Energy Transformations Facilitator Guide Convection in a House

It works well to have middle school students run the model individually to collect data and in pairs for the analysis and conclusion sections.  

It will be important for students to know how to run the model, including each of the model features and how they are activated or modified.  It is helpful to have students run the model in Collect Data I and then to address questions during/after running it for a few trials based upon student performance.  It is also important that students understand how to take snapshots of the model so they can analyze the data effectively.  Teacher modeling of how to take a snapshot will also be helpful for students using them for the first time.

The activity will take two days in this unit to allow time for effective data collection and analysis.  Then one day should be used for conclusions through group discussion presentations and whole class wrap-up.  Also, at least an additional day should be used for further investigation to allow students greater practice in engineering practices.     

This unit investigates the engineering concepts related to the conservation of energy and energy transfer.  The activity specifically investigates how different liquids and gases carry heat throughout a house, starting with the focus question, "How does air carry heat throughout a house?".
The big ideas for this activity are:
(a) The transfer of energy can be tracked as energy flows through a designed or natural system.

(b) Temperature is a measure of the average kinetic energy of particles of matter. Thermal energy of an object depends on its temperature, mass, and material.  The total energy of a system depends on its temperatures and the types, states, and amounts of matter present.

Thermal energy is transferred by three means.  It is transferred by (1) conduction through a material by the collisions of atoms within the material. Over time, the thermal energy tends to spread out through a material from one material to another if they are in contact moving from warmer areas to cooler ones. Thermal energy can also be transferred by (2) convection through currents in air, water, or other fluids. In addition, all materials (3) radiate thermal energy as light energy (electromagnetic waves) at a rate depending on their temperature; that light energy will be transformed back into thermal energy when the electromagnetic waves are absorbed by another material. Materials cool down when the net flow of energy is away from the material and warm when it is toward it. Materials vary in their capacity to hold and transfer heat energy.

Misconceptions
The common misconceptions that students have at the middle school level are (1) heat and temperature are the same thing, (2) thermal energy is a "substance" contained in an object and is not related to the material of the object, (3) inanimate objects do not have any thermal energy, and (4) gases do not have any thermal energy because gases do not have mass or are not matter.  

NGSS
Physical Science
MS-PS3-3 - Apply scientific principles to design, construct, and test a device that either minimizes or maximizes thermal energy transfer.

MS-PS3-5.	Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object.

Engineering, Technology, and Application
MS-ETS1-1.	Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.

MS-ETS1-4.	Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.

Standards From Activity
MS Physical Science 3.B: Conservation of Energy and Energy Transfer

Energy is transferred out of hotter regions or objects and into colder ones by the processes of conduction, convection, and radiation.

MS Physical Science 3.C: Relationship Between Energy and Forces

Design, construct, and test a device that either minimizes or maximizes thermal energy transfer by conduction, convection, and radiation.

MS Science and Engineering Practices: Constructing Explanations and Designing Solutions

Apply scientific knowledge to design, construct, and test a design of an object, tool, process or system.

Q: How is energy transferred?
A: Energy is transferred out of hotter regions or objects and into colder ones by (1) conduction (the transfer of thermal energy/heat by collisions between particles of matter), (2) convection (the transfer of thermal energy by the movement of particles within matter/the circulation of material caused by differences in temperature and density), and (3) radiation (energy carried by electromagnetic waves).

Q: If you put the barrier closer to the bottom what differences do you notice in the thermal image below and above the barrier?
A: When placed near the bottom, the area between the floor and the barrier reaches a uniform temperature quicker than if is is higher.

Q: What is the difference between natural and forced convection?
A: Natural convection is caused by density differences.  Forced convection refers to fluids being pushed around by outside forces.

Q; What do the colors represent on an infrared image?
A: Red or white shows hotter temperatures and blue shows cooler.

Q: Based upon what you learned in the unit how does natural and forced convection work in your home?

A: Natural convection is caused by density differences.  Therefore, warm air in a room quickly rises upward, and cold air sinks downward, even if the temperature differences are quite small.  Insulation/barriers in your home provide resistance to heat flow.  "Regardless of the mechanism, heat flows from warmer to cooler until there is no longer a temperature difference. In your home, this means that in winter, heat flows directly from all heated living spaces to adjacent unheated attics, garages, basements, and even to the outdoors. Heat flow can also move indirectly through interior ceilings, walls, and floors -- wherever there is a difference in temperature. During the cooling season, heat flows from the outdoors to the interior of a house." (http://energy.gov/energysaver/articles/insulation). Therefore, the more leaks in your home the greater probability of heat loss.

Forced convection or air being moved by external forces can increase the rate of temperature change of the surface of a building. Air colder then the surface will lower the temperature faster than still air (better known as wind chill effect) and increase the heat loss from the walls and windows. However, forced convection of air can also speed the distribution of warm or cool air in a building, making heating or cooling systems more efficient.

Energy is transferred out of hotter regions or objects and into colder ones by the processes of conduction (the transfer of thermal energy/heat by collisions between particles of matter), convection (the transfer of thermal energy by the movement of particles within matter/the circulation of material caused by differences in temperature and density), and radiation (energy carried by an electromagnetic wave). "Thermal Energy Transfer." Earth & Space: IScience. Columbus, OH: Glencoe/McGraw-Hill Education, 2012. 421-23. Web.

Convection is heat transfer by the mass motion of a fluid, for example water or air.  In the models, the motion is seen in the thermal images produced as in one location is heated.  This movement of warm air is seen and used in nature as well as in a building.  In nature thermal uplift (rising warm air) helps some species of birds soar for long periods with minimal work.  This same principle of convective lift is used by hot air balloon.  In the balloon a hot mass of air is created which provides the lift for the balloon.  To maintain lift the pilot may have to periodically reheat the air inside the balloon. (http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html)

Natural convection is caused by density differences.  Therefore, warm air in a room quickly rises upward, and cold air sinks downward, even if the temperature differences are quite small.  Insulation/barriers in your home provide resistance to heat flow.  "Regardless of the mechanism, heat flows from warmer to cooler until there is no longer a temperature difference. In your home, this means that in winter, heat flows directly from all heated living spaces to adjacent unheated attics, garages, basements, and even to the outdoors. Heat flow can also move indirectly through interior ceilings, walls, and floors -- wherever there is a difference in temperature. During the cooling season, heat flows from the outdoors to the interior of a house." (http://energy.gov/energysaver/articles/insulation). Therefore, the more leaks in your home the greater probability of heat loss.  Moving air currents (forced convection) lower the surface temperature of a building (wind chill effect) and increase the heat loss from the walls and windows.
(From NSDL Science Literacy Maps (http://strandmaps.nsdl.org/) and the Next Generation Science Standards

Q: What would it be like if there were no natural convection, that is, if air didn’t move around when heated or cooled?
A: There wouldn't be any circulation of fluids causing very hot or cold temperatures and meaning that circulation would have to be produced externally, making heat energy very costly.

Q: Give an example where heat is transferred by convection in a house. Use data, results or descriptions of your experiments or model-based activities to explain how heat is transferred by convection.
A: The upper levels are warmer than the lower levels which is evident in Data Collection I's infrared images because warm air in a room quickly rises upward, and cold air sinks downward. Heat flows from warmer to cooler until there is no longer a temperature difference in convection due to density differences.

Q: Convection might cause a building to lose heat when hot air leaks out through holes in the building (infiltration driven by the stack effect). Suggest how you might cut down on these forms of heat loss in a real house.
A: You can patch holes with insulation, caulk, or seal them.

Q: Moving air lowers the surface temperature of a building (wind chill effect) and increases the heat loss from the walls and windows. Air also enters the building through cracks and holes (infiltration). Think of other examples of the wind chill effect and how it is minimized.
A: You can use storm doors and/or windows and weather stripping.  Well-located trees and shrubs can also intercept the wind and cut heat loss.    

Students can complete a heat energy audit of their home and then formulate ways to cut down on heat loss and save money in their home.  They can also research heat energy efficient materials and then design even more heat energy efficient homes in the future to help them elaborate on engineering practices.

A link to a challenge for grades 3-8 to learn about energy conservation is located at http://www.homeenergychallenge.org/Default.aspx.  It is sponsored by the US Department of Energy, and administered by the National Science Teachers Association. "The challenge gives students the chance to learn about energy, develop techniques for reducing energy consumption, and save money in their own homes by reducing household energy use."

Since heat convection can also be seen in water you can set up a simple investigation with a pan, warm water, a paper cup, ice cubes, food coloring.  Fill the pan with warm water.  Poke a hole in the bottom of the cup.  Tape the cup to a corner of the pan so that the bottom is submerged but not sitting on the bottom pan.  Place in several ice cubes and a couple of squirts of food coloring.  Observe the thermal currents produced.