Thermodynamics 2 Flashcards
State the second law of thermodynamics (3)
-when objects of different temperatures are brought into thermal contact
-the spontaneous flow of heat that results is always from the high temperature object to the low temperature object
-spontaneous heat flow never proceeds in the reverse direction
State what a heat engine is and provide an example (2)
-heat engine is a device that converts heat into work
-a classic example is the steam engine
How does a steam engine function (4)
-fuel heats the water
-the vapor expands and does work against the piston
-the vapor condenses back into water again
-the cycle repeats
What do all heat engines have in common (3)
-a high temperature reservoir
-a low temperature reservoir
-a cyclical engine
Define the terms Qc and Qh in the context of a heat engine. How is the heat supplied from the hot reservoir (Qh) distributed in the engines operation (4)
-in heat engines, Qh represents the heat supplied from the hot reservoir
-and Qc is where the waste heat is released to the cold reservoir
-during the engines operation Qh is partially converted into work
-while the remaining heat Qc is released as waste heat
Explain the heat transfer process in a heat engine cycle, where Qh is supplied from the hot reservoir. What happens to this heat and how is it related to the work and waste heat in the cycle (4)
-the heat transfer process in a heat engine cycle involves Qh being supplied from the hot reservoir
-a portion of this heat is converted into useful work
-while the rest Qc is released as waste heat to the cold reservoir
-work done = Qh - Qc
Describe the role of the hot reservoir and the cold reservoir in a heat engines operation.
What is the outcome of the heat transfer process and how is it connected to the engine’s efficiency (3)
-in a heat engine the hot reservoir Qh and cold reservoir acts as a heat sink to receive waste heat
-the outcome of the heat transfer process is the production of useful work and the rejection of waste heat,
-the engines efficiency is a measure of how effectively it converts heat from the hot reservoir into useful work
Define efficiency and state its formula (2)
-is the fraction of the heat supplied to the engine that appears as work
E = w/Qh or 1-(Qc/Qh)
Why should there be a temperature difference in the engine (1)
In order for the engine to run there should be a temperature difference otherwise heat will not be transferred
Explain the key components of carnots theorem regarding maximum efficiency heat engine. What role does reversibility play in achieving maximum efficiency (3)
-carnots theorem specifies 2 crucial conditions for a heat engine to achieve maximum efficiency between 2 constant temperature reservoirs Tc and Th
-to have max efficiency, it must be an engine in which all processes are reversible
-all reversible engines operating between the same two temperatures Tc and Th should have the same efficiency
State the formula for maximum efficiency (1)
E = 1 - (Tc /Th)
Tc = tempt of cold reservoir
Th = tempt of hot reservoir
State Carnot’s theorem (3)
-if an engine operating between 2 constant temperature reservoirs is to have maximum efficiency
-it must be in which all processes are reversible
-in addition all reversible engines operating between the same 2 temperatures have the same efficiency
Can the max efficiency be 100%, if not, why ? (1)
-some of the heat is always wasted
State the formula for heat discharged (1)
= input - output
State the formula for the max work a heat engine can do (1)
= (1- Tc/Th) x Qh
How does a refrigerator function (3)
-it uses work to extract heat from the cold reservoir Qc (inside of the fridge) and exhausts to the kitchen
Qh = Qc +W
-more heat is released into the surrounding of the kitchen (hot reservoir) Qh
-as a fridge operates, it warms the kitchen at the same time that it cools the food stores in it
How does an ideal fridge function (1)
-it would remove most of the heat from the interior while requiring the smallest amount of work done
State the formula for COP
COP = Qc / W
Bigger the value of cop the better
How does an air conditioner function (3)
-air conditioner uses electrical energy to transfer the heat indoors to the outdoor envoirment cooling the indoor space
-the motors of the ac does the work
-more heat is removed to the hot reservoir (Qh) than is removed from the cold reservoir (Qc)
How does a heat pump function (4)
-heat is removed from the outside air (cold reservoir)
-and is transferred into the house
-keeping it warm
-the work that the pump does contributes to a warmer house
State the formulas used to determine how much work is needed to add HEAT Qh to a place (3)
W = Qh - Qc
W = Qh x ( 1 - Qc/Qh)
W = Qh x (1- Tc/Th)
Tc = tempt of cold reservoir
Th= tempt of hot reservoir
Qc = heat wasted
Qh= heat supplied
State the third law of thermodynamics (2)
-it is impossible to lower the temperature of an object to absolute zero
-in a finite number of steps
