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What is a heat engine?
A device that converts thermal energy into mechanical work?
What is the first law of thermodynamics?
Energy conservation in thermodynamic process, expressed as U= Q-W (change in internal energy = heat added to the system - work done by the system)
What is the second law of thermodynamics?
States that entropy of an isolated system always increases over time, and no process is 100% efficient.
What is heat engine operation?
Understanding how heat engines use a working substance to absorb heat Qh at a high temperature, do work W, and release heat Qc at low temperature.
What is a Carnot energy?
Theoretical engine with maximum possible efficiency
What is an internal combustion engine?
Common in cars, uses combustion of fuel to produce work
What is a steam engine?
uses steam pressure to move pistons or turbines
What is the formula for efficiency?
1 - Qc/Qh
Qh - is the heat absorbed
Qc - is the heat released
W - is the work done by the engine
What is the Carnot efficiency?
1 - Tc/Th
Th and Tc are the absolute temperatures of the hot and cold reservoirs, respectively
What is the Real vs. ideal efficiency
Ideal: The maximum possible efficiency that a heat engine can achieve under ideal conditions, without any energy losses
Real: The actual efficiency of a real heat engine, which is always lower than the ideal (Carnot) efficiency due to various real-world factors.
What is entropy
measure of disorder or randomness in a system. Second law states that in an isolated system, entropy will increase or remain constant over time.
What are examples of entropy?
ice melting: when ice melts into water, the molecules move from an ordered state (solid) to a less ordered state (liquid). The entropy of the state increased as the water becomes more disordered.
Mixing substances: When you mis two gases, the entropy of the combines system increases because the molecules of the two gases are now randomly distributed.
Spreading of heat: Heat will naturally move from a hotter object to a cooler object until thermal equilibrium is reached, increasing the entropy of the system as energy spreads out and becomes more disordered.
What is spontaneous processes
Processes that naturally increase the entropy of the universe
What is heat flow?
Heat flows from hot objects to cold objects, not the reverse, unless external work is done (refrigerators)
What is a refrigerator heat system look like
Transfers heat from the cold inside to the warmer outside
what is work input
Both devices require external energy to move heat against its natural direction
What is efficiency of real engines
How friction, heat loss and other factors affect real engine efficiency
How does friction affect the efficiency of real engines?
between moving parts in an angina, such as pistons, crankshafts, and bearings. Less energy from the fuel is used to generate motion, decreasing the overall efficiency.
How does heat loss affect a real engines?
By reducing the amount of thermal energy available to perform useful work. Real engines are not 100% efficient because a significant portion of the heat energy generated during combustion is lost to the surroundings rather than being converted into mechanical energy
What is the equation for the efficiency of heat engines?
W/Qh
What is the equation for Carnot efficiency
= 1- Tc/Th
What is work?
energy transferred to or from a system by mechanical means
What is Heat?
Energy transferred due to temperature difference
What is entropy
A measure of the disorder of a system
What is an isolated system
A system that does not exchange energy or matter with its surroundings
What is the adiabatic process
A process where no is heat is exchanged with the surroundings
What is the four reversible stages of a Carnot engine
The four reversible stages: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression
What is isothermal expansion
The engine absorbs heat from a high temperature reservoir. It then expands adiabatically (without heat exchange) and does work on the surroundings.
What is isothermal compression
The engine releases heat to a lower temperature reservoir, and in the adiabatic compression stage, it absorbs work to return to its original state
What is the Carnot engine the most effective engine?
The Carnot engine is considered the most efficient engine possible because it operates in a completely reversible cycle, meaning there are no energy losses due to friction, heat dissipation, or other inefficiencies. It sets the maximum theoretical limit for the efficiency of any heat engine.
How does the second law of thermodynamics explain why perpetual motion machines are impossible?
A perpetual machine of the first kind would violate the first law of thermodynamics by creating energy from nothing. A machine of the second kind would violate the second law by converting all the heat energy from a heat reservoir into work without any energy loss, which is impossible because it would mean that entropy could decrease, which contradicts the second law. Therefore, perpetual motion machines cannot exist because energy conversions will always involve some degree of energy loss, as often as heat waste.
Calculate the efficiency of an engine that absorbs 800 J of heat and does 300 J of work
Work output/ heat input x 100%
Work output = 800 J
Work input = 300 J
300/800 × 100% = 37.5%
if a heat engine operates between temperatures of 500 K and 300 K what is its Carnot efficiency?
1 - 300/500 = 40%
The Carnot efficiency is 40%
Efficiency of a heat engine:
A heat engine absorbs 1,500 J of heat from a high temperature reservoir and releases 500 J of heat to a low temperature reservoir. Calculate the efficiency of this heat
Output - 1500 J
Input - 500 J
1500-500 = 1000
1000/1500= 67%
Work done by a heat engine:
A heat engine absorbs 2000 J of heat from a hot reservoir and does 800 J of work. What is the heat rejected to the cold reservoir
Output - 2000
Work - 800
2000 - 800 = 1200 J
Carnot efficiency calculation:
A Carnot engine operates between two reservoirs, with the hot reservoir at 600 K and the cold reservoir at 300 K. What is the theoretical maximum efficiency of the engine?
Qh- 600K
Qc- 300K
1- 300/600 = 50%
Heat transfer and work output:
An engine operates between a high-temperature reservoir at 700k and a low temperature reservoir at 400k. If the engine absorbs 1,000 J heat from the hot reservoir, what is the maximum possible work output?
Qh - 700 k
Qc - 400k
Input - 1000 J
1 - 400/700 = 42.9%
Efficiency of a real engine:
A heat engine with a real efficiency of 25% absorbs 1200 J of heat from a high temperature reservoir. Calculate the work output of this engine and the amount of heat rejected to the cold reservoir
Efficiency: 25%
Input - 1200
(1200) (.25) = 300 J
1200 - 300 = 900 J
Temperature change and work done:
A heat engine absorbs 1000 J of heat from a hot reservoir at 600 K and rejects 300 J of heat to a cold reservoir. Calculate the work done by the engine and the efficiency.
Input - 1000
Qh - 600 K
Qc - 300 J
600-300 = 300
1- 300/600 = 50%
Comparing efficiencies:
Two engines are operating between the same high - temperature reservoir (500K) and different low - temperature reservoirs: one at 250K and another at 100 K. Calculate the efficiency of both engines and determine which one is more efficient
Qh - 500 K
Qc1 - 250K
Qc2 - 100K
1- 250/500= 50%
1- 100/500 = 80%
Engine 2 is more efficient
Finding heat input from work output:
A Carnot engine has an efficiency of 50% and does 1000 J of work. How much heat does it need to absorb from the hot reservoir to do this this amount of work?
1000/.50 = 2000 J
The efficiency of a heat pump:
A heat pump operates between two reservoirs at 280K and 350K. Calculate its coefficient performance (COP)
COP = Th/Th - Tc
350/350-280 = 5
Real world engine efficiency:
A steam engine operates with a heat input of 5000J and a heat output of 1500 J. What is the engines efficiency, and how much energy is lost as waste heat.
Input - 5000J
Output - 1500J
5000-1500 = 3500
3500/5000= 70%
Efficiency is 70% and the energy lost is 1500J
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