HEAT, WORK AND INTERNAL ENERGY GLOBAL WARMING?
THERMODYNAMICS: the science of energy, specifically heat and work, and how the transfer of energy effects the properties of materials.
A “system” is the “collection of objects on which attention is being focused” The “surroundings” are everything else in the environment The system and surroundings must be separated by walls which can either insulate or allow heat flow OPEN SYSTEM: Mass and energy freely moves in and out between the system and the surrounding ISOLATED SYSTEM: No interaction between the system and the surrounding CLOSED SYSTEM: fixed mass
Thermal Equilibrium Systems (or objects) are said to be in thermal equilibrium if there is no net flow of thermal energy from one to the other. A thermometer is in thermal equilibrium with the medium whose temperature it measures, for example. If two objects are in thermal equilibrium, they are at the same temperature.
RELATIONSHIP BETWEEN HEAT AND WORK W = F x d Pressure (P) = (Force) F or F = P A (Area) A Volume (V) = L x W x H or A x d d = V A Why does the volume of gas expands when it is heated? W = P AV = P V A
INTERNAL ENERGY (U or E) is the total of the kinetic energy due to the motion of molecules (translational, rotational, vibrational) and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals.
The First Law of Thermodynamics states that : The internal energy of a system changes from an initial value Ui to a final value Uf due to heat added (Q) and work done by the system (W) DU = Uf – Ui = Q – W Q is positive when the system gains heat, and negative when the system loses heat. W is positive when it is done BY the system, and negative when it is done ON the system
SPECIAL PROCESSES HOT DOGS!
Adiabatic Expansion of a Ideal Gas No heat transfer therefore no temperature change (Q=0). Generally obtained by surrounding the entire system with a strongly insulating material or by carrying out the process so quickly that there is no time for a significant heat transfer to take place.
Adiabatic Expansion of a Ideal Gas Both adiabatic expansion and compression of gases occur in only hundredths of a second in the cylinders of a car’s engine. Blowing air through wide open mouth results to warm air. Blowing through small opening results to cooler air due to adiabatic expansion. Compresses air leaking out through a small opening also results in adiabatic cooling.
PROCESS DIAGRAMS: visualize processes using properties (T, P, V, etc.) Area underneath the slope represents the amount of work done (P x V).
CYCLE: a system undergoes processes - returning to its initial state Area underneath the slope represents the amount of work done (P x V).
Refrigerators work by taking heat from the interior and depositing it on the exterior • The compressor raises the pressure and temperature of the refrigerant (freon or ammonia) while the coils OUTSIDE the refrigerator allow the now hot refrigerant to dissipate the heat • The warm refrigerant flows through an expansion valve from a high-pressure to a low-pressure zone, so it expands and evaporates • The coils INSIDE the refrigerator allow the cold refrigerant to absorb heat, cooling the interior • The cool refrigerant flows back to the compressor, and the cycle repeats
Can you beat the Second Law? • So, can you cool your kitchen by leaving the refrigerator door open • NO! • The heat removed from the interior of the refrigerator is deposited back into the kitchen by the coils on the back! • And to make matters worse, the Second Law of Thermodynamics says that work is needed to move the heat from cold to hot, so the actual amount of heat added to the kitchen is MORE than the amount removed from the refrigerator
Hopefully, you understand today’s lesson. Otherwise, you’ll end up like this cow.