Heat Transfer
Physics
HS
Heat Conduction
Heat Conduction: Molecular View
Thermal Radiation
Two of the activities are models only. These will work best if you can have as few members per group to work on the computers.
The third activity uses models as well as probes. You have to make a radiation detector to accompany the sensor, but it is not difficult. You can either do it yourself or have the students do it. If you are using black paint to coat the foil-faced insulation square, be aware of any possible allergies to the type of paint you use.
Students will also be using hot and boiling water. Care must be taken to make the students aware of the hazards of spilling the boiling water.
Since you are using water with the probe make sure the students keep the computers separated from the water.
You have to build a radiation sensor with foil, Styrofoam, black paint, and surface temperature probe. The activity shows you how to do it.
The two conduction labs could either be done as separate periods with other activities or done together in one 50 minute period with nothing else.
The radiation lab will need a whole period or possibly two if you want them to build the sensor themselves and depending on how many extra objects outside of the prescribed lab you want them to test.
This unit is motivated by the discovery questions:
Heat energy is constantly being exchanged throughout our world on all scales from the microscopic to the whole universe, flowing from warmer to cooler elements. Even in a system where the elements are at the same temperature, there is energy flow between them, but the net flow is zero.If there is a temperature gradient there will be a transfer of heat energy between elements. Heat energy is conveyed through three processes--conduction, convection, and radiation. Conduction transfers energy through direct contact, the jostling of molecules against each other with faster vibrating molecules exciting slower ones (heating), and slower ones slowing faster ones (cooling). Convection is the transfer of heat through the movement of matter usually through the expansion of warmer elements floating through cooler ones. Radiation is the transfer of energy through the emission and absorption of electromagnetic radiation. All materials emit radiation at a rate depending on their temperature with warmer elements radiating at a greater rate than cooler ones.
Probably the most common misconception is the relationship between heat and temperature. Many students have learned that "heat is the total internal energy of a system" and "temperature is the average internal energy of a system". If this was true then they would both need to have the same unit and they do not. Indeed, heat is a measure of total internal energy of a system, however temperature is a concept used to compare systems (not quantities of energy).If systems are at the same temperature (equilibrium) there will be no net flow of energy between them, and if they are at different temperatures, energy will flow from the warmer to the cooler. This can be difficult for students because they often think of the idea of “hot” and “cold” flowing, rather than the important principle that there is only one movement from hotter to cooler.
When you take a piece of metal out of the freezer your hand will get cold. Why?
The heat from your hand (higher temp) will flow from your hand into the metal (lower temp).
If you were going to heat up a cold object which method of heat transfer would be more efficient?
Efficiency depends upon the state of matter of the object, convection is generally more efficient in liquids and gases. Conduction is more efficient in solids due to direct contact. Radiation is the only way through empty space, since convection and conduction do not occur in the near-vacuum of empty space.
When you see a piece of paper or ash rise up out of a fire what caused that to happen?
Convection. The heated air was rising and the paper or ash was light enough to get moved by the air.
What process allows a bird like a hawk to get higher and higher in the air without ever flapping its wings?
Convection.
Will an object with a very low temperature radiate?
Yes, if an object is above absolute zero it will radiate.
When you collect heat data what additional heat sources do you need to try to eliminate so as to not affect the data?
Body heat, computer heat, and possibly heat from some lights.
In the models, why are the higher temperature molecules moving faster than the lower temperature molecules?
Answer: In the model, temperature is calculated as the average kinetic energy, which is related to the square of the velocity.
In the models what colors are used to represent the approximate amount of heat present?
Cold = Blue; Hot = Red
What properties affect how fast heat flows in the process of conduction?
Thick faster than thin; short faster than long; type of material.
What properties affect how fast heat flows in the process of radiation?
Color; Size; Temperature of the object radiating heat.
What process(es) is/are utilized to boil water in a pot on a stove top?
Conduction and convection.
Convection is a difficult process to do in the lab. The idea that heat rises and cold falls is easy to say, but can be difficult to fully understand. Showing video of a bird rising in the air or ash rising from a fire can help. However the mathematics is quite involved and calculus based due to changing densities and temperatures during movement.
Conduction is based on the equation ?Q/?t=-kA?T/?x (?Q = heat; k = thermal conductivity constant (specific by material); a = cross sectional area; ?T = change in temperature; ?t = elapsed time; ?x = length of what is conducting the heat). As you can see there are no variables raised to a power so the process is basically linear and can be a very good lab to work with graphs. The important part to remember is that ?T and L need to be utilized together. The ?T is based on the value you use for L. For example if you are using a metal bar you don't have to use the entire length of the bar, but the temperatures need to match to the length you are using.
Radiation is based on the equation Q = e* ? *A*T^4*t. (Q = heat; e = emissivity constant (value is between 0 and 1 and is based on the substance); ? = Steffan/Boltzman constant (5.67 X 10^-8 J / s*m^2*K^4); A = Surface area; T = Temperature (in Kelvin); t = elapsed time). Temperatures must be converted to Kelvin and if you don't know the emissivity constant for the object you are using and you are just comparing the same object for different amounts of time you can just use 1. A good extension lab is to collect data and find the e value for many different objects.
The amount of thermal energy (?Q) transferred through an area (A) in a time period (?t) is called the heat flux. In the case of conduction, heat flux is determined by the following relationship: ?Q/?t=-kA?T/?x where ?T is the temperature difference along a distance (?x) in the direction of conduction, and k is the thermal conductivity of the material. This is known as Fourier's Law of Heat Conduction. Explain in words what the equation expresses about the effect of area, thermal conductivity, and temperature difference.
All are directly related to the power (?Q/?t) conducted along the object and are inversely related to each other
Do the models agree with Fourier's Law? Explain your answer.
Yes. As k, A, and ?T were increased more heat was conducted per second and the graphs showed much steeper slopes
Think back to the kitchen situation. Give examples of the following: a material with high conductivity increases heat flow dramatically.
Answers will vary, but an example would be the metal pot will let a lot of heat to conduct through it and heat up quickly.
Think back to the kitchen situation. Give examples of the following: a material with low conductivity decreases heat flow dramatically.
Answers will vary, but the handle of the pot, usually not made of metal, will not get near as hot as the pot itself.
Think back to the kitchen situation. Give examples of the following: area is increased to increase heat flow.
A large pot is usually placed over a larger burner especially to heat water to boiling. When a large pot is placed on a smaller burner it takes much longer to heat up.
Area is decreased to decrease heat flow.
Some pots have metal handles and yet they don't seem to get as hot as the pot itself. In most cases these handles are connected to the pot by small welded connections. This dramatically reduces the area through which the heat can flow and so it takes much longer for the heat to flow into the handle.
Which conducts heat more quickly, a short bar or a long bar? Explain why on a molecular basis.
The short bar conducts more quickly because there is less distance for the molecules to pass the heat along.
Which conducts heat more quickly, a thin bar or a thick bar? Explain why on a molecular basis.
The thick bar conducts more quickly because there are more molecules present to move the heat from one end to the other.
Explain, on a molecular basis, why a vacuum does not conduct heat.
A vacuum will not conduct heat since there are no molecules present to transfer the heat. There must be matter for conduction to occur.
In the molecular model shown above, the two temperature graphs - one object cooling down, and the other one heating up - match perfectly. Looking only at the graphs, you might say that heat flows from a hotter to a colder object, or cold flows from a colder to a hotter object, or both. Which is the more accurate way to describe heat flow: from hot to cold, or from cold to hot? Defend your answer, using a molecular explanation.
Molecules that are moving quickly are "hot" and have more energy than molecules that are moving slowly and are "cold". If you watch the molecules the cold ones start to move faster and the hot ones start to move slower. Therefore the energy must be flowing from the "hot", faster molecules to the "cold", slower molecules. If the slow molecules transferred its energy to the faster ones they would have to get even faster and that did not happen.
Summarize your results. Compare the ability of the different materials to transmit visible and non-visible (infrared) electromagnetic radiation.
Student answers will vary. There does not have to be any direct correlation between visible radiation and infrared (heat).
Can the detector measure cold as well as hot? If so, how does that work? Is there such a thing as "cold" infrared rays?
Yes. Cold is just a term we use to match our perception. If an object has molecular movement then it has energy and therefore temperature. Cold is really just an object with less heat compared to something hot. If an object has heat it gives off infrared radiation just not has much as something hot.
How did the radiation from a fluorescent light compare to an incandescent light? Which one gives off more infrared radiation? Why?
The incandescent light gave off more infrared radiation due to the fact that it had the higher temperature that was recorded.
How can you be sure that the heating effect of the jars is by electromagnetic radiation and not conduction or convection by the air?
The sensor was normalized by pointing it up in the room before using it and convection might occur but the sensor is horizontal to the heat source and convection would take the heat vertically away from the sensor.
Why can't the Sun's energy come to Earth via conduction or convection?
Both processes need matter for them to occur and space is essentially a vacuum and therefore has no matter.
Give an example of heating something by radiation but NOT convection or conduction.
Student answers will vary. Some examples would be the Earth being heated by the sun, your body being warmed by a fire).
Using the following links have students collect data and see if they can match the constants on the table on the following sites.
http://www.thermoworks.com/emissivity_table.html -- EMISSIVITY
http://en.wikipedia.org/wiki/List_of_thermal_conductivities -- THERMAL CONDUCTIVITY