Thermodynamics Physics H Mr. Padilla

Thermodynamics The study of heat and its transformation into mechanical energy. Foundation – Conservation of energy – Heat flows from hot to cold Until 1840 heat was thought to be an invisible fluid called caloric.

Q, W, & U Work can transfer energy to a substance. That energy could then leave as heat. – The reverse is also possible A change in internal energy is apparent as a change in temperature or phase. This change in energy can take place on a substance or a combination of substances called a system.

Work done on/by a gas Previously: W=Fd When work is done on an enclosed gas: – W=Δ(PV) If P is held constant, – W = PΔV ΔV is + work is done by the gas (W is +) ΔV is - work is done on the gas (W is -) Work is only done if volume changes

Heat, Work, and Internal Energy If the gas expands, as shown in the figure,  V is positive, and the work done by the gas on the piston is positive. If the gas is compressed,  V is negative, and the work done by the gas on the piston is negative. (In other words, the piston does work on the gas.)

Knowledge about Pressure and Volume P = F/A (force/area) ΔV = Ad (area multiplied by the displacement) 1 Pa = 1 N/m 2

Sample Problem 11A An engine cylinder has a cross-sectional area of m 2. How much work can be done by a gas in the cylinder if the gas exerts a constant pressure of 7.5 x 10 5 Pa on the piston and moves the piston a distance of 0.040m?

Solution Choose the equation: Plug and Chug: W=(7.5x10 5 N/m 2 )(0.010 m 2 )(0.040 m) W= 3.0x10 2 J

Thermodynamic Processes No Work is done in a constant volume process. – Any changes to internal energy would be as a result of heat – Called: isovolumetric process Internal energy is constant in a constant-temp process. – Work can still be done if volume changes – Called isothermal process

Isovolumetric Processes A bomb calorimeter is a device where a small quantity of a substance undergoes a combustion reaction. The reaction causes a change in temperature and pressure, but because of the thick walls there is no volume change. Energy can only be transferred as heat.

Isothermal Processes Work is done inside by the molecules hitting the inside of the balloon (energy is lost as work). Energy from the outside is added as heat.

Adiabatic Process Process in which no heat enters or leaves a system. Changes of volume can be done rapidly so heat has little time to leave or can be insulated Changes in air temperature ~ pressure change – Temp of dry air drops 10°C for each 1km increase in altitude

1 st Law of Thermodynamics Whenever heat is added to a system, it transforms to an equal amount of some other form of energy – Energy, including heat cannot be created or destroyed Heat added does 1 or both of 2 things.. – 1) increases the internal energy of the system if it remains in the system – 2) does external work if it leaves the system

Energy Conservation If friction is taken into account, mechanical energy is not conserved. Consider the example of a roller coaster: – A steady decrease in the car’s total mechanical energy occurs because of work being done against the friction between the car’s axles and its bearings and between the car’s wheels and the coaster track. – If the internal energy for the roller coaster (the system) and the energy dissipated to the surrounding air (the environment) are taken into account, then the total energy will be constant.

Energy Conservation Notice that in the presence of friction the internal energy (U) of the roller coaster increases as KE + PE decreases.

Heat added = increase in internal energy + external work done by system If 10J of energy is added to a system that does no external work, by how much will the internal energy of that system be raised? 10J If 10J of energy is added to a system that does 4J of external work, by how much will the internal energy of that system be raised? 10J – 4J = 6J

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. The first law of thermodynamics can be expressed mathematically as follows:  U = Q – W Change in system’s internal energy = energy transferred to or from system as heat – energy transferred to or from system as work

Signs of Q and W for a system

1 St Law of Thermodynamics for Special Processes

Sample Problem 11B 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?

Solution Choose your equation:  U = Q – W Rearrange the equation: Q =  U + W

Solution Plug and Chug: Q = 114 J + (–135 J) Q = –21 J The sign for the value of Q is negative. This indicates that energy is transferred as heat from the refrigerant.

Absolute Temperature Absolute Zero: Lowest possible temperature, 273 degrees C below zero Other absolute temperatures…

Cyclic Process A thermodynamic process in which a system returns to the same conditions under which it started. – The change in internal energy of a system is zero in a cyclic process  U net = 0 and Q net = W net

The Steps of a Gasoline Engine Cycle

The Steps of a Refrigeration Cycle

Heat Engines use Heat to do Work A heat engine is able to do work by transferring energy from a high-temperature substance at T h to a substance at a lower temperature (the air surrounding the engine) at T c. W net = Q h – Q c The larger the difference between the amount of energy transferred as heat into the engine and out of the engine, the more work the engine can do.

2 nd Law of Thermodynamics Heat will never of itself flow from a cold object to a hot object. Heat can be made to flow the other way, but only by external effort. – Ex: Air Conditioner, Refrigerator

Heat Engines Changing heat completely into work can never be done. It is possible to change some. A heat engine is any device that changes external energy into mechanical work. Amount of work done – W net =Q h -Q c

Heat Engines Every heat engine will – 1) absorb heat from a reservoir of higher temp. increasing its internal energy – 2) convert some of this energy into mechanical work – 3) expel the remaining energy as heat to some lower-temp. reservoir, usually called a sink There is always heat exhaust

Thermodynamic Efficiency The ideal efficiency, or Carnot Efficiency, of a heat engine can be found using the formula… Ideal efficiency = W net /Q hot = Ideal Efficiency = T hot - T cold T hot Heat converted to useful work depends on the temp difference between the hot reservoir and the cold sink.

Carnot Efficiency What is the ideal efficiency of an engine having a hot reservoir at 300K and a cold reservoir at 150K? What is the ideal efficiency of an engine having a hot reservoir at 400K and a cold reservoir at 0K? (300K-150K)/300K =.5 50% (400K-0K)/400K = 1 100%

Sample Problem 11C 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.

Disorder Natural systems tend to proceed toward a state of greater disorder. This idea is called entropy Ex: Gas molecules in a bottle. What will happen when the bottle is opened? Entropy normally increases in natural systems. When work is input, entropy can decrease

Entropy Greater disorder means there is less energy to do work. If all gas particles moved toward the piston, all of the internal energy could be used to do work. This extremely well ordered system is highly improbable.

Entropy 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: The entropy of the universe increases in all natural processes. Entropy can decrease for parts of systems, provided this decrease is offset by a greater increase in entropy elsewhere in the universe.