Convection Current - Make your own heat waves in an aquarium

   

Convection Current

 

Make your own heat waves in an aquarium.

This demonstration gives you a simple and visually appealing way to show convection currents in water. Warmer water rising through cooler water creates turbulence effects that bend light, allowing you to project swirling shadows onto a screen. 

 

• One 6- or 12-volt lantern battery or a suitable low-voltage battery eliminator or power supply.
• A pencil lead.
• A clear plastic or glass container with rectangular flat sides. (A small aquarium works fine.)
• A light source. (Slide projectors, filmstrip projectors, or flashlights work well. A point-source flashlight, such as a MiniMaglite™ flashlight with the reflector removed, will produce sharp images.)
• Food coloring (in a small dropper bottle or an eyedropper).
• A projection screen or white posterboard.
• 2 electrical lead wires with alligator clips at both ends (available at Radio Shack).
• Tap water.
• Optional: Switch or dimmer switch (both available at hardware stores) or any sort of rheostat or variable resistor.
• Adult help.

(15 minutes or less)

Use one clip lead to attach the positive terminal of the battery to one end of the pencil lead, and the second clip lead to attach the negative terminal to the other end of the pencil lead. If you like, you may connect a simple switch, or a dimmer switch, in series. The switch makes using the device more convenient; the dimmer switch lets you vary the amount of current going through the carbon rod.

Fill the container with water and place the wires and pencil lead in it so that the pencil lead is positioned horizontally. Connect the two wires to the terminals of the battery, and allow the heating to start. Shine the projector through the liquid, projecting the light onto the screen or white posterboard.
 

(15 minutes or more)

Observe the convection currents. If you have a dimmer switch, vary the current and observe the effects of the various settings. If you are using a rheostat or variable resistor, you may have to try several settings to find which one works best. You can also vary the orientation of the pencil lead to see if this has any significant effect on the convection pattern. Add a few drops of food coloring and observe the effects.

Like air, water expands as it gets warmer and so becomes less dense. Since the water warmed by the current flowing through the carbon rod is less dense than the surrounding colder water, the warm water rises through the colder water to the surface, causing the food coloring to move along with it.

Since the cold and warm water have different densities, they have different indices of refraction. Light bends (refracts) as it passes from warmer to colder or colder to warmer. When light is bent onto an area of the screen, that area becomes brighter. When light is bent away from an area of the screen, that area becomes darker. The positions of warm and cold water are constantly changing, so the images projected on the screen shimmer and flow like heat waves in air.

A simpler method of doing this Snack, which allows people to perform it themselves, is simply to place a candle on a table and project this image onto a screen with a flashlight. The point source of a MiniMaglite™ projects clear images of convection when used on a small-scale desktop experiment. Changing the distance from a point light source to the candle will change the magnification of the image of the convection currents projected on the wall.

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Condiment Diver - The world's simplest Cartesian diver

 

Condiment Diver

 

The world's simplest Cartesian diver

Squeezing a plastic bottle filled with water and a condiment packet makes the packet sink. Letting go of the bottle makes the packet rise. 

 

• Squeeze condiment packet (soy sauce, ketchup, etc.)
• Clear plastic bottle with tight-fitting lid
• A glass or cup of water

First, you have to figure out if your condiment packet is a good Cartesian diver candidate. Fill a glass with water and drop in your packet. The best packets are ones that just barely float. 

After you have found the proper packet, fill an empty, clear plastic bottle to the top with water. Shove your unopened condiment packet into the bottle. Replace the cap... and you're done! Squeeze the bottle to make the diver sink, and release to make it rise. Amazing!

Many sauces are denser than water, but it is the air bubble at the top of the sauce that determines whether the packet will sink or swim. Squeezing the bottle causes the bubble to shrink. This smaller bubble is less buoyant and the packet sinks.

By Eric Muller
Orginally published in The Physics Teacher, May 1996

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Colored Shadows - Not all shadows are black.

Colored Shadows

 

Not all shadows are black.

When two different-colored lights shine on the same spot on a white screen, the light reflecting from that spot to your eyes is called an additive mixture because it contains the colors from both lights. We can learn about human color perception by using colored lights to make additive color mixtures. 

 

• White surface. (A white wall, white posterboard, or white paper taped to stiff cardboard works well. Do not use a beaded or metal slide projection screen.)
• Red, green, and blue lightbulbs or floodlamps, one of each color. Sylvania #11 colored lightbulbs or General Electric Dichrocolor Dichroic Floodlamps (150 watt) work well. We have even obtained excellent results with clear-colored Christmas tree lights. Smaller or dimmer bulbs are fine for tabletop use by a few students, but larger, brighter bulbs allow a larger-scale demonstration.
• 3 light sockets of any type or arrangement that will get the light from the three bulbs simultaneously directed onto the same area of a white surface.
• Any solid object such as a pencil, ruler, correction fluid bottle, finger, etc.
• Adult help.

 

(15 minutes or less)

Set up the bulbs and screen in such a way that the light from all three bulbs falls on the same area of the screen and all bulbs are approximately the same distance from the screen. For best results, put the green bulb in between the red and the blue bulbs. 
 

Turn on the lights, and adjust the positions of the bulbs until you obtain the "whitest" light on the area of the screen where the three lights mix. For best results, make the room as dark as possible.

Place a narrow opaque object, like a pencil, fairly close to the screen. Adjust the distance from the screen until you see three distinct colored shadows.

Remove the object, turn off one of the colored lights, and notice how the color on the screen changes. Then replace the object in front of the screen and notice the color of the shadows. Move the object close to the screen until the shadows overlap. Notice the color of these combined shadows.

Repeat the previous step with a different light turned off while the other two remain on, and then a third time so you have tried all combinations. Repeat again with only one color at a time on, and then with all three on. Vary the size of the object and the distance from the screen. Try using your hand as an object. 
 

The retina of the human eye has three receptors for colored light: One type of receptor is most sensitive to red light, one to green light, and one to blue light. With these three color receptors we are able to perceive more than a million different shades of color.

When a red light, a blue light, and a green light are all shining on the screen, the screen looks white because these three colored lights stimulate all three color receptors on your retinas approximately equally, giving us the sensation of white. Red, green, and blue are therefore called additive primaries of light.

With these three lights you can make shadows of seven different colors: blue, red, green, black, cyan (blue-green), magenta (a mixture of blue and red), and yellow (a mixture of red and green). If you block two of the three lights, you get a shadow of the third color: Block the red and green lights, for example, and you get a blue shadow. If you block all three lights, you get a black shadow. And if you block one of the three lights, you get a shadow whose color is a mixture of the two other colors. If the blue and green mix, they make cyan; red and blue make magenta; red and green make yellow.

If you turn off the red light, leaving only the blue and green lights on, the lights mix and the screen appears to be cyan, a bluegreen color. When you hold the object in front of this cyan screen, you will see two shadows: one blue and one green. In one place the object blocks the light coming from the green bulb and therefore leaves a blue shadow; in another place it blocks the light from the blue bulb to make a green shadow. When you move the object close to the screen you will get a very dark (black) shadow, where the object blocks both lights.

When you turn off the green light, leaving the red and blue lights on, the screen will appear to be magenta, a mixture of red and blue. The shadows will be red and blue.

When you turn off the blue light, leaving the red and green lights on, the screen will appear to be yellow. The shadows will be red and green.

It may seem strange that a red light and a green light mix to make yellow light on a white screen. A mixture of red and green light stimulates the red and green receptors on the retina of your eye. Those same receptors are also stimulated by yellow light --- that is, by light from the yellow portion of the rainbow. When the red and green receptors in your eye are stimulated, whether by a mixture of red and green light, or by yellow light alone, you will see the color yellow.

Find out what happens when you use different colored paper for the screen. Try yellow, green, blue, red, purple, and so on.

If you let light from the three bulbs shine through a hole in a card that is held an appropriate distance from the screen, you will see three separate patches of colored light on the screen, one from each lamp. (Make the hole large enough to get a patch of color you can really see.) If you move the card closer to the screen, the patches of light will eventually overlap and you will see the mixtures of each pair of colors.

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சென்சார் டெஸ்ட் மூலம் சர்க்கரை நோய் மற்றும் ஹார்ட் பிரச்னைகளுக்கு சிகிச்சை!

இப்பொதெல்லாம்  தினம்தோறும் உருவாகும் புது வியாதிகள் மட்டுமின்றி பழைய வியாதிகளைக் குணப் படுத்த அல்ல்து கட்டுப் படுத்த புதிய வகை கருவிகளையும் கண்டுபிடித்துக் கொண்டேதான் இருக்கிறார்கள். அந்த வகையில் இரத்த அழுத்தத்தை அளவிடும் முறையில் கருவி ஒன்றை ஆராய்ச்சியாளர்கள் கண்டுபிடித்துள்ளனர்.


தற்போது உபயோகத்தில் இருக்கும் கருவிக்கு மாற்று கருவியாக இதை கண்டறிந்துள்ளனர். கடிகாரம் போன்ற இக்கருவியில் உள்ள சென்சார் இரத்த குழாயின் நாடித்துடிப்பு அலையை அளவிட்டு கணிக்கும் விபரங்களை பழைய தோள்பட்டை கருவி மூலம் அனுப்புகிறது.


இதன் மூலம் இதயத்துக்கு அருகிலுள்ள அழுத்தத்தை ஏயார்டா மூலம் அறியலாம். ஏயார்டா என்பது இதயத்திலிருந்து மில்லிமீட்டர் அளவு அருகில் இருப்பது. தோள் பட்டையை விட இவ்விடத்தில் அழுத்தம் அதிகம்.ஏயார்ட்டாவின் அழுத்தத்தை அளவிட்டால் மாத்திரமே சிகிச்சை பூரணமாக இருக்கும் என்றும் மேலும் மூளை மற்றும் இதயத்திற்கு அருகில் உள்ள இரத்த அழுத்த அளவை அறிவது இதய நோய்கள் மற்றும் மாரடைப்பு சிகிச்சைகளில் அவசியம் என்றும் விஞ்ஞானிகள் சொல்லி வந்தனர்..


இப்படியாக இன்றைய நிலையில் மருத்துவத் துறைகளிலும் எத்தனையோ வியத்தகு கண்டுபிடிப்புகள் வந்துவிட்டன. அவற்றுடன் புதிய வரவாக இந்த சென்சார் மருத்துவ முறையும் பெரிய அளவில் இடம்பெறக் கூடும். வருடக்கணக்காக ராணுவத்தினர் தண்ணீருக்கடியில் உபயோகப்படுத்தும் சோனார் கருவியின் செயல்பாடுகளை ஒத்த அல்ட்ரா சவுண்டு தொழில் நுட்பத்தை மருத்துவ சிகிச்சைமுறையில் கொண்டுவர, அமெரிக்காவின் பஃபலோ பல்கலைக்கழகத்தின் ஆராய்ச்சியாளர்கள் தற்போது ஈடுபட்டுள்ளனர். சோனாரின் சிறியதாக்கப்பட்ட வடிவைமைப்புடன் உள்ள கருவியை மனித உடலினுள் பொருத்துவதன் மூலம் சர்க்கரை நோய், இதய நோய் போன்றவற்றிற்கு சிகிச்சை அளிக்க முடியும் என்று இவர்கள் கூறுகின்றனர்.


உயிரியல் மருத்துவத்தின் முன்னேற்றமான இந்தக் கண்டுபிடிப்பு, நோயாளிகளை கவனித்துக் கொள்ளும் விதத்தில் புரட்சிகரமான மாறுதல்களை ஏற்படுத்தக் கூடும் என்று டாம்மாசோ மெலோடியா என்ற மின்பொறியியல் துணைப் பேராசிரியர் கருத்து தெரிவித்துள்ளார்.


இத்தகைய தொழில்நுட்பம் பத்து வருடங்களுக்கு முன்னரே வளர்ச்சி அடையத் துவங்கியது. ஆனால், ஆப்போது, ரேடியோ மின் காந்த அதிர்வெண் அலைகள் மூலம் சென்சார்கள் பரிசோதனைக்கு உட்படுத்தப்பட்டன. இவற்றின் மூலம் அதிக வெப்பம் வெளிப்பட்டது. மேலும், இத்தகைய மின்காந்த அலைகள், மனித தோல், தசை, திசுக்கள் போன்றவற்றில் ஊடுருவிச் செல்ல அதிக சக்தி தேவைப்பட்டது.


அத்துடன் மனிதனின் உடலில் 65 சதவிகிதம் நீர்ச்சத்து உள்ளதால், அல்ட்ரா சவுண்ட் கதிர்களின் பயன்பாடு எளிதாக இருந்தது. அதனால், இந்தத் தொழில்நுட்பத்தின் மூலம் மனித உடலினுள் பொருத்தப்படும் பேஸ்மேக்கர், ரத்தத்தில் ஆக்சிஜன் அளவைக் கணிக்கப் பயன்படும் கருவி போன்றவற்றை இயக்குவது எளிதாக இருக்கும் என்று டாம்மாசோ தெரிவித்தார்.


இதனை ஒத்த முறையில், ரத்தத்தில் குளுகோஸ் அளவை அறிவிக்கும் சென்சார்களை பயன்படுத்துவதன் மூலம், இன்சுலின் அளவை கண்காணித்து அத்துடன் இணைக்கப்பட்டுள்ள சிறிய இன்சுலின் கருவியிலிருந்து தேவைப்பட்ட இன்சுலினை ரத்தத்தில் கலக்கச் செய்தல் சாத்தியமாகக் கூடும். இந்தத் தொழில்நுட்பத்தில் சாத்தியமாக்கக்கூடிய பயன்பாடுகள் அதிகம் உள்ளன. நாம் இவற்றிலிருந்து என்ன பெறமுடியும் என்று ஆராய்ந்து கொண்டிருக்கின்றோம் என்றும் டாம்மாசோ கூறினார்.


Soon, wireless body sensors to treat diabetes, heart failure!


 
 For decades, the military has used sonar for underwater communication.Now, researchers at the University at Buffalo are developing a miniaturized version of the same technology to be applied inside the human body to treat diseases such as diabetes and heart failure in real time.The advancement relies on sensors that use ultrasounds – the same inaudible sound waves used by the navy for sonar and doctors for sonograms – to wirelessly share information between medical devices implanted in or worn by people.

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Cold Metal - "Cold" metal and "warm" wood may be the same temperature.

Cold Metal

 

"Cold" metal and "warm" wood may be the same temperature.

Your hand is not always a good thermometer. When you touch a variety of materials, some will seem warmer or colder than others, even when they are at the same temperature. 

 

• Various materials (metal, wood, Styrofoam, glass, plastic, cardboard, etc.) with one flat surface larger than the size of your hand.
• A thermometer (liquid crystal thermometer cards work well).

 

(15 minutes or less)

There is no actual assembly. Be sure that there are many different surfaces and that they are large enough for you to touch easily. Allow the materials to come to room temperature before you begin. 
 

(15 minutes or more)

Place your palms flat on the various surfaces and compare how cold they feel. Arrange the materials in order from cold to warm. Then place the liquid crystal thermometer, or regular thermometer, on each surface. Notice that all the materials are at the same temperature.

The temperature-sensitive nerve endings in your skin detect the difference between your inside body temperature and your outside skin temperature. When your skin cools down, your temperaturesensitive nerves tell you that the object you are touching is cold. An object that feels cold must be colder than your hand, and it must carry your body heat away so that your skin cools down.

Styrofoam and metal are two materials that work well for this Snack. They both start at room temperature and are both colder than your hand. They do not feel equally cold because they carry heat away from your hand at different rates.

Styrofoam is an insulator, a very poor conductor of heat. When your hand touches the Styrofoam, heat flows from your hand to the Styrofoam and warms the Styrofoam surface. Because this heat is not conducted away quickly, the surface of the Styrofoam soon becomes as warm as your hand, so little or no additional heat leaves your hand. There is no difference in temperature between the inside of your body and the outside of your skin, so the temperature-sensitive nerves detect no difference in temperature. The Styrofoam feels warm.

The metal, in contrast, carries heat away quickly. Metal is a good conductor of heat. Heat flows from your hand into the metal and then is conducted rapidly away into the bulk of the metal, leaving the metal surface and your skin surface relatively cool. That's why metal feels cool.

Metals will warm to above room temperature after just a few rounds of being touched. The surfaces should be allowed to cool for a few moments between each person's turn. It might be useful to have multiple metal samples. While you are using one sample, the extras have time to cool back to room temperature.

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Circles of Magnetism IV - Two parallel, current-carrying wires exert forces on each other.

 

Circles of Magnetism IV

 

Two parallel, current-carrying wires exert forces on each other.

When an electric current flows through a wire, a magnetic field is created around the wire. If you place two current-carrying wires near each other, the magnetic field around each wire exerts a force on the current flowing in the other wire. These forces can push two current-carrying wires apart, or pull them together. 

 

• One 6-volt lantern battery or an equivalent current supply.
• 2 electrical lead wires with alligator clips at both ends (available at Radio Shack).
• Tinkertoys™ or wood for a stand.
• Masking tape or transparent tape.
• Light aluminum foil.
• Adult help.

 

(15 minutes or less)

Make a stand from wood or Tinkertoys™ (see the photo on page 14 and the diagrams below), or build a stand of your own design from available materials.

Cut a strip of aluminum foil measuring about 2 feet (60 cm) long and 1/2 inch (1.3 cm) wide. Tape one end of the foil strip to your support. Run the strip down and back up to the support, making a loop, then tape the other end in place. Be sure the ends of the strip do not touch.

Attach one clip lead to each battery terminal, but do not attach the other ends of the lead wires to the strip yet. 
 

(15 minutes or more)

Touch the two clip leads to the ends of the foil strip. The descending and ascending portions of the loop will repel each other. The closer you can hang the descending and ascending portions of the loop to each other - without allowing them to touch - the larger the repulsion.

Now hang the foil strip from the support with the two ends overlapping, so they make a good electrical contact. Connect one of the clip leads to these overlapping ends. Separate the two sides of the loop and briefly touch the other clip to the bottom of the loop. Notice that the sides of the loop are attracted to each other when the current flows. (This step requires a little coordination and a delicate touch to clearly demonstrate that it is the current flow in the strips that makes them move together and not forces that you create when you touch the clip to the bottom of the loop.) 
 

A current-carrying wire generates a magnetic field that circles the wire (See the "Circles of Magnetism I" Snack.)

When a current flows in a magnetic field, the field exerts forces on that current. (See the "Motor Effect" Snack.) So each current-carrying wire in this Snack generates a magnetic field at the position of the other wire and thus exerts a force on the current in the other wire. Two parallel wires will either attract or repel each other, depending on the direction of current flow in each wire. If both currents flow in the same direction, the wires will attract; if they flow in opposite directions, they will repel.

The forces produced on the aluminum foil are small. This is because the electrical current flowing through the foil is small, only a couple of amperes. Larger currents produce larger forces. The Exploratorium exhibit, for example, uses wires carrying 400 amperes, which produces forces that are more than 10,000 times stronger than the forces you produce with this Snack.

The ampere, the fundamental unit of electrical current, is defined by the force exerted by one wire on another. The definition of the ampere is as follows: A current of 1 ampere flowing in each of two infinitely long parallel wires separated by 1 meter will produce an attractive force of 2 x 10^-7 newton on each 1-meter length of wire. For comparison, a force of 1 newton is approximately the weight of a quarterpound of hamburger.

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