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Thermodynamics

Thermodynamics. The study of energy conversions between heat and other forms. Pressure: force / area. Units of Pressure: N/m 2 Also known as Pascals (Pa). Work = F( Δ h) = (F/A) ( Δ h*A) = P ( Δ V). If a piston compresses its volume by .03 m 3 at a constant pressure of 10 4 Pa,

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Thermodynamics

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  1. Thermodynamics The study of energy conversions between heat and other forms.

  2. Pressure: force / area Units of Pressure: N/m2 Also known as Pascals (Pa)

  3. Work = F(Δh) = (F/A) (Δh*A) = P (ΔV)

  4. If a piston compresses its volume by .03 m3 at a constant pressure of 104 Pa, A.) How much work does it do? B.) Does the gas gain or lose energy? Work = P (ΔV) = 104 (.03) = 300 J A Gas being compressed gains energy

  5. Do Now:

  6. Identify the type of process: Compressed air expands when released from airhorn A sealed 2L bottle of Pepsi warms in a room. The piston in a diesel engine compresses quickly and elevates the Temperature enough to ignite the fuel. The water in a cooking pot expands slightly as it heats up before beginning to boil.

  7. The First Law A total of 135 J of work is done on a gaseous refrigerant as it is compressed. If internal energy increases by 114 J, what is the total amount of energy transferred as heat? W = -135 J ΔU = +114 J ΔU = Q - W +114 J = Q – (-135 J) Q = -21 J Energy removed as heat

  8. The First Law If the internal energy of a gas decreases by 3500 J in a isovolumetric process, how much heat is transferred? Is heat transferred to or from the gas? W = 0 ΔU = -3500 J ΔU = Q - W -3500 J = Q – 0 Q = -3500 J Energy removed as heat

  9. Part 2 If the internal energy of a gas decreases by 3500 J in a isovolumetric process, by how much does the temperature of the gas change if there is 2.0 kg of gas and the specific heat is 350 J/kgoC ? Cp = 350 Q = -3500 J Q = CpM∆T -3500 J = (350)(2.0)∆T ∆T = -5.0 oC Temperature decreases

  10. The First Law A gas is trapped in a small metal cylinder with a movable piston and is submerged in a large amount of ice water. The initial temperature of the gas is 0°C. A total of 1200 J of work is done on the gas by a force that slowly pushes the piston inward. How much energy is transferred as heat between the gas and the ice water? “slow” process means it has time to stay in thermal equilibrium with the surrounding ice water. ΔU = 0 J 0 = Q – W Q = W

  11. Cyclic Processes The process is cyclic, so the working fluid returns to its initial temperature and pressure. U = 0 so Qnet = Wnet -Heat engines take heat from a substance at a high temperature, use that energy to do some work, and then expel heat from the substance at a lower temperature. -Internal combustion engines are heat engines. -Hot gases do work on the piston and then are expelled to a lower-temperature environment.

  12. Heat Engines -Heat Qh: Enters engine (burning fuel) -Heat Qc: Leaves engine (exhaust) -The difference is used to do work

  13. Do now: Explain why a cold air-conditioned room feels colder if you enter it from outside on a summer day and warmer if you enter it from a walk-in freezer?

  14. 2nd Law of Thermodynamics – No cyclic process can be 100% efficient at converting heat to energy.

  15. These are the maximum possible efficiencies based on how much heat is “wasted” as exhaust. Friction and conduction of heat reduce the efficiency even further.

  16. Efficiency Problem In one cycle, an engine absorbs 4,540 J of heat from a high temperature reservoir and expels 1,430 J of heat as exhaust to a cold temperature reservoir. What is the efficiency of the engine? During the cycle, the engine’s piston compresses the gas by a volume of .06 m3. What is the average pressure during the cycle? (hint: how much work is done?)

  17. A typical diesel engine has an ideal efficiency of 56%. If it releases 2,500 J of energy as exhaust per cycle, how much work will it do per cycle?

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