DAY 115
Essential Question: How does thermal energy flow?
GOALS: SWBAT...
1. define entropy
2. diagram how a refrigerator works
3. Compare and contrast a refrigerator, heat pump, and heat engine
WARM-UP 115 (4 min)
1.can you warm your house by using the oven?
2. should you? Why yes or why no?
3. can you cool your house by using the open refrigerator?
4. Should you?
CLASSWORK
1. Review of Yesterday's Classwork & the Laws of Thermodynamics
2. Review 115A: Entropy
Definition: a measure of the level of disorder. Greater disorder = greater entropy. Entropy is related to the second law of thermodynamics.
The second law - The level of disorder in the universe is steadily increasing. Systems tend to move from ordered behavior to more random behavior.
One implication of the second law is that heat flows spontaneously from a hotter region to a cooler region, but will not flow spontaneously the other way. This applies to anything that flows: it will naturally flow downhill rather than uphill.
If you watched a film forwards and backwards, you would almost certainly be able to tell which way was which because of the way things happen. A pendulum will gradually lose energy and come to a stop, but it doesn't pick up energy spontaneously; an ice cube melts to form a puddle, but a puddle never spontaneously transforms itself into an ice cube; a glass falling off a table might shatter when it hits the ground, but the pieces will never spontaneously jump back together to form the glass again. Many processes are irreversible, and any irreversible process increases the level of disorder. One of the most important implications of the second law is that it indicates which way time goes - time naturally flows in a way that increases disorder.
Also, objects with greater entropy are generally less able to do work. Example: Air. In what state is air? Is air a good conductor (aka, can it pass energy from one object to another)?
examples (wood+fire=ash; solid water+heat=liquid water; energy+popcorn kernels=popcorn)
Common Quote: "The universe tends towards disorder"
3. 115B: How Does a Refrigerator Work?
https://www.youtube.com/watch?v=EIP3pSio7-M
Consider the following for your notes
1. Diagram the flow of refrigerant as it moves through the refrigerator
2. How does heat flow during evaporation (similar to vaporization)?
3. How does heat flow during condensation?
4. What does the expansion device do to the refrigerant?
5. The cool refrigerant flows through the refrigerator and absorbs heat. Which Law of Thermodynamics does this heat flow illustrate?
6. What is the function of the coils on the back of the refrigerator?
4. 115C: What is the difference between a refrigerator and a heat pump and a heat engine?
First of all, what is a heat pump? Generally, if you've stayed in a hotel, you've used a heat pump. It's a device which can be used as either a heater OR an A/C. Pretty cool, as it's reversible. Here's a photo of one: https://article.images.consumerreports.org/f_auto/prod/content/dam/cro/news_articles/appliances/CR-BG-Heat-Pump-Ductless
Here's a video of how they work (watch the first 5:30 minutes): https://www.youtube.com/watch?v=QykwWs3L1W8
What's a heat engine? A great example is an internal combustion engine in a car. It uses heat to cause movement (aka, to do work). Heat flows from high to low (2nd law of thermodynamics)
Create a Venn Diagram comparing a contrasting a refrigerator, heat pump, and heat engine.
Some answers to the above question:
ANSWER #1: All three devices work on the basis of heat flow.
In a heat engine, heat is transferred from a higher temperature level called source to a lower temperature level called sink. Work is obtained during this process.
A refrigerator is a reversed heat engine. Heat is transferred from the lower temperature level to higher temperature by applying external work to maintain the temperature below atmospheric temperature.
A heat pump is similar to a refrigerator. The only point of difference between the two is of the operating temperatures. The working temperatures in a refrigerator are of the colder level and room temperature, whereas working temperatures in heat pump are of hotter level and room temperature.
ANSWER #2
Heat flows in one direction; from high to low. A heat engine is any device that uses heat to do work. Internal combustion engine, steam engine, even a convection device used to move air or mix something, would be heat engines. It gives off heat as a by product. The gases or liquids are heated or burned in a way that causes things to move.
A refrigerator, which is mechanically similar to an air conditioner, has a "flash gas" or metering device, that's basically an atomizer, that is used to separate the high pressure piping from the low pressure piping so that when refrigerant is force by the high side pressure through the metering device to the low pressure side, the sudden expansion and evaporation of the refrigerant forces the absorption of heat from the surrounding area as it flows through the evaporator. The absorption of heat is perceived as cooling. The refrigerant, having absorbed the heat, is drawn from the low side by a compressor and is highly compressed into the high side (compression drives up the temperature) where it flows through a condensing unit releasing all that heat back into the surroundings. In short, evaporators cool and condensers heat. Evaporators use the surrounding heat to boil the refrigerant and turn it into a vapor that is sucked up by a compressor, compressed into the condensing unit which gives off heat to the surrounding environment to facilitate condensing the vapor back into a liquid that's pushed through the metering device to start the cycle over again.
A refrigerator and air conditioner are basically the same device, except the evaporator is inside the refrigerator compartment and cools a lot lower than an air conditioner while the condenser is just on the outside of the refrigerator releasing the heat into the room it's in. An air conditioner has the evaporator coil inside the house and the condensing coil outside the house.
A heat pump is essentially two air conditioners in one. There is either a special valve or a couple of adjustable metering devices that allow the evaporator and condenser to switch jobs and cool the outside air (even though its already cold) and heat the inside of the house. The flash gassing of a refrigerant can draw heat even when the surrounding temperature is well below freezing. Unfortunately this can cause some icing problems that interfere with the effectiveness of a heat pump.
Exit Question:
1. Which of the above three heat transfer devices uses energy to push heat from low (cold) to high (hot)?
LEARNING AT HOME (HW)
Quiz Wednesday. Math problems related to temperature change, phase change, and heat
Exam Friday. Review Days 101-114
Essential Question: What is Heat?
GOALS: SWBAT...
1. answer thermo related questions and define thermo terms
WARM-UP 115 (4 min)
1. Describe how a thermometer works and what it measures.
2. Does ice have thermal energy?
3. What is entropy?
4. What has more entropy, a liquid or solid?
CLASSWORK
1. Review 115A
What's on the exam? Everything from Day 101-114
What should I study? See above, and below.
https://sites.google.com/site/mralexandersscienceweb/physics/daily-agenda-classwork/day-101
Note: for 19 and 20, I won't test you on deposition and sublimation
Between the following two documents, you'll find the answers to the 20 Knowledge Inventory questions. You'll also find some extra stuff (example, we didn't use a hand boiler)
LINK 1: https://drive.google.com/file/d/0B-AiPYEb3nFVUVh0MU9HeGZ6Umc
LINK 2: https://docs.google.com/file/d/0B-AiPYEb3nFVbFI4WnlJTTFpR2c
TERMS: Need to review that vocabulary? Here are many of the terms, in flashcard form. https://quizlet.com/636382/thermodynamics-review-flash-cards/
Note: I won't test you on enthalpy
PRACTICE TEST #1: https://www.proprofs.com/quiz-school/story.php?title=unit-test-thermodynamics
Complete the following: 1-3, 10-12, 20 (it's a true false question)
PRACTICE TEST #2: https://quizizz.com/admin/quiz/5907d1ed51a74d14001ec3cf/thermodynamics
Complete the following: 1, 2, 4, 5, 10-14, 17, 18
PRACTICE TEST #3: https://www.ducksters.com/science/quiz/heat_questions.php
Complete #1-10
PRACTICE TEST #4: https://quizizz.com/admin/quiz/569f9e4032b7545d46f035c7
Complete #2-50 (skip 36)
HEAT PUMP vs. HEAT ENGINE vs REFRIGERATOR: https://www.quora.com/What-is-the-difference-between-a-refrigerator-and-a-heat-pump-and-a-heat-engine
PHASE CHANGES & COOLING vs WARMING PROCESSES: http://www.theweatherprediction.com/habyhints2/524/
TEMPERATURE SCALES: You should know absolute zero, as well as the boiling/condensing and freezing/melting temperatures of water, in Kelvin and Celsius. You should also know how to convert from K to C, and vise versa.
LEARNING AT HOME (HW)
Exam Thursday. Review Days 101-114
States of Matter Lab is due Wednesday 11:59pm
What are we doing this week?
Ending Thermodynamics. beginning a review of modeling Conservation of Energy, and of Simple Machines.
You: What are simple machines?
Me: https://www.youtube.com/watch?v=Av07QiqmsoA
Also me: https://www.youtube.com/watch?v=3jQXN8SMewo
Tues: Entropy/Knowledge Inventory revisit
Wed: exam review
Thurs: exam
Fri: Screw, Incline, etc (6 components). Define. explain how each works. Give examples. Put info graphic organizer
Next week: Make Rube Goldberg Machines in groups