 1.4 atm = 3.4 atm T 2 = T 1 (V 1 /V 2 ) γ-1 = 292K = 359 K The gas is now cooled back to 292 K at constant pressure. What is its final volume? Constant pressure: p= nrt/v, or T/V, is constant V 3 =V 2 (T 3 /T 2 ) = 0.5 l (292 K/ 359 K) = 0.41 l 3!

4 One mole of an ideal diatomic gas undergoes a transition from a to c along the diagonal path in the figure. The temperature of the gas at point a is 1200 K. (a) What is the change in internal energy? [-5000 J] (b) How much heat is added to the gas? [2000J] (c) How much heat is added to the gas if it goes back to the original state a through the point b? [-5000J] 4!

5 pv=nrt 5!

6 A free expansion process happened when Q = W = 0, for an isolated system. Since internal energy is constant, temperature is constant. Pressure and volume change, but the quantity pv = nrt is constant The curve in a pv diagram is an isotherm (but no work is done or heat is exchanged in the process!). This is a non-reversible process!! 6!

7 We can easily tell the direction of the arrow of time in irreversible processes which, if spontaneous, only happen one way: spilling fluids gas expansion breaking solids Conservation of energy doesn t forbid these processes; there is another quantity, entropy, which measures the degree of disorder : S = k log W 7!

8 If an irreversible process occurs in a closed system, the entropy S of the system always increases; it never decreases. If any process occurs in a closed system, the entropy of the system increases for irreversible processes and remains constant for reversible processes. It never decreases! ΔS 0 8!

9 ΔS = S f S i = f i dq T 9!

10 An 8.0g ice cube at -10 o C is put in a Thermos flask containing 100 cm 3 of water at 20 o C. The specific heat of ice is 2220 J/kgK, the specific heat of water is 4190 J/kgK, and the heat of fusion of ice-water is 330 kj/kg. What s the final equilibrium temperature? Use ΔQ = 0 = c W m W (T f -20 o C) + c ice m ice (0 o C-(-10 o C)) + m ice L f + c W m ice (T f -0 o C) Answer: o C= K What is the change in entropy of the ice-water system? Use ΔS= dq/t = mcdt/t = mc ln(t f /T i ) for changes in temperature, and ΔS= dq/t =Q/T = ml f /T for changes of phase Answer: ΔS w = J/K ΔS ice = J/K ΔS = 0.64 J/K 10!

11 Any isothermal process: ΔS= dq/t= (1/T) dq = ΔQ/T Any reversible process: dq = de int + dw = nc V dt + p dv dq/t= nc V dt/t + pdv/t = nc V dt/t + pdv/(pv/nr) = nc V dt/t + nrdv/v E int = (f/2)nrt = n C V T W = p dv pv = nrt ΔS = dq/t = nc V ln (T f /T i ) + nr ln (V f /V i ) 11!

12 Ideal heat engines use a cycle of reversible thermodynamic processes. A heat engine transforms energy extracted as heat from thermal reservoirs, into mechanical work. Consider a Carnot engine: a cycle with two isothermal processes at a high temperature T H (a b) and and a low temperature T L (c d), and two adiabatic processes (b c, d a). ΔE int = 0 = Q W W = Q = Q H - Q L > 0 ΔS = 0 = Q H /T H - Q L /T L flashlets/carnot.htm 12!

13 We use the heat Q H to get work W done, so efficiency is defined as ε = W / Q H For a Carnot engine, W = Q H - Q L Q H /T H = Q L /T L so ε C = ( Q H - Q L )/ Q H = 1- Q L / Q H = 1- T L /T H < 1 Carnot s theorem: The most efficient cycle with maximum temperature T H and minimum temperature T L, is the Carnot cycle. 13!

14 14!

15 Three ideal Carnot engines operate between (a) 400K and 500K, (b) 500K and 600K, and (c) 400K and 600K. 1. Rank them according to their efficiencies, greatest first. 2. If they all extract the same amount of energy per cycle from the high temperature reservoir, rank them according to the work per cycle done by the engines, greatest first. 15!

16 Three ideal Carnot engines operate between (a) 400K and 500K, (b) 500K and 600K, and (c) 400K and 600K. 1. Rank them according to their efficiencies, greatest first. ANS: ε=1-t L /T H : ε c > ε a > ε b a) 20% (1-4/5=0.2) b) 16% (1-5/6=0.16) c) 33% (1-4/6=0.33) 2. If they all extract the same amount of energy per cycle from the high temperature reservoir, rank them according to the work per cycle done by the engines, greatest first. ANS: ε = W / Q H, so ranking according to W is the same as according to efficiency (above). 16!