Tag - heat transfer

Liquid Crystal Sheet Demos [W/Video]

Safely demonstrating the principles and properties of heat can sometimes be a risky proposition. But it needn’t be. Watch as Physics Teacher James Lincoln uses the safe, simple, and inexpensive Liquid Crystal Sheet to visually display some very cool things about heat we usually only TELL our students about.

Introduction
These Liquid Crystal Sheets are heat sensitive and offer a wide-range of possibility for experiments. Because they change color based on temperature, they can be read visually and quickly, at a distance, allowing the whole class to enjoy your demonstrations. The sheets easily warm to the touch and as they do, will display the visible spectrum ROYGBIV – with blues and purple signifying the warmest temperatures. If they get too hot, they will become dark again. The color change is caused by tiny crystal layers – on the micron scale – twisting as they warm.

heat_article-1 hand on sheet

The thin layer responds rapidly to the touch. These sheets offer a wide range of possibilities.

 

 

 

 

 

 

 

 

Fun and Easy Explorations
After warming a sheet with your hand, try putting some cold water or ice on it. Then, you can put some hot water on it and demonstrate evaporative cooling (blow air on the hot water and it cools rapidly). The thin design of the sheet allows for rapid color changes. Another cool thing to try is to compare the hands of male students to female students. Male hands are usually warmer because of higher blood pressure and surface area-to-volume ratios. This is plain to see using the Liquid Crystal Sheet.

heat_article-2

A streak of warm water beautifully demonstrates the color changes. Notice the far end is already demonstrating evaporative cooling.

 

 

 

 

 

 

 

 

Efficiency of Light Sources
One experiment you should conduct is to compare the heat output of various light sources. Of course the candle puts out an enormous amount of heat. This is easily tested by holding the Liquid Crystal Sheet above the light source, since heated air rises by convection. The classic incandescent light bulb is next and it puts out a medium amount of heat, because it has to heat up to glow. But the fluorescent bulbs barely put out any heat at all compared to these other sources – they are much more efficient. This topic is appropriate for Physics and Environmental Science. The more modern light sources are light-emitting diodes (LEDs), and they are the most efficient of all, putting out almost no heat. You can get one of the new, futuristic diode bulbs at your local hardware store.

Insulation and Conduction
You can demonstrate insulation using a piece of Styrofoam. Place the Styrofoam below the sheet to delay the transfer of heat to the tabletop and hold the thermal images longer. Once you have your image, use a piece of metal to demonstrate thermal conductivity. The metal takes the heat away faster. You can also demonstrate that friction generates heat when the metal is applied to the surface.

heat_article-4

Placing the sheet on Styrofoam will lengthen the duration of the thermal image.

Thermocline
As an example of how well the sheets can display ‘what’s hot & what’s not”, I offer the thermocline. Cold water and warm water will sort themselves based on temperature due to density differences, called a thermocline. Thermoclines occur in swimming pools, lakes, and the ocean. Generally, the warm water rises and the cold water sinks. However, in the ocean there are also haloclines, which are density differences caused by salt content. This is a good chance to probe the water line and determine which color corresponds to which temperature.

Measuring the Microwave’s Wavelength
In this experiment, we are going to measure the wavelength of a microwave with a ruler, a piece of Styrofoam, and the Liquid Crystal Sheets. Insert two sheets on a plate into the microwave; make sure that they do not rotate by using a tube or rack to hold the plate up, then let it cook for only a few seconds. When you take the sheets out the pattern you will see patterns in the shape of the microwaves. You can measure the wavelength of the microwave by measuring the distance between these hot spots. You will probably notice two different distances between spots. Between two close spots measure only half the wavelength (antinode to antinode distance) but when spots are far apart, it is a full wavelength (think cosine). The full wavelength is about 12 cm. Using the microwave oven’s frequency (usually stamped on the back or the inside), you can calculate the speed of light. My microwave oven is 2450 MHz. Multiplying this by .12 meters, and using v = λ f gives us the speed of light: 3 x 108 m/s.

Lasers
I have found that these sheets absorb heat well from red, and sometimes blue, lasers. Though I haven’t thoroughly tested the effect of different colored lasers, my belief is that the red is absorbed more readily than the green, for example, and so it can be used to write messages. Awesome!

heat_article-7

With a red laser you can write not-so-secret messages. Light waves transfer energy.

 


  James Lincoln

Tarbut V’ Torah High School

Irvine, CA, USA

James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years.  James has consulted on TV’s “The Big Bang Theory” and WebTV’s “This vs. That”  and  the UCLA Physics Video Project.

Contact: [email protected]


 

Recommended Tools

Liquid Crystal Sheet

In Stock SKU: P1-2100
$11.95

Strip Thermometer

In Stock SKU: P1-2060-02
$1.25

Giant Drinking Bird

In Stock SKU: P3-5014
$35.00

Dye Sensitized Solar Cell Kit

In Stock SKU: P6-2110
$110.00

Ice Melting Blocks - Thermal Conductivity

In Stock SKU: P6-7060
$17.00
Read more...

Heat Transfer Radiation

Radiant heat travels in the form of waves. No physical contact between objects is needed to transfer heat by radiation. Students will fill black, white, and silver cans with water, and observe the temperature change in the water when the cans are placed under a bright lamp. The cans are being heated by radiation, and the color of the outer finish of the cans determines the efficiency with which they absorb the heat energy. The white and silver cans reflect most of the rays that strike them (visible and non-visible), so they will heat much more slowly than the black can.

Required Equipment

Radiation Cans, Student Thermometer, Stop Watch, Heat Lamp or Bulb Base 200 Watts with Bulb.

Download Teacher Notes and Student Worksheets

Radiation Cans, Set of 3

In Stock SKU: PX-2084
$18.00

Student Thermometer

In Stock SKU: 68-6202
$3.75

Bulb Base 200 Watts

In Stock SKU: PX-1208
$9.25

Digital Stopwatch Timer

In Stock SKU: 52-3200
$9.95

The “Heat Lamp” required for this lab is readily available at your local hardware store. If you instead choose to use the 200 Watt Bulb Base than it is necessary to obtain a 200 watt bulb which can be found at your local hardware or convenience store.

Read more...

Heat Transfer

heat transfer

Ice Melting Blocks! Heat can be transferred by conduction, convection, or radiation. In this experiment, students will discover the different rates at which materials can conduct heat. Aluminum is a better conductor of heat than high-density foam.

Required Equipment
Ice Melting Blocks, Ice Cubes.

Download Teacher Notes and Student Worksheets

Ice Melting Blocks - Thermal Conductivity

In Stock SKU: P6-7060
$17.00

Infrared Laser Thermometer

In Stock SKU: 68-6505
$39.00
Read more...

Lab #22.8 Heat: Chapter 22 I’m Melting! I’m Melting!

PurposeIn this activity, you will investigate curious heat transfer ability of different surfaces.

Discussion
If you walk around the house with bare feet, you probably notice that a tile floor feels much colder than a carpeted floor or rug. It’s hard to believe that they might actually have the same temperature. The tile feels colder because it is a better conductor than carpet. Heat is conducted from your warmer feet to the cooler floor faster when the floor is tile than when the floor is carpet. So your feet are cooled faster by tile than they are by carpet at the same cool temperature. In this activity, you will see which kinds of surfaces transfer heat most rapidly.

Required Equipment

2- Black Defrosting trays, 2- Black Foam Trays, 4- Ice Cubes, Paper Towel.

Download

Student Worksheet   Teacher Notes

Ice Melting Blocks - Thermal Conductivity

In Stock SKU: P6-7060
$17.00

The “ice cubes” used in this experiment are any pieces of ice that you may have available. The “paper towel” is any paper towels or other absorbent material you may have. This is used for wiping up the melting ice cubes so you may want a couple of them.

Read more...

Lab #22.5 Heat: Chapter 22 Canned Heat: Heating Up

Purpose

In this experiment, you will compare the ability of different surfaces to absorb thermal radiation.

Discussion
Does the color of a surface make a difference in how well it absorbs thermal radiation? If so, how? The answer to these questions could help you decide what to wear on a hot, sunny day and what color to paint your house if you live in a hot, sunny climate. In this experiment, we will compare the thermal absorption ability of three surfaces: silver, black, and white. We’ll do this by filling cans with these surfaces with water, then exposing the cans to heat lamps. We’ll measure the temperature of the water in each of the cans while they’re being exposed to the heat and see if there’s a difference in the rate at which the temperatures increase.

Required Equipment

Heat Lamp and Base, Radiation Cans (Silver, Black, White), Thermometer, Cold Water, Paper Towel, Graph Paper.

Download

Student Worksheet   Teacher Notes

Radiation Cans, Set of 3

In Stock SKU: PX-2084
$18.00

Student Thermometer

In Stock SKU: 68-6202
$3.75

The “cold water” used in this experiment is just “tap water”. The “paper towels” required for this lab are any paper towels that you may already have. The “graph paper” required for this lab is readily available at your office or school supply store. Each lab would require one sheet.

Read more...

Thermodynamics: The heat is on!

Thermodynamics and the thermal properties of matter are inextricably linked to daily living. Most everyone has an innate sense of hot and cold, checks the temperature outdoors before deciding what to wear, knows that spilled water will eventually evaporate and not to expect a snow storm on a hot July day. We all understand that a cold drink will become warmer and a hot drink cooler if left sitting on the counter. On the other hand, we know that both hot and cold liquids retain their respective temperatures for quite some time when placed in a thermos bottle. These everyday experiences of hot and cold, evaporation and freezing, and the transfer of heat are the province of thermal physics.

On a grander scale, the universe and everything in it – heat engines, the weather, cabbages and kings – are subject to rules known as the laws of thermodynamics. In simplest terms, the laws of thermodynamics dictate the specifics for the transfer and transformation of energy. The first law of thermodynamics is an expression of the law of conservation of energy and identifies heat transfer as a form of energy transfer. The second law limits the efficiency of engines. While mechanical energy can always be converted entirely into heat, heat cannot be converted entirely into mechanical energy. Attempts to convert heat completely into energy always produce some waste heat.

Perhaps the British scientist and author C.P. Snow stated the laws of thermodynamics most simply:

  • You cannot win (that is, you cannot get something for nothing because matter and energy are conserved).
  • You cannot break even (useful energy degenerates into non-useable, disorganized energy, because there is always an increase in disorder).

Research tells us that students learn best when they are allowed to ask questions of nature through exploration and experimentation. With that in mind, this edition of CoolStuff offers several activities that invite students to ask questions and find answers regarding the thermal properties of matter and the laws of thermodynamics.

see the largest Drinking Birds in the world. Artist Daniel Reynolds created these incredible Drinking Birds as an art exhibit in Manhattan NY. The idea came from watching Mr. Wizard…

Concept:
Heat is transferred in three common ways: convection, conduction and radiation.

Convection:
The process in fluids, i.e., gases and liquids, in which heat is transferred by the motion of the fluid itself.

Making Convection Visible
To witness convection occurring in a gas, look over the surface of a hot plate or, if possible, above the hot hood of a car. In both instances, the heated surfaces cause the air to expand and rise. The convection currents are visible because cold and hot air have different indices of refraction.

Convection in a liquid may be demonstrated by passing an electrical current through a length of pencil lead which is submerged in a clear tank of water such as an aquarium. Connect two electrical leads to the two ends of a piece of pencil lead approximately three inches in length. After placing the pencil lead into the tank, connect the ends of the other two electrical leads to a six-volt battery or other DC power supply.


Image courtesy of The Exploratorium.

(Note: An inexpensive immersion heater used for heating water for coffee or tea may be used in lieu of the pencil lead heater.) Shine light from a slide projector or flashlight through the tank and onto a white screen. As the pencil lead heats up, convection currents will appear as moving shadows on the screen (See figures above and below).

Image courtesy Montana State University
Physics Department

Conduction: The process whereby heat energy is transmitted through a material as a result of molecular collisions.

Demonstrating Conduction

Fire Proof Paper

Paul Hewitt suggests the following dramatic demonstration of conduction by solids. Tightly wrap a piece of paper around a thick cylindrical metal rod. When the paper is placed in the flame, it will not catch fire. The metal conducts heat away from the paper, preventing the paper from reaching the temperature required for ignition. (Note: Do not hold the end of the rod in your bare hand. It will rapidly become very hot.)

Fire Proof Balloon

The transfer of heat by conduction and convection may be demonstrated with two similar balloons, one filled with air, the other with water. When a match is brought close to the air-filled balloon, the balloon ruptures. However, when a match is brought within the same distance of the water-filled balloon, the balloon remains in tact. Conduction and convection carry the heat away from the balloon before the temperature required for melting occurs. A second reason the balloon doesn’t burst is related to the heat capacity of water. Water is able to absorb a great deal of heat with little change in temperature.

Ice Melting Blocks

Touch these two black blocks, and one feels cooler. Place an ice cube on each block. One cube completely melts before your eyes, while the other stays frozen! Surprisingly, the “cooler” block melts the ice faster! This discrepant event introduces many concepts, including heat transfer, change of state, and thermal conductivity.

Ice Melting Blocks - Thermal Conductivity

In Stock SKU: P6-7060
$17.00

Boiling Water in a Paper Cup

A striking demonstration illustrating thermal conductivity, heat capacity of water and the cooling effect of boiling can be performed with nothing more than a paper cup filled with water and a Bunsen or alcohol burner. When exposed to an open flame, the water in the cup will come to a boil. The cup will not burn until the water has completely evaporated.

Radiation:

The transfer of energy by electromagnetic waves.

We experience the sensation of heat via radiant energy from the sun, a light bulb, or a fireplace.

Absorbers & Reflectors

Good absorbers of radiant energy are also good emitters. To demonstrate how surface color affects the emission and absorption of radiant energy, you will need two or three containers of the same size and shape. One container should be white or shiny; the other container should be black.

Radiation cans, such as those shown here, work well. Soup cans with labels removed may be also be used. After painting one can black, fill both cans with warm water. Thermometers placed in the cans will reveal that the water in the black can cools faster. If ice water is used instead and the cans are exposed to direct sunlight or light from a heat lamp, the dark can will warm faster.

Radiation Cans, Set of 3

In Stock SKU: PX-2084
$18.00

Radiometer

The radiometer, invented in 1873 by the chemist Sir William Crookes, consists of a rotating shaft with four vanes. The shaft and vanes are sealed in a glass container which has over 99% of the air removed. The vanes are painted black on one side, silver on the other. As we have seen, when exposed to radiant energy, the black surfaces will become warmer than the silver surfaces.

In the presence of light, the air molecules that remain inside the bulb begin to move faster as they absorb energy from the light. At the same time, the light warms the vanes, the black surfaces more so than the silver. As the randomly moving molecules strike the vanes from all sides, the ones striking the cooler, reflective silver vanes take on very little additional energy. However, the molecules coming in contact with the warmer, dark vanes do gain energy and leave with considerably higher speeds. As a consequence, the molecules push on the dark vanes harder than they do on the silver vanes, producing rotation.

You may wish to try these demos!

  • In the absence of bright light, the radiometer can be made to turn by directing a hair dryer at the vanes. The infrared radiation from the heating element will produce results similar to those obtained with visible light. This demonstration never ceases to amaze those that witness it.
  • If you cool the glass quickly in the absence of a bright light source by placing ice on the glass, the vanes will turn backwards (i.e. the silver sides are trailing).

Radiometer

In Stock SKU: P3-8105
$10.95

Concept:
Work may be completely converted into heat, as briskly rubbing your hands together demonstrates, but it is not possible to completely convert heat into useful work.
Metal Balls
Large ball bearings may be used to demonstrate the eventual fate of most forms of energy. In this case, mechanical energy is converted into internal energy.

  • Place a piece of paper between the two large spheres. Now smash them together. After the spheres collide, smell the area on the paper where they came into contact. What do you detect? What do you suppose caused this odor?
  • Did the paper get hot? That is, where did the energy that was responsible for the heating come from? As best you can, describe the energy transformation that took place as a result of the collision.
  • Examine the point of contact. Do you observe any signs of scorching on the paper? What can you conclude about the increase in the paper’s temperature? In theory, what could you do to cause combustion?

Smelling a piece of paper placed between colliding spheres reveals that the kinetic energy possessed by the moving spheres has been converted into heat.

Colliding Steel Spheres

In Stock SKU: P6-6070
$28.00

Heating up a Hanger

The conversion of mechanical energy into heat may be dramatically demonstrated by simply bending a coat hanger. First cut a 30-cm length of coat hanger with wire cutters. Grab the ends of the wire in each hand and rapidly bend it back and forth several times. Now touch the point on the wire where the bending occurred. (Caution! the coat hanger can sometimes get surprisingly hot, so only touch the hot spot briefly.)
Concept:
An adiabatic process is one in which no heat is added to or removed from a system. This does not mean that the temperature of the system necessarily remains constant, for even in the absence of external interactions with the surroundings, the system is free to exchange energy between thermal (internal energy) and mechanical forms. Adiabatic conditions are closely approximated when the process happens so quickly that there is no time to transfer heat, or if the system is very well insulated from its surroundings.

Stretching Exercise

Place a rubber band loosely looped over the index fingers in contact with skin just above your upper lip. Now quickly stretch the rubber band. What do you experience? Now let the rubber band relax quickly. What do you feel now?

When the rubber band is stretched quickly, work is done on it, causing its internal energy to rise. This rise reveals itself as a small increase in temperature. When the rubber band is allowed to quickly contract, it performs work and suffers a reduction in internal energy which produces a cooling sensation.

Fire in the Hole!
As everyone who has pumped up a bicycle tire knows, if you rapidly compress a gas, it gets hot. The fire syringe is a device that dramatically demonstrates just how much heat may be produced when a volume of air is rapidly compressed. Temperatures of over 260 degrees C (500 degrees F) may be produced with this simple device. Since paper burns at 235 degrees C (454 degrees F), a small piece of tissue paper is easily ignited when the plunger is rapidly inserted into a glass cylinder.

It is believed that Rudolf Diesel knew of this demonstration when he began work on his compression engine.

Fire Syringe

In Stock SKU: P1-2020
$19.00

Chill Out!

Adiabatic cooling may be easily demonstrated with an inflated bicycle tire. When the air is released from the tire, the escaping air will be noticeably cooler than the tire. After all the air has escaped, the valve stem will be cold to the touch. The effect can also be demonstrated with using any aerosol spray can or CO2 fire extinguisher. When gas escapes from either container, it cools dramatically. In the case of the aerosol spray can, the dispensed gas is cold. The escaping CO2 from a fire extinguisher is so cold that it solidifies and forms dry ice. Students love to see this solid form of CO2 falling like snow.

Cloud in a Bottle

Adiabatic processes are very important in the atmosphere. In fact, adiabatic cooling of rising air is the dominant cause of cloud formation. Using a gallon jar, a rubber glove, some water and a match, you can demonstrate adiabatic expansion and in the process, produce your own cloud.First cover the bottom of the jar with a thin layer of water. Drop a lit match into the jar. Quickly place the fingers of the glove inside the jar and stretch the open end of the glove over the mouth of the jar. Put your fingers in the glove and pull the glove outside the jar.

This rapid expansion of the gas inside the bottle results in adiabatic cooling. Presto! You should see a wispy cloud inside the jar.

Little Heat Engines That Can

Concept:
A heat engine is any device that converts internal energy into mechanical work. All heat engines – steam engines, jet engines, and internal combustion engines – extract useful energy as heat flows from a higher temperature to a lower temperature. As the second law of thermodynamics states, while it is possible to convert work completely into heat, it is not possible to convert heat completely into useful work.

You may demonstrate heat engine essentials with several simple devices. Some of these devices may be easily constructed; others may be purchased at modest cost. The devices described here may not seem like heat engines since they do not resemble the engines that power our cars, lawnmowers, or airplanes. However, analysis will show that all take in heat, convert some of this energy into mechanical work, and expel the rest.

Hero’s Engine
In about 100 BC, Hero of Alexandria invented a heat engine that used steam for propulsion (see figure right). Water in the lower vessel is converting into steam. After passing through two supporting pipes, the steam is expelled through ports on opposite sides of the sphere. Unbeknownst to Hero, he had produced the first steam turbine, versions of which are still used to this day.

Make Your Own…

A very simple version of Hero’s engine may be constructed with an empty soda can and a heat source such as a Bunsen burner or a can of Sterno.

  • Place an empty soda can, with its opener lever still intact, on its side. Using a nail or pin, make two holes opposite each other in the side of the can, about one half inch above the bottom. Before removing the nail or pin from each hole, bend it to the right and parallel to the rim. The slanted holes will insure that the escaping steam will leave tangentially with respect to the sides of the can.
  • Bend the can’s opener lever so that it is perpendicular to the top of the can and tie a short length of fish line through the hole in the lever. After adding water so that it just covers the bottom of the can, tie the other end of the fish line to a support such as a ring stand.
  • Place the bottom of the can over the heat source. When the water starts boiling, the can will begin to spin. If the fish line offers too much resistance and impedes the motion of the can, place a snap swivel, used for fishing and available in any sporting goods store, between the fish line and the opener lever.

Watch a demonstration of Hero’s engine

The Drinking Bird

The classic toy the drinking bird exemplifies a thermal engine described by the Rankine cycle. (Rankine cycles describe the operation of steam heat engines commonly found in power generation plants.) The toy consists of two hollow glass spheres connected by a hollow tube. The upper sphere, the head of the bird, is coated with felt. The bottom sphere is partially filled with a liquid, methylene chloride, which evaporates very rapidly. The tube extends into the liquid and provides a passageway between the two spheres.


The engine is set into operation by moistening the head with water. As the water evaporates, the head is cooled. The vapor pressure of the methylene chloride within the head decreases as it is cooled. The greater vapor pressure in the lower sphere pushes the liquid up the tube, thus shifting the bird’s center of gravity. Tipping downward, the bird’s felt-covered beak enters water. As the bird dunks, the head is re-moistened, the liquid returns to its tail, and the process begins again.

Drinking Bird

In Stock SKU: P3-5001
$5.95

The Mysterious Rubber Band Heat Engine

Most materials expand when heated. Rubber, being a rather peculiar substance, contracts when heat is applied. As the figure below illustrates, this may be demonstrated by heating a rubber band that supports a mass.

Interesting Links:

Exploratorium Science Snacks http://www.exploratorium.edu/snacks/

Let’s Build a Can Stirling Engine http://www.bobblick.com/techref/projects/stirling/can/can.html

Thermodynamics: Building Simple Heat Engines http://sci-toys.com/scitoys/scitoys/thermo/thermo.html#hero

Thermodynamics Demonstrations http://buphy.bu.edu/~duffy/thermodynamics.html

CoolStuff Archives A complete list of all the CoolStuff Newsletters. Find student activities for specific topics.

Read more...