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Thermodynamics

Thermodynamics. The Ideal Gas Law. The internal energy of an ideal gas is entirely kinetic energy. There are no intermolecular forces (potential energy) between the gas atoms. The temperature is directly proportional to the average kinetic energy of the gas particles.

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Thermodynamics

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  1. Thermodynamics

  2. The Ideal Gas Law • The internal energy of an ideal gas is entirely kinetic energy. • There are no intermolecular forces (potential energy) between the gas atoms. • The temperature is directly proportional to the average kinetic energy of the gas particles. • One mole of an ideal gas contains 6.02x1023 particles and occupies 22.4dm3(L) at Standard Temperature Pressure. (STP T=0ºC and P=1.01x105Pa.)

  3. Ideal Gas Law Equation P = Pressure (N/m2 = Pa) V = Volume n = # of moles R = Universal Gas Constant (8.31Jmol-1K-1) T = Temperature (K) simulation

  4. Constant Pressure Constant Temperature Constant Volume P P V T T V Graphical relationships between pressure, volume and temperature.

  5. Combined Gas Law

  6. Example 1: • The internal volume of a gas cylinder is 3.0x10-2 m3. An ideal gas is pumped into the cylinder until the pressure is 15MPa at a temperature of 25ºC. • Determine the number of moles of the gas in the cylinder • Determine the number of gas atoms in the cylinder? • Determine the average volume occupied by one atom of the gas. • Estimate the average separation of the gas atoms.

  7. Example 2: A sample of gas is contained in a vessel at 20ºC at a pressure P. What temperature does the gas need to be heated to in order for the pressure of the gas to be doubled if the volume remains constant?

  8. Gas A ΔV Δx Work done by a gas on a piston.

  9. The First Law of Thermodynamics • The study of processes in which thermal energy is transferred as heat and work. • Applies to engines that convert thermal energy to mechanical energy. • Macroscopic view of pressure, volume, temperature and internal energy in determining the state of a system.

  10. Q=Thermal Energy (Heat) System Engine (piston) WORK ΔU ↑ System Engine (piston) ΔU = The change in internal Energy, which is an increase in temperature of the System.

  11. First Law of Thermodynamics All quantities are measured in joules. Statement of conservation of ENERGY Q = Heat added to system (+) or removed from system (-) W = Work done by system (+) or Work done on system (-). Work is done when there is a change in volume. ΔU = increase in internal energy (+) or decrease in internal energy (-). ΔU represents a temperature change.

  12. Specific Processes and their corresponding PV graphs • Isobaric Process – Pressure remains constant and work is done by (+ΔV) or on the system (-ΔV). • Isochoric (isovolumetric) Process - Volume remains constant. No work is done, so there must be a change in internal energy. • Isothermal Process – Temperature is constant and the pressure and volume vary inversely. • Adiabatic Process – No thermal energy is added or removed from the system. (Q=0)

  13. W P P P V V V

  14. Heat Heat Heat Engine P D C D A C B A B V simulation

  15. Net work is done by the gas Cycle is clockwise Net work is done on the gas Cycle is counter-clockwise. Heat Pump or Refrigerator Heat Engine

  16. Efficiency Qh = Input Heat (Joules) Qc = Exhaust Heat (Joules)

  17. Maximum Efficiency – Carnot Cycle Maximum Efficiency Th = Maximum temperature in Kelvin Tc = Minimum temperature in Kelvin

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