Energy in Thermal Systems

1. Energy in Thermal Systems 5.4

2. Internal Energy • A. The total energy of the sum of kinetic and potential energies of all the particles in an object. • B. Depends on: • 1. material composition • 2. mass • 3. temperature • 4. physical state

3. C. Internal energy changes because of heat transfers due to a temperature difference D. Internal energy changes because of friction (work done on a system) E. Thermodynamics—the science dealing with the relationships between internal energy, heat, and work

4. II. The First Law of Thermodynamics A. The law of conservation of energy B. ΔU = Q – W C. Change in the internal energy of a system = net heat input to the system – work done by the system 1. Heat is positive if it enters the system and negative when it leaves. 2. Work is positive when the system does the work and negative if work is done on the system.

5. D. Adiabatic process—there is no heat transfer to or from a system 1. isolate the system from its surroundings (insulation) 2. work quickly enough that there is no time for heat transfer to take place.

6. III. Heat Engines A. Device that converts thermal energy into mechanical energy. B. Automobiles, nuclear reactions, human body C. Every heat engine: 1. Absorbs thermal energy from a high- temperature source 2. Converts some of the thermal energy into work. 3. Discards the remaining thermal energy into a low-temperature “sink”. (reservoir)

7. D. Four-stroke gasoline engine operates in a cycle 1. intake stroke 2. compression stroke 3. ignition 4. power stroke 5. exhaust stroke E. Gasoline engine must 1. overcome friction 2. keep the engine operating 3. operate equipment 4. move the vehicle

8. IV. Refrigerators and Heat Pumps A. Cycle is reverse of a heat engine (net heat input is negative, so work is negative) B. Steps 1. Refrigerant enters the compressor as a low-pressure gas. A piston compresses the gas, so temperature and pressure is increased. 2. The compressed gas flows into a condenser, where it is cooled and changed from a gas to a liquid. (releases thermal energy) Heat is transferred away from the condenser to the high-temp. reservoir using air- or water-cooling of the condenser.

9. 3. The refrigerant leaves the condenser as a high-pressure liquid. The liquid flows through an expansion valve, decreasing the pressure. Some liquid becomes gas. 4. The remaining liquid is vaporized in an evaporator. The liquid absorbs thermal energy, and is transferred back to the compressor. C. Heat Pump acts as an air conditioner in the summer by extracting heat from the interior of a house and exhausting it outside. In winter, the system reverses.

10. The Second Law of Thermodynamics • A. Entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. • B. Entropy—a measure of the disorder of a system—engines cannot be 100% efficient because some energy is “lost” (unusable) • C. Carnot efficiency—the maximum efficiency of a heat engine—depends on the absolute temperatures (Kelvin scale) of the hot and cold reservoirs • D. Absolute zero = 0 K or -273°C

11. VI. Energy Dissipation A. Energy no longer available to do work B. Occurs in all processes C. Every time energy is used, some is transformed into unusable energy.