To begin class, I asked the students to name some sources of heat. These can include the sun, geothermal heat, and fuels such as wood, charcoal, coal, oil, gasoline, and natural gas. One of the first answers given by a student was friction which was a great segue into my next question: "Can friction be a source of heat?" :)
I asked the students what would happen if they pressed their hand against a cool tile wall or the cool metal cabinet in our classroom. They knew the cold object would make their hands cold. I told them that if they then moved their hand away and touched the same spot with their other hand, that the wall or cabinet would actually feel a little warmer. I then explained this happens due to the flow of heat from the students' warm hands to the cool wall or cabinet.
We then discussed how two drinks that were originally at very different temperatures could reach the same temperature over time. I asked the students to imagine they were working in a restaurant. Two customers had just left and you are cleaning up the table. One of the customers had a hot chocolate and the other had an ice-cold soda. When you picked up the drink containers, though, you noticed they are now the same temperature. How did this happen? The students realized the heat from the hot chocolate had transferred out to the cooler air while the warm air had transferred into the icy soda. Heat generally flows from a warm object to a cooler object.
Here's a fun lab to try at home (with minimal materials!)
"Heat - Energy Extraordinaire!"
Mix a dish soap and water solution in a cup (use 1/2 tsp. of liquid dish soap and 1 tbsp. of hot tap water).Add hot water to a second cup so it's about half-full.
Lower the mouth of a plastic bottle into the detergent solution so you have a nice film that covers the mouth of the bottle (think of the film you want when dipping a bubble wand into bubble solution).
Now, slowly push the bottom of the plastic bottle into the cup of hot water.
Observe.
As we learned, heat energy likes to travel from warmer areas to cooler areas. When you push the plastic bottle into the cup, the heat from the water travels into the bottle. This warms up the air that is in the bottle causing the air molecules to get excited and move faster. This increases the air pressure causing it to create a bubble at the mouth of the bottle.
We switched gears at this point to learn about conductivity. Conduction is a point-by-point process of heat transfer. What that really means is that if one part of an object (say, the bowl of a metal spoon) is heated by direct conduct (or contact) with a heat source (say, a bowl of hot soup), the neighbouring parts of the object (in this case, the stem of the spoon) will also become heated. Heat travels along or through an object by conduction.
The molecules in any object, as we learned last week, are in constant motion. When one part of an object is heated up, the molecules start moving faster. This causes them to bounce off each other more and transfer heat as they bounce. Eventually, through bouncing and hitting and heating lots of other molecules, the temperature of the entire object increases. Through the process of conduction, all the molecules pass heat around to each other until they are all hot.
"Cold Foot" experiment:
For this lab, we placed a bathroom rug and a piece of aluminum foil on the floor and left them undisturbed for about 10 minutes (while we completed the next lab, "Heat Conductors"). Each student stood with one bare foot on the foil and the other bare foot on the rug. They then compared how their feet felt.A good conductor of heat (the foil) will allow for the flow of heat. This happens due to those bouncing molecules passing heat around to each other. A poor conductor (also called an insulator) does not allow for this flow of heat. In fact, insulators work to keep the heat from escaping the warm object and transferring to a cooler object.
The aluminum foil felt colder than the carpet because, being a good conductor, it allowed for heat to flow from the students' warm feet. The carpet is a poor conductor so it kept the students' feet warm by trapping their heat in.
For this lab, each student was given three spoons: a metal spoon, a ceramic spoon, and a wooden spoon. I asked them which spoons they thought would make good conductors of heat and they gave their predictions. They were also each given a foam cup that was about half-full of very hot water. The students placed the three spoons in the hot water at the same time and let them sit for a few minutes. They then pulled the spoons out one-by-one and felt the temperature differences. The metal spoon was the clear winner/the best conductor. The ceramic spoon came in second since its stem was a little warmer but nowhere near as warm as the stem of the metal spoon. The wood spoon's stem did not heat up at all.
I then gave some examples of conductors and insulators. Metal, of course, is a great conductor of heat. I also pointed out that it is a good conductor of electricity which is why wires are metal. Plastics (such as the foam cup) and ceramics are often used as insulators since they are poor conductors. We talked about the use ceramics in cars (brake pads) and space shuttles to help protect engine parts from heat. I showed the students a pan with a copper bottom and asked why they thought the pan had that on the bottom. We also discussed whether it would be a good idea to stir hot soup on the stove with a metal spoon as well as a better alternative for this job.
To finish up the lesson, I taught the difference between endothermic and exothermic reactions. I wrote the two words on the board along with the words "thermal energy" and asked the class to point out the similarity. We talked about how "therm" relates to heat.
I then circled the prefix "endo" and explained that this means "inner" or "to draw into." We discussed the endoskeleton that humans and all other mammals have in common. I repeated this with the prefix "exo," explaining this one means "outer" or "to give off." We talked about the exoskeleton that ants have.
Endothermic reactions take in or absorb heat while exothermic reactions give off heat. A very simple example of an endothermic reaction is the process of ice changing from a solid to a liquid. In order for an ice cube to melt, it has to take in heat from its surroundings. It melts because thermal energy flows from a warmer object to the cold ice cube.
A basic exothermic reaction would be lighting a match. Striking a match causes the release of heat from the chemical on the match head into the air.
"Heat Changes" lab:
This lab demonstrated an exothermic reaction.For this lab, I filled a baby food jar with water and then added a teaspoon of powdered bleach. I stirred the mixture and then inserting a thermometer. The students checked the thermometer every few minutes.
Because bleach contains oxygen, adding water to it creates a reaction that gives off oxygen gas. Heat is also a product of this reaction.
Next week: Simple Machines
The lab Heat - Energy Extraordinaire! is available as a PDF from the American Chemical Society (www.acs.org or www.acs.org/kids). A link to the PDF can be found on this website:
http://search.acs.org/search?q=Heat-Energy+Extraordinaire&client=acs_r2&output=xml_no_dtd&proxystylesheet=acs_r2&sort=date%3AD%3AL%3Ad1&entqr=3&oe=UTF-8&ie=UTF-8&ud=1&site=acs&x=0&y=0
The lab Heat Conductors was found on this website:
http://www.kids-science-experiments.com/heatconductors.html
Cold Foot is from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.
Heat Changes is from Chemistry for Every Kid.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.
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