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Kinetic energy into thermal energy [W/Video]

Posted on January 17, 2014 by Dr. Brian Thomas There have been 7 comment(s)

Watch how silly putty and simple steel spheres can be used to demonstrate the conversion of kinetic energy into thermal energy! (And you thought Silly Putty was for copying comics from the newspaper!)
 

 
One of the most important concepts in physics is the principle of energy conservation, especially the idea that energy can change forms and be transferred from one object to another. An important application of this idea is that kinetic energy can be transferred to heat, which is just another form of energy. One way to demonstrate this idea is to have students simply rub their hands together, showing that friction transfers energy into heat. However, it may be less obvious to students that impacts also generate heat, since the role of friction may not be as clear when the objects are not actively rubbing together. One easy way to demonstrate this is with a set of Colliding Steel Spheres. When I use this demo in class I ask for a “brave student volunteer.” I have the student volunteer hold a sheet of paper vertically and then I dramatically smash the spheres together with the paper in between. A small hole results and I have the student examine the hole, in particular noticing that there is a smell of burning paper. This clearly shows the class that the kinetic energy of the moving spheres is changed into heat energy when the spheres collide. I then typically make several more collisions to drive home the point, and sometimes allow students to try it for themselves.

swiss_cheese_paper_600

Another dramatic and visual way to demonstrate this concept uses silly putty, a hammer, and a temperature probe connected to computer data acquisition system. Simply embed the temperature probe inside the silly putty, start the data acquisition, let the temperature come to equilibrium and then smash the hammer down on the silly putty. You should then see a rise in temperature recorded on the screen, again showing how kinetic energy is transferred into heat energy, resulting in a rise in temperature of the silly putty.

fire syringe
Another great way to show energy conservation, and connect to thermodynamics concepts, is the Fire Syringe. This is basically a piston that you can use to rapidly compress a column of air with a bit of cotton ball; the rapid compression heats the air and ignites the cotton ball. This again is an illustration of converting kinetic energy into heat; you can also present this in the context of the idea gas law, where the compression increases the temperature. A couple of things to watch out for when using this device: the seal needs to be good, so the o-rings should be lightly lubricated and when you compress the piston it should bounce back rapidly; use a small bit of cotton ball and pull it apart so there is a lot of surface area to heat. You need to give the piston a quick compression, and it helps if the cotton fibers are near, but not sitting on, the bottom of the column.

These demos are visual and, for many students, surprising, and can really drive home the idea that energy can change forms and particularly that kinetic energy can be changed into heat energy.

 
thomasBrian Thomas is an Associate Professor of Physics & Astronomy at Washburn University's College of Arts & Sciences His Career Accomplishments include but are not limited to: Principle Investigator on a $500,000 research project funded by NASA’s Astrobiology program, in collaboration with the Smithsonian Environmental Research Center, author or co-author on approximately 20 peer-reviewed articles. Invited speaker for several international conferences. His work has been featured in news articles, as well as in several television documentaries.
http://www.washburn.edu/our-faculty/brian-thomas

 
 
 
 
 


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This post was posted in CoolStuff Newsletters, Energy, Measurement and was tagged with energy


7 Responses to Kinetic energy into thermal energy [W/Video]

  • Thank you very much for this and other demos that I find very useful.

    Posted on January 19, 2014 at 8:11 am

  • I am concerned about the loose usage of the terms "heat" and "thermal energy" in this video. The First Law of Thermodynamics is subtle and requires us to be very careful and specific about our language to ensure that we are not promoting misconceptions.

    Posted on January 19, 2014 at 8:57 am

  • Scott says:

    By heat energy, I assume you mean kinetic energy? What is "heat energy"?

    Posted on January 19, 2014 at 10:10 am

  • Another good demonstration is to take a wire coat hanger and bend it back and forth until it breaks. Then have a student lightly tap the wire to feel the heat. If you grasp it you can occasionally burn yourself.

    Posted on January 19, 2014 at 4:09 pm

  • Brian says:

    Thanks for the comments about "heat" and "thermal energy." I'm certainly aware of the complexities there, but for this context I have deliberately avoided getting into those details. When I use these demos in class it is in the context of a much more detailed and precise discussion of these ideas, as appropriate for the audience, and anyone is of course free to take and modify as they see fit.

    Posted on January 20, 2014 at 11:32 am

  • Two more demonstrations of the same idea, which may attack misconceptions:

    Use a 10 cm by 10 cm square of "Fickle Foam," a liquid crystal film that changes colors around 20 to 30 degrees Celsius, on a foam backing, and a large rubber mallet. Show the students that the film is one color at room temperature and a patch of it changes through different colors as it rises to the temperature of a fingertip. Show that the film cools off again. Let a student testify that the mallet is at room temperature. When the mallet is pressed hard onto the temperature-sensing surface, should the temperature change? Let the students answer: No, because no work is being done; pressure is altogether different from temperature. Show that the liquid crystal is still in its low-temperature state. Now have a student bash a spot on the foam hard with the mallet. Show that the temperature of the spot has gone up. Often it is a centimeter-scale crescent-shaped area of bright color. What made the temperature go up? The conversion of energy of organized motion into energy of disorganized motion on the molecular scale. Or, work input turning into internal energy with no heat involved. Heat output makes the temperature drop back down again.

    All of these demonstrations made the temperature change large enough to notice by making the sample size very small. To show that one joule typically produces a negligible temperature change, or that we work with much larger quantities of energy when we study calorimetry in lab instead of rolling carts (or that the internal energy in the air above Allegheny County, New York, equals that in the world's annual use of fossil fuels) do this demonstration: Obtain a brace from a carpenter's brace and bit, a large nail, and a "dry calorimeter" like Science First 612-1330, consisting of a 300-g aluminum cylinder inside a styrofoam jacket. Have the students put an electronic temperature probe in the small hole provided in the aluminum cylinder and display a running graph of its temperature on a computer screen. Mount the nail head downward in the chuck of the brace. Observe that the nail head will not cut into the bottom of the large blind hole in the aluminum block, but will just rub there when the brace is turned. Let the students predict what will happen to the temperature--in a second, in a minute, or in five minutes. Aluminum is a fast conductor of heat, so the whole block will rise in temperature nearly uniformly. Now have the students turn the brace. What happens to the rate of temperature increase when they turn it slowly? fast? What if they do not press the nail head against the aluminum? what if they press hard? Negotiate with the students for the conclusion that both force and speed are required for high power, evidenced by a rapid temperature increase (Which beefy young man can make it go up steepest on the compute screen?) and that both force and distance are required for a high energy input, evidenced by a high final temperature (Which hardworking, faithful, and tiring team can get their block hottest?)

    Posted on January 21, 2014 at 10:37 am

  • I would like to suggest that Benjamin Spicer's comments not be dismissed so cavalierly. We confuse students, no matter what the audience, when we interchange terms that should not be interchanged. One does not avoid the issue by interchanging terms. I'm not suggesting that one must get into the difference between heat and thermal energy, but rather just use the correct terms. I would also like to point out that, while transfer of energy is associated with conservation of energy, none of the demos in the video illustrate conservation of energy. Perhaps better not to mention conservation of energy when that goes far beyond the context of the demos?

    Posted on February 10, 2014 at 7:46 pm

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