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radioactivity in my heart

I took radioactive technetium willingly? Yes, it’s a good diagnostic for blood flow around the heart, and not as risky as you might expect. It also makes for a dramatic display near a Geiger counter! Here’s the link for the story.

Radiation straight from the heart.  In 2017 I had a diagnostic procedure done, in which I exercised and then got injected with the radioactive element (radionuclide) technetium-99 metastable. The gamma rays came flying out of my body to create an image on a screen.  I was still radioactively “hot” hours later, as a Geiger counter showed.

VIDEO TO COME SOON

The Tc-99 was chemically bound to a ligand that migrates preferentially to the heart.

Science Direct

The idea was to map the blood vessels around my heart – all normal, as it turned out. I was exposed to many gamma rays but for a fairly short time; the Tc-99 has a half-life of 6 hours.  My total exposure was [let me find my notes again], a fraction of my normal dose from cosmic rays, internal potassium load, and other sources.  It was enough to increase my lifetime chance of cancer by 1%. Anyway, about 6 hours after the test I wanted to show how radioactive I was.  I put a Geiger counter I had built from a kit on a table and walked up to it, while taking a video.  I pegged the Geiger counter output when my chest reached it. Fortunately, not only does Tc-99 decay fast, but the gamma rays don’t deposit much radiation in my body on the way out; if they did do so, they’d make a too-fuzzy image.  Note that Tc-99 is made “on-site” in the doctor’s office.  He or she gets a device loaded with molybdenum-99 that’s decaying to the metastable state of Tc-99.  When a diagnostic dose is needed the Tc-99 is quickly separated out chemically and bound to a special chemical, the ligand.

Diagram of gammas escaping to be imaged

 

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Seeing in the thermal infrared

Let’s “see” thermal radiation, and see who’s (what’s) how or cold, and why. You’ll need a thermal imager that you can attach to your smartphone. It’s a bit expensive but it offers a lot of learning and even utility around the house to find heat or water “leaks.” Shiny metals and the clear sky both offer some real surprises.

The clear sky is so cold! Read temperatures at a distance, and with surprises.

Equipment: simple, except for one expensive infrared imager (ca. $400)

You can look at the temperature of things – your arm, a chair, a puddle of water, a piece of hot sunlit metal, even the sky. Here are two images we’ll look over in a bit:

Grab from science 2021-22

You can look into reasons why different objects attain different temperatures, and even why some objects seem not to register their “correct” temperature. We can take a picture of an object or even a whole scene and see the different temperatures of all the parts.

This is a demo that’s best done with a thermal imaging camera, not a common device, but you may find a scientist willing to do the demo. I have a FLIR ONE PRO, which is a small device that attaches to a smartphone, in this case, my Android phone. The FLIR has two imagers, one a standard light-sensitive camera and the other an array of 19,200 tiny bolometers that are sensitive to thermal radiation focused onto the array through a separate lens made of light-opaque germanium. The two imagers look at the same scene and are close enough to see overlapping images, one in the visible spectrum and one in the thermal infrared. The thermal infrared or TIR is a range of electromagnetic waves that are less energetic and of longer wavelength than ordinary light. Ordinary visible light is composed of waves with wavelengths of between 400 and 700 nm (nanometers, billionths of a meter), or about 1/100 the diameter of human hairs. Thermal infrared has much longer wavelengths; the FLIR captures waves between 8 and 15 μm (micrometers, millionths of a meter). That’s about 20 times longer than visible light, and carrying about 1/20th as much energy as sunlight’s particles of light or photons. That’s appropriate, since the energy of thermal motion at Earth’s conditions (a temperature of about 20°C or 68°F) is about 1/20th of that on the Sun, which is about 20 times hotter in absolute temperature (see below).

The core idea is that every object, even gases in the air, give off TIR at a rate that is proportional to the fourth power of their temperature on the absolute scale. The absolute scale is measured from absolute zero, where thermal motion stops (and only quantum mechanical zero-point motion remains – and interesting topic). Scientists use the Kelvin scale, which starts at -273.15 degrees Celsius; the freezing point of water on that scale is then +273.15 K (Kelvin, not degrees Kelvin). An increment of 1 K is the same as 1 degree Celsius (1°C). In the US (almost only the US!) people tend to use the ancient Fahrenheit scale, which we’ll peek at now and then. OK, let’s compare two bodies on Earth: a tree near you at 20°C or 68°F, and your face at a comfy 33°C (91.4°F). The absolute temperatures are 293.15 K and 306.15 K. The ratio of their fourth powers is (306.15/293.15) to the fourth power, which is 1.19 – that is, an area of your face gives off 19% more TIR than the same area of leaves. The FLIR detects these differences as signals that get converted to digital signals; the FLIR software processes these signals to temperatures that get presented as different colors. You can choose the color scale, with a common one being red for hot, through orange, yellow, green, to blue (as continuous hues or colors). That’s a nice picture each time. You can set the FLIR to present numerical values of the temperature at selected sites in the image.

There are some very amusing scenes to image, such as a person with hot, dark clothing and maybe a cool forehead just wiped with a wet towel and eyeglasses that stand out frames vs . lenses. I presented two scenes earlier that are also very informative. One is an image the includes a person, some shrubs, some soil, and a bit of sky. The person is warm, the sunlit soil is hot – it absorbs sunlight well and can’t shed heat very well to the air. The shrub stays cool as air flows well over its small leaves, taking away much heat deposited by sunlight. The sky looks cold, and it is. Let me explain. The air is at a mild temperature, about that of the shrubs, but air can’t radiate TIR well. The major gases, nitrogen and oxygen, are extremely poor radiators, for reasons that lie in the wonders of quantum mechanics. The only good radiator in the air is water vapor, and there isn’t much of it in the dry time that this picture was taken in Las Cruces, New Mexico (we are in the USA, though many people don’t know it). So, little TIR energy arrives at the FLIR thermal imager and it registers a low temperature. We often see sky “radiative temperatures” that are 40°C (72°F) below air temperature! There are interesting consequences. On a clear night we cool to the sky but get very little TIR back from the sky. Our bare heads feel colder than they “should.” This imbalance in radiation affects plants, too. On a night with air temperature just above freezing the plant’s leaves cool by radiation enough to get below freezing. That’s called a radiation frost. Farmers are well aware of it.

The second scene shows interesting properties of shiny metals such as the aluminum here. Those metals don’t absorb too much sunlight because they are so reflective in visible light. However, they do get very hot because they can’t unload that little heat energy very well; they have a low “thermal emissivity” or ability to emit TIR. Anyone who left a shiny tool in the sun knows the result! Shiny metas are about the only common objects that have this low emissivity. We can “trick” them into emitting TIR well by giving them a thin coat of something that emits TIR well. I coated one strip of aluminum with nail polish, and it stayed cool. Maybe backyard mechanics should buy some nail polish for their tools!

So, this is a long demo. You can make it into a series of real experimental tests if you think about it. Good luck.

 

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pencil lead conducts electricity

PICTURES COMING SOON

Pencil “lead” does conduct electricity. You can show this with a simple LED, some wire, and a 9-volt battery. You can make it more quantitative with different “drawings” and even more so with an inexpensive electronic multimeter.

Pencil “lead” does conduct electricity

Equipment: an ordinary no. 2 pencil (other hardnesses work, too); a sheet of paper; one LED (light-emitting diode; almost any color or size; pennies); a 9-volt battery (even a run-down one may work; free, or maybe $3); wire leads(“leeds”) for the 9V battery (there are snap-on connectors, as shown ($0.50 but hard to purchase singly); you can also twist ordinary wires around the battery terminals and hold them on, say with tape.

Why pencil lead conducts electricity: Pencil lead is, of course, not metallic lead as was sometimes used to make marks centuries ago. It is graphite mixed with clay and fired to make it hard. Graphite is a form of carbon, as is charcoal (rather impure) and diamond. It conducts electricity, with some amazing special properties having been found recently, but those are long stories. Graphite is in the form of single sheets, one atom thick! Bulk graphite is made of perhaps thousands of sheets stacked together. The sheets easily slide apart, which makes graphite easy to write with as the sheets smear across the paper. Fortunately for this demo, the sheets overlap enough, even with that clay spreading around, so that they maintain connections of some of the sheets as far as you write. So, a nice, thick pencil mark acts as an electrical conductor, if a weak one… but good enough to carry a current to light an LED!

Setting up the demo: Wrap the lead from the positive terminal of the 9V battery to the longer lead of the LED. This is the lead that’s intended to carry a positive current into the LED to light it up. See the note at the end about positive and negative. Be sure you have the correct lead on the LED; applying 9V the wrong way might burn out the LED. Make the wrapping good and tight; expose more bar wire on the battery lead if you need to. You can strip off the insulation with a sharp knife held almost parallel to the wire – CAREFULLY. Strip the insulation off on two or three sides and then pull off the remainder. You’re now ready.

PICTUREs, with annotation

Lighting the LED: Place the LED down on the sheet of paper. Press down both the negative lead of the ED (the shorter lead, not wired) and the negative wire from the 9V battery. Put them any handy distance apart. DO NOT let the LED lead and the battery wire touch each other or you will instantly flow out your LED; it is not meant to carry huge currents that would result. Oops, did it? Just get another LED and start over. Now scribble with the pencil to bridge the gap between the LED lead and the 9V battery wire. The LED will start to glow! The heavier you make the pencil mark the more the LED will glow. Heavier marks make more conductive paths for the electricity. Of course, you can erase part or all or your pencil mark and see the effects.

Different pencil marks: Move to a clean section of the paper and try using smaller or larger distances between the LED lead and the battery lead. See how that affects the intensity of the glow. You’re learning about electrical resistance in “series,” just as you effectively learned about electrical resistance in “parallel” by making thicker or wider pencil marks the first time.

If you have some other equipment: You can measure the electrical resistance of the pencil mark or marks that you make with a multimeter (cheap – as low as $5, though fancier ones up to, say, $50, have many more functions, more sensitivity, and greater accuracy). Here’s a picture of one being used to make the measurement:

PICTURE

You can put the LED and battery off to the side and just examine the pencil marks with the multimeter. Set the function dial to ohms (with the symbol Ω, or word “ohms”). Press one lead of the multimeter to one end of the pencil mark and the other lead to the other end. Any modern multimeter will automatically choose the appropriate “range” of resistance. In the case shown, the reading is XXXkΩ. The “k” stands for “kilo,” or thousand. That’s a pretty high resistance but it does pass enough current. The resistance is high because most of the contacts between pieces of graphite have been interrupted by smudges of clay. You can figure out how much electrical current flows through that resistance. The LED “drops” 2 to 3 volts of electrical potential, depending on its color. The rest of the 9V drops across your pencil mark (the drop across wires is negligible). So, if you picked a red LED and a battery that has an actual 9V, then there’s 7V or so to pass across the pencil mark. The current is then those 7V divided by the resistance. That’s 7V/50,000 Ω or about 0.00014 amperes; amperes are the measure of current. We can express the current also as 0.14 thousandths of an ampere, or 0.14 milliamperes (mA). A typical LED at maximal current flow might pass 20 mA, or, for a bigger one, 50 mA… and there are huge LEDs, too, that pass more than a full ampere. Even at small current flows that you got with the pencil marks you can see enough light from the LED.

About positive and negative things in electricity. There’s an unfortunate terminology in electricity. In most electrical conductors, it’s the electrons that move. They have a negative charge. However, by convention, the flow of electricity or current is called positive when it moves in the direction opposite to the flow of electrons. Think of it this way: negative things moving to the left achieve the same transfer of electrical charge as positive things moving to the right. This way of thinking becomes automatic as you work in electricity and electronics. You usually don’t even worry if it’s electrons or ions moving.

 

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Newton’s cradle: momentum running around

PICTURES COMING SOON

What does Isaac Newton teach us with an intriguing “toy,” Newton’s cradle. The quantity of motion, or momentum, is conserved. When steel balls collide, there are many possible patterns of which ones go and how fast. It’s addictive to try many patterns.

Newton’s cradle: momentum running around

Equipment: a Newton’s cradle ($18)

Newton’s cradle is a set of 5 steel balls hanging by light monofilament line. At rest, all 5 balls lie in a straight line, touching each other. One can then take any number of balls away to the side and release them, in order to watch the responses of all the balls, both those released and those initially at rest, They collide, move apart, collide again and again, while energy losses slow them (we’ll talk about where those losses are, later).

Newton’s cradle

Static

Videos for frame grabs – 1L; 2L; 3L; 1L+1R; 1L + 2R; 1+2L to stick to 3

The object lesson is learning the conservation of momentum, as well as the conservation of energy. Moving objects carry momentum; that’s the quantity of motion and it equals the mass of the object multiplied by its velocity. Yes, velocity has direction, not only magnitude or size, and so does momentum, but we know we’re only talking about motion in one direction, so let’s just call it speed. With mass m and speed v, the momentum is calculated simply as mv, that is, m multiplied by v.

The object also carries energy of motion, called kinetic energy. You need a bit more physics, but let’s just use the result that kinetic energy, KE, equals ½ the mass multiplied by the square of the speed, or ½ mv2.

When “elastic” objects such as steel ball bearings collide in the Newton’s cradle, momentum and energy both go into other balls. This tells us what the pattern of motion is after the collision.

Consider lifting the leftmost two balls up and to the left, then releasing them. They hit the three stationary balls. Then, the ball in the middle stays put and the two rightmost balls fly off to the right, a mirror image of the starting case. Those two balls are then moving at the same speed as the leftmost balls had when they contacted the other balls. It’s easy to prove mathematically that the only state of motion that conserves momentum and that also conserves kinetic energy is this fling to the right by two balls. You won’t get all three originally stationary balls moving off at various speeds; only two balls move. (The exact solution in physics gives a slightly different result, but minimally different when we take account of the tiny starting separations of the balls and the elastic deformations of the balls; yes, the steel balls deform a tiny bit as they collide! And, yes, steel is elastic, meaning that it deforms under a force and then regains its form almost exactly as the force is released. It’s a bit more complex that that, but steel and other elastic items don’t lose much energy in being hit and deformed.)

PICTURES – see list at top

Everyone likes to try different combinations – release only one ball from the left (only one ball leaves on the right); release three balls from the left (three balls leave on the right); release balls from opposite sides, one from the left and one from the right, both at the same speed (they both simply bounce back); release one ball from the left at twice the speed of a ball released from the right (check this out yourself). You can also make balls act as single units. Put a bit of modeling clay between two balls at the left and release them as a single unit. (Two balls shoot off to the right, the same as if the balls were not stuck together.) Try releasing those two balls at the same time as releasing a ball from the right at the same speed…. and then at twice the speed. Try putting clay on the side of the two balls that will hit the three stationary balls. What happens? You can see the results. If you want to get into the math, you can analyze all these cases. You could go to the exact solution, but that’s pretty heavy-duty math.

The exchange of momentum is very clear. The exchange of energy is a deeper story. In fact, we have to consider a second kind of energy called potential energy. In moving balls to the left or to the right while holding them “tight” on their supporting strings, we have to lift them up a bit against the force of gravity. That is storing potential energy. As the balls move, the potential energy is continuously converted into kinetic energy; gravity pulls on the balls to increase their movement, or accelerate them. The pattern in time of how potential energy changes into kinetic energy is an advanced topic that you’ll find in physics courses; the math is fun.

The motion of the balls slows down over time. Clearly, energy has been lost, both as kinetic energy and as potential energy. Where did it go? It got converted into two other forms. One is kinetic energy of air molecules pushed around as the balls move – they’re creating a little wind that carries energy farther and farther away. A second one is heat in the balls. Steel is not perfectly elastic. At each collision, some of the obvious or macroscopic motion gets changed into rapid internal motions among all of its atoms -sound waves run around in the ball and then slowly degrade into random motions called heat. You would find it vey hard to measure the rise in temperature of the balls after collisions. It’s very tiny, but real.

The challenge with the Newton’s cradle is that everyone wants to try their own releases. Leave more time for this than for most demos! It can be a good learning experience for students to sketch the resuls of all the different trials.

 

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Make your own electric motor

Make your own small electric motor with a loop of wire, a magnet, a AA dry cell, and a few odds and ends. It works the same way as large motors, though don’t count on powering a Tesla this way.

Make an electric motor with a simple magnet and a loop of wire getting current through it from a common household “battery” (really called a cell, a dry cell): The essence of an electric motor is having magnetic fields repeatedly attract and repel each other.  In this simple motor there is a fixed, strong, rare-earth magnet at the base, and a magnet field produced in a rotating part or rotor by the flow of a direct current. That current makes the rotor into a modest electromagnet.

The rotor is a coil of wire with two free ends that have been stripped to bare metal (use a knife or side-cutters carefully) so that the ends make electrical contact in a manner seen below.  Key thing: Lay the coil flat and insulate one side of the bare wire by rubbing a good marker on it. This makes sure that the current gets interrupted on part of the cycle and keeps the rotor going in one direction.

Use a simple AA dry cell as the current source.  Take two bare-metal paper clips (that it, not plastic-coated) and straighten out a length on each to make a vertical support for the rotor, as below.  Affix the paper clips to the battery, one on each end, so that they stand vertically. Stiff duct tape should work. Brace the AA cell against rolling, such as with lumps of clay:

Now put a strong rare-earth magnet on top of the AA cell.  Slide the rotor into position between the paper clip ends, and bend one or both ends to keep the rotor from slipping out. The rotor should start spinning vigorously, perhaps needing an initial spin by hand!

Hints: Make the rotor with wire that’s stiff but not too heavy.  Use only solid wire, not stranded wire, which goes limp.

How it works: As the rotor rotates over the magnet, it is at times attracted to the magnet and at times repelled.  The diagram below shows eight phases of a complete rotation as we look down the axis of the rotor. At point A there’s a repulsion of the rotor as an electromagnet but no torque or twisting action. The rotor will keep moving, however, if it has been rotating; it has momentum (angular momentum).  At point B the rotor and the fixed magnet repel each other, forcing the rotor to rotate faster.  At point C the effect is neutral but the rotor has angular momentum to keep rotating.  At point D the fixed magnet “sees” the opposite pole of the rotor and attracts it, helping to pull the rotor around further.  At later points F, G, H we want the current off so that the rotor is not pulled the opposite way to bring it ultimately to a standstill.   To do this we have put the insulating marker coating on one side (say, up to one half) of the circumference of the bare wire end.

Here’s how the motor is assembled. The images below are numbered; the numbers and the text only show when you click an image to see it full size (a quirk of WordPress)

You can go further with this demo, making it more of an experiment.  You can find the magnetic polarity (north and south directionality) of both the fixed magnet and the rotor.  Use a simple compass to see which end of the compass needle moves toward the rotor (energized but held from rotating) or the fixed magnet. The rotor’s polarity depends on the direction of the current (you can swap it end for end to change the direction of rotation).  Its polarity also depends on which way you wrapped the rotor, clockwise of counterclockwise as viewed from the end; you can even predict the polarity from the “right-hand rule” relating current and the magnetic field (look this up). The direction should depend on the polarity of the fixed magnet, too; flip it over and watch again.

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The Magdeburg sphere

PICTURES COMING SOON

Explore how really big air pressure is with a device first made 368 years ago – the Magdeburg sphere. An inexpensive “toy” can defeat some very strong people trying to pull it apart after someone sucks out some air.

Explore how really big air pressure is: evacuate a Magdeburg sphere:

Equipment: “toy” Magdeburg sphere. You can buy one in cast iron for about $25.

Note: You can do this without a vacuum pump; with a pump you can make the effect stronger.

This goes back to a dramatic demonstration 368 years ago in the town of Magdeburg, Germany. The town mayor, Otto von Guericke, had invented an effective air pump. He used it to pump the air out of a sphere made of two halves joined with an air-tight gasket. He then showed that the pressure of normal air outside was so great that two teams of 15 horses each pulling on the two halves could not separate the sphere! The sphere was ½ meter in diameter, so the area of each half-sphere was about 0.2 square meters. If all the air had been pumped out, the force on each half would be 20,000 newtons, that of a 2,000 kilogram mass in Earth’s gravity. Multiply by two for the two halves. You get the same force as exerted by a 4,000 kilogram mass, about 8,800 pounds!

You can do this on a small scale with the “toy” Magdeburg sphere. Here’s a picture:

PICTURES from above

Clean the rims of both halves well and then apply some thick grease, ideally vacuum grease such as you get with the vacuum pump. Join the halves and now suck out the air. First trial: do this by mouth (take care re hygiene; only one person does this, or you wipe down the tube with alcohol each time). Open the valve, put the exit tube in your mouth, and suck the air out. Using “cheek power” you can readily pull a vacuum that’s about 1/3 of normal air pressure. Close the valve. On a sphere of 8 cm internal diameter, you have an area of both halves that’s about 100 square cm or 0.01 square meter. The force on the halves at 1/3 of air pressure difference between outside and inside is then about 0.33*100,000*0.01 newtons or 330 newtons. That’s the force exerted by a 33 kg weight, or about 70 pounds. Get two people to try to pull the sphere apart; there are handles. Some strong, older students might be able to exert enough force to do this. They’ll fall in opposite directions suddenly; do this on a safe, soft surface such as a lawn, or pea gravel as we have at our school. Now boost the demo: connect the exit tube to the vacuum tubing on the vacuum pump and pump the air out. Now the effective force is 3x greater, that of a 100 kg weight. No two people can pull the sphere apart while standing, or maybe even foot-to-foot lying down!

YouTube link

 

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Squeeze and sink

PICTURES COMING SOON

Control a tiny “submarine” with a hard squeeze… and learn the principle of how anything floats, from an ice cube to a ship. All you need is a handy plastic bottle, a test tube, some water, and a ruler.

Cartesian diver – squeeze a bottle and make it sink:

Equipment: minimal, cheap.

The Cartesian diver is a small item such as a test tube with an air space that just barely floats because it has an air bubble giving it buoyancy; it sinks when the air pressure on it increases to compress the bubble. To create the higher pressure, the diver is contained in a sealed vessel such as a plastic drink bottle. Squeezing the bottle (with its cap on!) creates the extra pressure. The Cartesian diver illustrates the principle of hydrostatics, that the upward force on an immersed object is equal to the weight of water (I presume you’ll use water) that the object displaces. With the right-sized air bubble, the object and its contained air bubble displaces a greater weight of water than its own weight. Squeezing the bottle compresses the air bubble and allow water to enter. Now the diver is not displacing as much weight of water as its own weight; it sinks. Relieving the pressure lets the diver rise. The cycle can be repeated endlessly. The hydrostatic principle applies to SCUBA divers, submarines, fish with swim bladders, and more. A note: I do mean weight, which is mass multiplied by the acceleration of gravity. People often confuse weight and mass, and, here, it’s actual weight.

Here are two images of the diver, afloat and sunk:

PICTURES

My combined sketch

Photo, filled bottle

Photo, dropping in the test tube

Photo, test tube floating

Photo? Measuring the float height

Photo, filling in the test tube

Video, quickly slipping in the partly filled test tube, then seeing it barely float, then squeezing it to make it sink, then letting up P to have it float again

Needed:

* A squeezable plastic bottle with its cap available.

* A “diver” – here, I use a simple glass test tube. You can use any clear item that will float with the air bubble up (not overturning). Be sure the test tube is glass and not plastic. Plastic won’t ever sink. Use a thin-walled glass test tube; a heavy one might not float, even empty.

* Water to fill the plastic bottle and to fill part of the volume of the diver.

* A short ruler

* A place to do this, away from computers and anything else that can be damaged by water

Fill the bottle with water nearly to the top. Do this over a basin because you’re going to spill some water.

  • Fill the bottle all the way to the top. Holding the test tube upside down, slip it into the bottle. It will bob up so that about half of it will be above the water line. We’ll get to why it reaches that level, later.
  • Now measure the height from the top of the water to the top of the tube. You’re going to want to take the test tube out of the water and fill it with almost the same volume of water as there was empty (air) space bobbing up; that will cancel the extra displacement of water that keeps the tube floating. So, suppose you had a 10 cm (4”) test tube and it floated with 4 cm above the water.
  • Take the test tube out and, holding it upright with the opening at the top, fill it with a little less water, say, 3.6 cm or 90% of the empty space. You can measure out the water slowly or be a little cavalier and then shake the tube sideways as needed to reduce any overfilling.
  • Now insert the test tube, again upside down, into the bottle. You want to keep all the water in the test tube, so do this quickly. Hold the water bottle at an angle just short of spilling out its water. Hold the test tube also at an angle, facing the water bottle. Swiftly push the test tube into the bottle.
  • If you didn’t spill any significant amount of water, the test tube, your diver, should just barely float. Now cap the bottle, tightly. When you squeeze the bottle tightly enough, the diver will sink to the bottom. Release the pressure and the diver will rise. You can do this indefinitely.
  • If the diver floats too high, it won’t sink with the highest pressure squeeze you can make. Take the diver out (easy: squeeze the bottle to bring the diver just above the rim of the bottle). If the diver sinks, retrieve it and fill it with less water. The easiest or least messy way is to hold the bottle upside down over a basin. Let your thumb off the bottle opening and grab the divers as it begins to come out. Refill the water bottle again.

A little more detail: Why use a glass test tube: Glass is denser than water; about 2.5 times more. By itself, it will sink. You have to give it a bit of air with water in its interior to make its filled weight match the weight of water it displaces. Plastic, on the other hand, is less dense than water and you can never get it to sink.

Making it a more quantitative demo: Calculate the level of air you need in the diver. You need to get the “weight” (mass) of the test tube, either using a small electronic scale or using geometry and the density of glass. Contact me if you’d like to see the sample calculations using the test tube dimensions.

 

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A magnet fighting its own fall

PICTURES ARE COMING SOON

A magnet that fights its own fall? In a copper tube that conducts electricity so well, a magnet can do this.

The slow magnet: Dropping a rare-earth magnet in a copper tube: buy or borrow a pure copper tube with 3/4″ inside diameter and about 18″ long (hardware stores, alas, don’t yet do metric!). Also get a strong rare-earth magnet about 1/2″ in diameter.

It’s important to have it as long as or longer than its diameter so that it won’t tumble going down.  Stack a bunch of button magnets, if need be.  Hold the Cu tube over a soft pad (so that the magnets don’t crack hitting the desk or floor). Drop a nonmagnetic piece of similar shape down the tube; it comes out fast (about  0.3 sec.). Drop the magnet down the tube, especially while a student watches from above.  Better yet, let the student do it. The magnet takes several seconds to fall.  As it moves down the tube, its magnetic field “cuts” the copper tube to create an electrical current circulating around the circumference.  That creates it own magnetic field that opposes that of the magnet, slowing down its fall. You might ask if the fall could be stopped completely (with an answer that should be obvious) or several other questions.

PICTURE, VIDEO

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gallium, the weird metal

PICTURES COMING SOON

Gallium: a weird liquid metal in your hand! It’s as shiny and mobile as mercury but so safe… unless you’re a Coke can. It’s a liquid wire to carry electricity. It acts like ice on freezing. With a simple AAA cell you can make it take extremely different shapes. It’s easy and inexpensive to buy, for much fun.

Equipment and supplies: For many parts of this demo It depends on how many of the 6 parts of the whole demo you wish to do. Foremost, you need gallium ($14 or more; see below). Get some simple containers, cups or chemical beakers, to hold warm or cold water. Get a piece of glass (careful of the edges) or a glass bottle for demo 2. For demo 3 get an empty aluminum can and a piece of sandpaper. For demo 4 get a small, clear, rigid tube – glass or plastic. For demo 5 make a small trough, even in modeling clay, and get the use of an electronic multimeter ($10 or more) to measure electrical resistance. For demo 6 get a beaker, some sodium hydroxide ($10, or less if you pick the lye crystals out of some Drano carefully, not with bare hands), some wire, a piece of aluminum (maybe even a bit of aluminum foil), and a AAA cell.

Many of us, chemists and members of the general public, remember mercury, the liquid metal. You could pour it out, even onto your hand, as a beautiful, silvery, shimmering liquid. While you can still do that, people will look askance at you for the potential health hazard. There is such a hazard but almost solely if you let certain bacteria at it in oxygen-free environments to make astoundingly toxic methyl mercury or dimethyl mercury. So, avoid the problem and get some gallium. I bought some several years ago along with also fascinating bismuth metal, for a total of $55. You can get it from Luciteria.com, where you can get almost any chemical element! You can get 20 grams for about $14.

Demo 1: Melt it, even in your hand: The gallium will sit in the container as a silvery solid like any nice metal. However, warm it up a little and it melts. If you’re patient and your hands are not cold, it will melt in your hand. Its melting point is 30°C, which is 86°F. If you’re less patient, warm up some warm and insert the container.

Demo 2: The shimmer and the streak: Carefully pour some out onto your hand. No worries. It’s essentially totally nontoxic. Roll it around and let it slosh around. Try not to spill any; it will leave a gray streak on lots of surfaces, even on your hand. The streak is readily washed off with a little soap and water. You can paint on glass with gallium, with a little practice. Get some gallium on your finger and rub it onto a clean glass surface. I did this on old wine bottles. Only a little suffices to create a fine, silvery mirror.

PICTURE(S)

Demo 3: Eating a coke can! Find an empty coke can or other aluminum can. Empty, I say, for reasons that become clear. Have your gallium liquid. Take a bit of sandpaper and scrape an area on the lid of the can. Deposit some liquid gallium on the area. Come back in an hour or two. The gallium will look odd and the can, more so. You can now push your finger easily through the lid! Gallium atoms have diffused into the aluminum metal, moving on their own just as a drop of food coloring diffuses slowly into still water. Gallium is a chemical analog of aluminum – it shares many of the same physical and chemical properties, but its crystal structure as a solid is spaced differently from that of aluminum. The presence of gallium pushes the crystals of aluminum apart, massively weakening the lid (yes, crystals, as are all solid metals in many small places). You’ll also notice that the decay of the aluminum lid extends well beyond the place where you applied the gallium; the diffusion kept going.

PICTURE

Demo 4: It’s slow to freeze and it does an odd thing: Gallium is one of the very few liquids that expands on freezing, just as water ice does. As also with ice, you can demonstrate this by putting a little bit of it as a liquid, a few grams, into a tube and letting it cool, say, in a bath of cool water. If you have a thermometer you can try baths of different temperatures. You’ll find that it takes a bath a number of degrees below the nominal melting point of 30°C for gallium to freeze. It will “supercool” and then suddenly start turning solid when you’ve pushed supercooling too far. The same supercooling phenomenon occurs in water. Some clouds have water droplets cooled far below 0°C. An airplane flying through such a cloud can collect these droplets that instantly become ice, and that’s usually a growing problem! Also, you can mark the tube at the height of the liquid and then see that it’s higher when the solid forms. The expansion of the liquid on freezing can create great force, as is readily shown by freezing water inside a metal container filled to the top (a soft drink so frozen has additional things going on, though it also explodes).

Demo 5: The liquid is a fine electrical conductor: Most metals conduct electricity much more poorly as liquids than as solids. The disorder of the locations of atoms in the liquid phase causes the electrons moving through it to scatter. They lose momentum and it takes more force, more voltage, to move a current. To see this, solidify some gallium in a “boat” shape – that is, some little trough with closed ends. Take a simple electronic multimeter that you’ve set to the setting of resistance (ohms, Ω). Touch the two probe tips to the two ends of the solid gallium and record the resistance in ohms. Now, melt the gallium in the trough by any handy means, even a hair dryer on low heat held at a distance so as not to splatter the liquid. When the gallium is liquid, repeat the measurement. It’s lower!

PICTURE

Demo 6: Gallium has a huge surface tension: When you put a drop of water on a surface that water doesn’t wet, such as clean glass or most plastics or some plant leaves, it forms into a ball, a bead. The attraction of the water molecules for themselves is far stronger than their attraction for the molecules of the surface. The best way to reach the lowest energy state is to stay as compact as possible, toward a spherical shape. That reduces the surface created. Surface tension is a measure of how much new force is needed to expose more surface. There’s a lot to explore in that concept. For now, let’s just note that gallium has seven times higher surface tension than water! Hmm. So why does it flow nicely over my hand? Well, when it’s exposed to air, gallium combines with oxygen gas in the air to make an extremely thin layer of gallium oxide, Ga2O3. Aluminum also makes an oxide layer, as you might expect from Ga and Al being in the same column of the periodic table of the chemical elements. Anyway, we can eliminate that layer.

Here we go: Get a chemical beaker. Pour in some water that is warm enough to keep the gallium liquid. Dissolve a bit of sodium hydroxide, known as lye, in the water. Do this carefully, since lye is corrosive and hazardous to human tissue (on your hand, it will first just make it slippery because it breaks down fats around your cells. Don’t leave it there. Don’t let it get to your eye! Wash it off; wash your eye with running water for several minutes if you get any lye in it. To continue, make a simple electrical circuit. Attach a wire to a piece of aluminum and suspend at least part of the aluminum in the liquid. Run another wire to the liquid gallium at the bottom of the beaker. Now, touch the two ends of the wires you have outside the liquid to the two ends of a 1.5 volt battery, such as a AAA cell. One of two things will happen. If you put the wire that runs to the gallium on the bottom of the AAA cell, you have made the gallium negative electrically. That chemically reduces the oxide coating to pure gallium. It then exhibits its maximum surface tension and really balls up. Now, switch the leads, making the gallium positive. It gets oxidized… .and it spreads out into fantastic shapes called fractals. It spreads even though the oxide coating is something of a restraint. The coating keeps getting dissolved by the lye, allowing the metal to spread!

Picture

End notes – some history: Where did the name gallium come from? The existence of gallium as a chemical element was predicted by the great Russian chemist Dmitri Mendeleev in 1867. He had data from the world chemistry community (hard to get in pre-Internet days!) about 48 chemical elements, including the relative mass of the atoms in each element. He put them in a 2-by-2 chart according to repeating properties. There was a gap below aluminum, so he boldly predicted there was a metallic element acting a lot like aluminum chemically with an atomic mass of about 68 with a density of 5.9 grams per cubic centimeter. In 1875 the fine French chemist Paul-Émile Lecoq de Boisbaudran used the relatively new method of spectroscopy to determine there was another, distinct chemical element in some metal samples. He purified it and there was gallium. It’s atomic mass of 69.7 was close to Mendeleev’s prediction, and so was its density of 5.91 grams per cubic centimeter. Being French, Boisbaudran honored his home country by giving the element a name derived from the Latin name of France, Gallia. Boisbaudran is also credited with discovering the elements samarium and dysprosium. His name is pronounced as bwah boh drahn, or more precisely in international phonetic symbols that I haven’t copied here. The “r” is lightly trilled and the “ah” is breathy.

 

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colored flames

PICTURES COMING SOON

Flames in many colors: Just add the right chemical element and you can get red, orange, yellow, green, blue, or violet. Some of these elements occur in common household chemicals. A propane or butane torch, an old spray bottle, and some safety precautions and you’re good to go. There’s interesting quantum physics behind it all, too.

Many-colored flames

Equipment: a butane or propane torch ($15; be sure only a responsible adult handles it!); a selection of chemical compounds (some cheap, others pricey; I’ll note the safety issues, which can be handled readily).

This demo can be done as a short “Gee, whiz” demo, rapidly showing the flame colors. It’s far more interesting with all the context I develop here; I offer a lot of it here for fun and education.

We’ve all seen flames with different colors – the blue and yellow of a candle flame, some pure blue from a natural gas flame, deep red in a charcoal fire. Some of us have seen exceptional flames, such as burning magnesium (blindingly white, literally – never keep looking at it).

Getting new colors: We can look at what causes different flame colors. More than that, we can get many different colors in a flame by adding simple chemicals in small amounts. Take ordinary table salt, sodium chloride. Dissolve salt in some water. Put it in a small spray bottle (say, from eyeglass cleaner or nasal spray, cleaned out and dried). Set up the butane torch. Safety:

* Have an adult do this

* Be sure that the torch is very stable, on a table or desk; a torch knocked over can be a serious hazard.

* Have a fire extinguisher handy.

Get the butane torch burning nice and light blue. Spray the solution steadily into the flame about halfway along its length. You’ll get a vivid yellow! Why? To answer this we’ll talk about how electrons are running around in atoms (and molecules) and how changes in their states relate to energy, thus, to the color of light.

We’ll talk a bit more about safety of the chemicals at the end. First, please note that no one should inhale the spray! Toxicities are low but non-zero. There should be no problem with the small amounts sprayed into the flame and reaching the air in the room.

PICTURES

Why the colors appear: In chemistry or physics you learn that the atom has a tiny, very dense nucleus with a positive charge and a set of electrons of negative charge moving around them. The actual motion is complex and even beautiful while often simplified as the electrons moving in circular or elliptical orbits. The real patterns still retain the interpretation that electrons in an atom “at rest” occur at distinct or discrete energy levels. Let’s build up a sodium atom from a bare nucleus. We’ll need to eventually add in 11 electrons to match the 11 positive charges in the nucleus; sodium is thus chemical element number 11. The first electron ends up in an “orbit” or state called the 1s. One more electron can fit in this state, to give a atomic state we denote as 1s2, where the superscript “2” is counting those electrons. The next two electrons go into the state called 2s. Six more can go into three similar states all lumped as 2p. The eleventh and final electron goes into the 3s state. This is the state of lowest energy of the sodium atom in isolation… and that’s a common state for sodium that’s been wafted into the flame after the chloride partner gives back one electron to what started as a sodium atom missing one electron, an ion.

The flame is a state of high energy as disordered thermal energy or heat. Atoms and molecules in the flame bounce into each other with extreme frequency. Sometimes an electron on the sodium atom gets bumped up in energy from the 3s state to the 3p state. That’s not a stable state for various reasons. The electron can fall back to the 3s level. In doing that it loses a lot of energy. Ah, but energy is conserved! The energy is taken up in creating a particle of light, a photon. This photon, when it hits our eyes, gives us the sensation of yellow. There is an exact mathematical relation between the energy of the photon and its wavelength and, therefore, its color. (Light is a vibration of electrical and magnetic fields in space, as realized by the brilliant James Clerk Maxwell in 1860, and the idea carries into modern “quantum” physics, with interesting ties to the Nobel Prize for Albert Einstein! Light has a wavelength for its vibration, just as does sound or water ripples. There are lots of stories here.)

Let’s get some other colors: There are many metals that we can dissolve as salts and put into a flame. Each has its own electronic energy levels in both “ground” and “excited” states, and this gives their electronic transitions a special color. There may be several colors, with several transitions happening to

different atoms at the same time. To get nice, rather pure colors, we want the metal atoms to be combined with other atoms that don’t give another color to confuse the result. Many metals can be dissolved in, say hydrochloric acid to give chloride salts. Chlorine atoms don’t give light that we can see.

Let’s look at different metals, the colors they give, and the chemical compounds we can obtain and dissolve in water nicely. After this we’ll get back to the blues, yellows, oranges, and reds of common flames.

Element name Symbol Flame color Compound to use
Copper Cu Green Copper chloride or sulfate
Potassium K Violet Potassium chloride
Rubidium Rb Violet Rubidium chloride
Cesium Cs Blue Cesium chloride
Calcium Ca Orange/red Calcium chloride
Manganese Mn Lime green Manganese dioxide (see below) or nitrate
Lithium Li Deep red Lithium chloride
Iron Fe Orange Ferrous sulfate (ferric is insoluble)
Indium In Blue Indium chloride
Zinc Zn Aquamarine Zinc chloride or sulfate
Strontium Sr Red Strontium chloride or sulfate
Boron B Green Boric acid (borax is not as good)

The last element, boron, is not a metal, but it’s handy and it’s colorful, and the same ideas about electrons changing state apply. The particular salts to use, chloride or sulfates, are not critical; you might find some others, but avoid salts where the other part, the anion, imparts its own color that might mask the color you want. Obviously, avoid borates of the metals, or you’ll mostly see the green of boron.

Dissolve as much of a compound as possible. You’ll be using just a tiny amount sprayed into the flame, so you won’t waste much – and you can keep the solution and its solids for later, anyway. Some of these compounds are much more soluble than others. E.g., only about 6 grams of boric acid dissolve in 100 g of water, while 129 g of manganese nitrate will dissolve in 100 g of water! In any case, all the above are usefully soluble.

Getting these chemicals and using them safely: Some of these are easy to get and cheap, others are more specialized. Start with the easy ones:

* Copper: copper sulfate is sold as a root killer in garden or hardware stores (it kills roots that invade your oudoor plumbing. Cheap. Copper sulfate is toxic in fairly small amounts, even 1 gram. Don’t let anyone taste the pretty blue crystals!

* Sodium: this is ordinary table salt. Super cheap.

* Potassium: pharmacies and supermarkets sell potassium chloride as a “salt” substitute for people with high blood pressure. Make sure it has no sodium chloride at all, or the sodium color will dominate. Cheap.

* Calcium: Calcium chloride is sold in big bags to melt ice in cold climates. It is also sold in small amount, rather pure, for making some cheeses and can be bought online.

* Manganese is in alkaline batteries. You can cut an old battery open (careful with the sharp edges!). However, the manganese is as the insoluble dioxide. To make a soluble form, you can dissolve a pinch of the dioxide in a pinch of hydrochloric acid, HCl, also sold in hardware stores as muriatic acid (from the Latin muria, brine, since the chlorine is made ultimately from sea salt). BE CAREFUL; HCl is volatile, so that HCl gas wafts off the liquid. Its suffocating odor is a deterrent; its effect on lungs is terrible. Don’t breathe over it, and discard any leftover acid (in small amounts) by diluting it with lots of water.

If you can get ahold of manganese nitrate, you’re set, without any processing.

* Iron: You can get essentially pure ferrous sulfate as “iron pills” at a pharmacy or supermarket. They’re sold to treat iron deficiency in humans. Note: While iron is a critical element in our nutrition (usually obtained from food), an excess is toxic and even deadly, as it drives the destruction of organs. The most common accidental poisoning of children is from their naïve consumption of iron pills. Take care.

* Boron: boric acid is sold as a roach and ant killer. Get the pure stuff, not mixed with other chemicals. It is toxic to humans, too, at the level of a few grams. Don’t let anyone taste it!

The other chemicals are less available. Get your friendly local university chemist to give you a few grams, or have him or her do those demonstrations. You might also get them from a chemical supply house or Carolina Biological Supply.

Controlling the torch flame:

Why was the butane flame blue and orange to start with? Combustion of a hydrocarbon such as butane is a very complicated set of chemical reactions, all going on at the same time in a very short time as the butane and its products shoot out. The combustion is always a little bit incomplete, creating chemicals that have their own glow or luminosity. Very incomplete combustion, similar to that in a candle flame with its low temperature, creates balls of mostly carbon, called soot. When in a flame, they glow from what’s called blackbody radiation (the name has a long and fascinating history, even involved with Nobel Prizes). This is radiation of all wavelengths or colors. Its intensity rises with temperature. Its color changes with temperature, from red at low temperatures on up to blue at temperatures of stars, far higher than in any flame. To prevent the orange glow from masking the colors of the metals that you want to see, adjust the torch flame so it burns blue.

Finally, why is there blue in the torch flame? This color comes from some unusual and unstable chemicals forming in the flame. One such chemical is the radical CH, one carbon and one hydrogen bonded together. It’s very unstable and gets burned up eventually, but not before it glowed. Another compound is C2, just two carbons bonded together. Same deal – it’s unstable and it disappears, but it did glow on its way out.