Presentation is loading. Please wait.

Presentation is loading. Please wait.

Thermodynamics The study of heat energy through random systems.

Similar presentations


Presentation on theme: "Thermodynamics The study of heat energy through random systems."— Presentation transcript:

1 Thermodynamics The study of heat energy through random systems

2 Laws of Thermodynamics 0th Law: If Ta=Tb and Tb=Tc then Ta=Tc 0th Law: If Ta=Tb and Tb=Tc then Ta=Tc 1st Law: ΔE=W+Q (Conservation of Energy) 1st Law: ΔE=W+Q (Conservation of Energy) The increase in thermal energy of a system is determined by the heat of the system and the work added to it. The increase in thermal energy of a system is determined by the heat of the system and the work added to it. The idea behind the Heat Engine The idea behind the Heat Engine

3 Efficiency W net =Q net =Q hot -Q cold W net =Q net =Q hot -Q cold or Eff=W net /Q hot =1-(Q cold /Q hot ) or Eff=W net /Q hot =1-(Q cold /Q hot ) Ex: A steam engine absorbs 1.98 x10 5 J and expels 1.49 x10 5 J in each cycle. Assume that all the remaining energy is used to do work. Ex: A steam engine absorbs 1.98 x10 5 J and expels 1.49 x10 5 J in each cycle. Assume that all the remaining energy is used to do work. a. What is the engine’s efficiency? a. What is the engine’s efficiency? b. How much work is done in each cycle? b. How much work is done in each cycle?

4 a. Eff=0.247 a. Eff=0.247 b. W=4.9 x10 4 J b. W=4.9 x10 4 J

5 2nd Law: A system can be defined by its entropy. In a closed system entropy tends to increase. 2nd Law: A system can be defined by its entropy. In a closed system entropy tends to increase. Entropy: A measure of the disorder (randomness) of the system Entropy: A measure of the disorder (randomness) of the system 3rd Law: Temperature and Entropy are absolute scales. 3rd Law: Temperature and Entropy are absolute scales. At some point no temperature or entropy exists. At some point no temperature or entropy exists. T → 0 K and S → S 0 T → 0 K and S → S 0

6 Heat Engines Engines allow heat energy to be transformed into work or mechanical energy. Engines allow heat energy to be transformed into work or mechanical energy. Work or Energy  Heat increases Work or Energy  Heat increases no big deal. Friction does this. no big deal. Friction does this. Heat  Work or Energy Heat  Work or Energy is a big deal. Heat is easy to move around. You could just bring heat wherever you needed work done and “Boom!” you wouldn’t have to do the work, a machine could. is a big deal. Heat is easy to move around. You could just bring heat wherever you needed work done and “Boom!” you wouldn’t have to do the work, a machine could. Work or Energy  Heat decreases Work or Energy  Heat decreases is also a big deal. Making food cold preserves it and allows it to be moved readily. Less spoilage means less disease. is also a big deal. Making food cold preserves it and allows it to be moved readily. Less spoilage means less disease.

7 Any heat engine works on the same properties. Any heat engine works on the same properties. A hot reservoir is the source of the energy. A hot reservoir is the source of the energy. Both words mean something. Hot means that there is plenty of heat energy and reservoir means that if heat is removed the temperature doesn’t drop much. Both words mean something. Hot means that there is plenty of heat energy and reservoir means that if heat is removed the temperature doesn’t drop much. There is also a need for a cold reservoir. Again, both words mean something. There is also a need for a cold reservoir. Again, both words mean something. Cold because it is at a lower temperature than the hot reservoir and reservoir because it must be large enough that you can dump heat into it without appreciably raising the temperature. Cold because it is at a lower temperature than the hot reservoir and reservoir because it must be large enough that you can dump heat into it without appreciably raising the temperature.

8 What happens if we put a hot and cold reservoir in contact? Thermal Equilibrium is not the answer! Heat transfer (or flow) is the answer. Heat transfer (or flow) is the answer. Remember that these are reservoirs so it would take a long time for them to come into thermal equilibrium. Remember that these are reservoirs so it would take a long time for them to come into thermal equilibrium. This is great, but we don’t get any work out of it. This is great, but we don’t get any work out of it. We need to “steal” some of the energy leaving the hot reservoir and make it do work for us. We need to “steal” some of the energy leaving the hot reservoir and make it do work for us.

9 Heat Engine Stealing some energy to do work High Temp Low Temp ΔE Q W

10 Types of Heat Engine Steam Engine Steam Engine

11 Internal Combustion Engine

12 Unfortunately we don’t get an even trade! We lose energy to randomness/Entropy We lose energy to randomness/Entropy This is the 2nd Law of Thermodynamics This is the 2nd Law of Thermodynamics Automobile engines are only about 15% efficient. That means for every 100J of heat energy, 15J worth of work is done on the piston and 85J of heat are discarded. Still, this is the source of energy for most of our transportation. Automobile engines are only about 15% efficient. That means for every 100J of heat energy, 15J worth of work is done on the piston and 85J of heat are discarded. Still, this is the source of energy for most of our transportation.

13

14 If a steam engine takes in 2.254 x 10 4 kJ of heat and gives up 1.915 x 10 4 kJ of heat to the exhaust, what is the engines efficiency? If a steam engine takes in 2.254 x 10 4 kJ of heat and gives up 1.915 x 10 4 kJ of heat to the exhaust, what is the engines efficiency?

15 We can go backwards! This is a refrigerator or air conditioner High Temp Low Temp ΔE Q W

16 This requires Energy/Work This is why your refrigerator must be plugged in. This is why your refrigerator must be plugged in. It is constantly dumping heat into your kitchen It is constantly dumping heat into your kitchen Due to the 2nd Law of Thermodynamics more heat is dumped than is removed Due to the 2nd Law of Thermodynamics more heat is dumped than is removed If you left the refrigerator door open you would heat up the kitchen If you left the refrigerator door open you would heat up the kitchen

17 Perfect Heat Engine Q cold Q hot ΔEΔE W

18 Q cold Q hot ΔEΔE W

19 Efficiency W net =Q net =Q hot -Q cold W net =Q net =Q hot -Q cold or Eff=W net /Q hot =(1-Q cold /Q hot ) or Eff=W net /Q hot =(1-Q cold /Q hot ) Ex: A steam engine absorbs 1.98 x10 5 J and expels 1.49 x10 5 J in each cycle. Assume that all the remaining energy is used to do work. Ex: A steam engine absorbs 1.98 x10 5 J and expels 1.49 x10 5 J in each cycle. Assume that all the remaining energy is used to do work. a. What is the engine’s efficiency? a. What is the engine’s efficiency? b. How much work is done in each cycle? b. How much work is done in each cycle?

20 a. Eff=0.247 a. Eff=0.247 b. W=4.9 x10 4 J b. W=4.9 x10 4 J

21 More 2 nd Law Entropy Entropy S=entropy S=entropy Q=Heat (Joules) Q=Heat (Joules) T=Temperature (In Kelvin) T=Temperature (In Kelvin) Entropy in a system must increase or at least stay the same!!!!!!!!!!!! Entropy in a system must increase or at least stay the same!!!!!!!!!!!!

22 An engine has a hot reservoir at 1000 K and uses the atmosphere at 300 K as the cold reservoir. You take 2500 J from the hot reservoir to do 1900 J of work. An engine has a hot reservoir at 1000 K and uses the atmosphere at 300 K as the cold reservoir. You take 2500 J from the hot reservoir to do 1900 J of work. A. How much heat goes into the atmosphere? A. How much heat goes into the atmosphere? B. Is this engine possible? (Does the entropy increase?) B. Is this engine possible? (Does the entropy increase?)

23 W net =Q net =Q hot -Q cold Q cold =Q hot -W net =2500J -1900J =600J W net =Q net =Q hot -Q cold Q cold =Q hot -W net =2500J -1900J =600J Entropy Engine is not possible. Entropy Engine is not possible. What is the maximum amount of work we can take out? What is the maximum amount of work we can take out? How much work is done? How much work is done?

24 Carnot Engine Maximum efficiency Maximum efficiency In theory could run backwards In theory could run backwards All temperatures in Kelvin All temperatures in Kelvin What is the efficiency of an ideal steam engine with steam at 685 K and exhaust at 298 K? What is the efficiency of an ideal steam engine with steam at 685 K and exhaust at 298 K? What is Q cold ? What is Q cold ?

25 Carnot Engine Maximum efficiency Maximum efficiency In theory could run backwards In theory could run backwards All temperatures in Kelvin All temperatures in Kelvin What is the efficiency of an ideal steam engine with steam at 685 K and exhaust at 298 K? What is the efficiency of an ideal steam engine with steam at 685 K and exhaust at 298 K? What is Q hot if Q cold is 450J? What is Q hot if Q cold is 450J?


Download ppt "Thermodynamics The study of heat energy through random systems."

Similar presentations


Ads by Google