post ¥ AP PHYSICS _ Thermodynamics Practice 1* Law Chapter 15 GROFF. ‘The frst aw of thermodynamics is simply a statement of conservation of energy. Qw Uis the internal energy ofthe gs in question Qi the heat added tothe gas, and Wis the work cone on the {92 when the gas is compressed. NAMI ‘The signs ofeach term inthe first law are extremely important. I heat is ade to the gas, Qs postive, and if heat is taken from the gas, Qs negative. I the gas's internal energy (and, thus, its temperature) goes up, then QU is postive; f the gas's temperature goes down, then AUis negative, The sign of Wis more dificult to conceptualize. Postve 1 represents work done on a gas. However, ia gas's volume expands, then work was not done on the ga, work was done by the gas. In the case of work done by a gas, Wis given a negative sign Practice Problems. 1. Agasis kept in a cylinder that can be compressed by pushing down on apiston. You add 2500} of heat to the system, and then you push the piston 1.0m down with a constant force of 1800N. What is the change in the gas’s internal energy? Please sketch the situation to give yourself a fea a bus &4W ene ' Q= + Cheat added) W= + (work done on system) Rush Piston = [Meter Force = 19000) The Pusu! sete k fe '800N Work =ExD forte = 1800 Force © 1B00N_ Dist =m f werk = 1p00Teules Ineter + DU= 2500,+ 190Q = 1Z00Toules = Ce2. A gas is enclosed in a metal container with a moveable piston on top. Heat is added to the gas by placing a candle flame in contact with the container's bottom. Which of the following is true about the temperature of the gas? Explain your choice. @rne temperature must go up if the piston remains stationary. XThe temperature must go up if the piston is pulled out dramatically. The temperature must go up no matter what happens to the piston. The temperature must go down no matter what happens to the piston. The temperature must go down if the piston is compressed dramatically. A. Heat ts adleledt. Tame of the GAS ull increase iF piston remains stationary duc to +e fest that work is hell constont ip geek Tl abledeesl fay Cexparsion) aban Du well revue eo 2a. et: Tenp will not inerense due jo expensing @ gq. DUO +@ -W_ if exper vf rer, 3.A child takes a long, partially inflated, cylindrical balloon, 4.cm in deter ‘ and 50 cm long, into the bathtub with them. As the balloon floats on the surface, the air in the balloon receives 34.9 J of energy in the form of heat and expands until itis 58 cm Jong (and still the same width). How much does the internal energy of the balloon increase? [Hint: what determines the pressure of the air inside the balloon?] Please sketch the situation to give yourself a mental picture. Bollaon . trem in dia Sdem long Vol egtinder = (02m) fren qptmmber = Tr (5.4 Yt) = goles x lagth, oo Volp= 6-28x10- m3 Volg= ,20fase x lengths 1.26 xlo—“ ms ABvel= 1, Mend Au= Q-W Wwe Pavel Seer AU = ©-(Pow!) Heat addect = 24.4coules Q=* Expands W >> DU= BY. Wouter ~ (/,01110%Pe + oxlo'n”) B44 - teu DU= 24.8 Tevles4. Agas is kept in a cylinder that can be compressed by pushing down on apiston. You remove 1575] of heat from the system, and then you push the piston 1.0m down with a constant force of 900N. What is the change in the gas’s internal energy? Please sketch the situation to give yourself a mental picture. \" = 1 IST of heat lost (remmed) p> t : Tim push piston = (meter pos2 ¥ Foree push = 9000 Aue? wt = Work done om te systens buU=Q tw QV eee wort = FxD Q0ON x Imetery = 900Tsules DUE —ISIST +900T = OTS Teoules + Even though we did 9005 of work Cdepottel 1900), we wire traveferes! isis4) , in heat fae! Our interme] ena newt clown 67S Towler. A