Tag: science

Counter Timer

Posted on by Brian Garthwaite
Pulse counter, interval timer
Pulse counter / tedium saver

Behold: a box which counts! That’s it, for the most part. It counts pulses of positive voltage. Very quickly, and you can set some thresholds to tell it to count certain values but not others.

It also gates over an interval you set, so you can tell how many pulses it receives over, say, one second. It counts, displays the total, then counts again. Displays the new number.

We use these for our wave/particle duality lab experiment, which relies on counting individual photons. Yes, those. The teeny, massless quantum packets of energy, the messenger particles of electromagnetism. Light. It acts in non-intuitive ways, and the students who think “that’s amazing and I want more!” sometimes become Physics majors.

Part of using this box – just one aspect – is helping convince those students that only one photon at a time can be reaching the photomultiplier tube sensor. At the speed they move, a mind-boggling number of photons can zip through that meter-long box without bunching up. c in air isn’t all that far from c in a vacuum, so if your one-second counts aren’t remotely near 299,792,458 (adjusted for PMT sensitivity and other losses), you know some of those photons are pretty lonesome. Sometimes you need a little math to make sense of things you can’t directly sense.

Pulse counter, back
Crickets.

One other fun aspect is a little switch hidden on the back: cricket. It’s the volume switch, letting the box emit a little beep for every pulse it counts.

If you’re counting pulses from a radioactive source, which arrive randomly, it can be informative to hear these irregular signals, gated and grouped into numbers which show a decaying curve.

If you’re counting 100,000 photons every second, in a room of other lab benches also counting thousands of photons? Less informative, more irritating.

Spectra

Posted on by Brian Garthwaite
Spectroscope
Old school. $1,455.68 in today’s fun bucks.

Sometimes, we have old equipment which is rarely, if ever used. Case in point: the mid-20th-century spectroscopes which have been supplanted by digital spectrometers. They’re both effective tools for examining a spectrum of light, one by eye and the other fed by a USB cable. Using a diffraction grating, they split light into its constituent spectrum – its rainbow, more or less – and can identify the presence of individual wavelengths. Not something our eyes can do, as they blend everything together, though that’s very helpful in most situations, such as reading this on your screen.

Summing bands of reddish, greenish, and bluish into a broad rainbow of colors is one neat-o trick.

With a diffraction grating, reflection grating, or prism, you can refract light out along a range of angles which correspond to its constituent wavelengths. Put a sensor at a known angle – your eye or a semiconductor exhibiting the photoelectric effect – and you know the wavelength if you sense a photon. It’s a simple piece of information which can be used to unlock a staggering amount of interesting, related information about what you’re observing.

Hydrogen diffraction
Hydrogen.

You can also use a diffraction grating to get a quick sense of the entire visible spectrum of a source by holding it off to the side. Remember: the angle of the light’s path change as it refracts, so you’re trying to angle it back to your eye. Hydrogen has a distinctly pinkish-purplish hue when excited at high voltage, and you can see the dominant red and blue lines in its spectrum. With just that one electron to absorb energy and emit photons, the spectrum can only be so complicated.

Helium diffraction.
Helium.

That’s in contrast to helium, with its two electrons. The spectrum doesn’t look white, per se, but is much more filled out than hydrogen. Look at those spectral lines, and there are so many more! They’re distinct, measurable, and provide a “fingerprint” that can be immensely useful for scientific study. Or for just looking cool.

Neodymium magnets

Posted on by Brian Garthwaite
Cube of cylinders: squaring the circle?

‘Tis that most joyous of days in the beginning of the semester: physics toy kit day! A bag full of odds and ends, perfect for playing, experimenting, and providing tactile bits to use when working through physics problems. Batteries, compasses, various wires, polarizing filters, nails, magnets, and balloons. Always balloons.

Each kit contains two small neodymium magnets, because magnets are amazing. First, you’re bound to stick them together, then spin one around and feel them repel. Surprisingly strong such wee little cylinders. Then check what they stick to around you: whether or not they feel attracted to stainless steel is always intriguing. (The answer is: depends on the type of steel and how it was formed.) Stick them together across a string and let it hang: you’ve built a compass!

Pay attention to the time and location of the sun – or Polaris if you’re pulling an all-nighter – and you can tell which pole of your magnets is which. Maybe it’ll come in handy?

Beats

Posted on by Brian Garthwaite
wobblewobblewobble

In acoustics, there’s a phenomenon known as beats, which is when two similar tones generate an interference pattern that sounds like a pulsing beat. It happens with waves of all kinds, waves being moving energy and all that, but sometimes it’s easier to really get the sensation when you hear it. Graphing out the sinusoids and showing the constructive and destructive interference helps explain it. Hearing that wubwubwubwubwub cements it.

We have what looks, at first glance, like a glockenspiel, with its metal bars all in a row. Tap one with the mallet, and it sounds almost the same as the one next to it. Almost. Tap two at once, and you get the beats.

At one end, it’s 440 Hz. Then 439, 438, 437, 436, and 435 Hz. Not only can you hear the beats, but you can very clearly hear the change in beat frequency as you combine tones in different combinations. It’s very cool.

Also quite unnerving after a while. woobwoobwoobwoobwoob

Pendulum bobs

Posted on by Brian Garthwaite
Aluminum and brass pendulum bobs.
Shiny!

Does the mass of a simple pendulum affect its period of oscillation? The small-angle formula doesn’t include mass, just the length from the pivot to the center of mass and g, the gravitational constant. It’s an approximation that’s pretty good for angles up to 15-20°, and after that it’s into introductory differential equations. Which still don’t use the mass, as it cancels out when applying Newtonian mechanics.

That, however, is for an ideal pendulum, with a massless string and point mass bob in a system without friction and other losses. We’re all out of massless string at the moment, and those point masses are proving elusive. And as neat as it might be to swing a pendulum in a vacuum, the setup sounds like a real challenge.

On top of that, it’s an interesting question that’s really addressing a student’s understanding of measurement and uncertainty. Equations and models illustrate principles, and sometimes do an excellent job of making sense of the world. It’s just that the real world is messier, and wading into that mess – even a little bit – can be enlightening.

Two new pendulum masses, machined to the same dimensions, or close enough that you won’t notice without accurate calipers. Threaded to screw on and off. Aluminum (2.7 g/cm³), checking in slightly under 50 grams each. Brass (8.7 g/cm³), a little over three times the mass, a shade above 150 grams apiece. If we can work with the material, we could make more from anything available.

Lightweight plastics, like Delrin acetal (1.4 g/cm³)? Sure. Denser stuff, like lead (11.3 g/cm³)? Not impossible, but okay, well, no. Even denser? Tungsten, gold, and depleted uranium are all in the 19 g/cm³ range. McMaster-Carr has a range of tungsten alloy rods in stock! (For a small fortune.)

For now, though, it’s two masses, a string, and a stopwatch. Real physics in action.

Inverted balloons

Posted on by Brian Garthwaite
Balloon on flask.
It’s like a hat.

What can you do with a large Erlenmeyer flask, a kettle of boiling water, and a bag of balloons? Science, of course!

Get the flask hot with a pour of boiling water first, then pour out. Add a small amount of boiling water – as hot as can be – and quickly seal with a balloon. We’re looking for steam, and lots of it.

Then wait and see what happens!

Balloon in flask.
Cool!

Spectrometer

Posted on by Brian Garthwaite
Spectrometer and sodium lamp.
Rainbows!

Astronomy is roughly 98% figuring out how to look at stuff better than our eyes can do it.

Gathering more light with large-aperture lenses and reflectors. Gathering more light with long camera exposures. Using detectors for light outside the visible spectrum, from radio waves to gamma rays. Launching telescopes a million miles into space to get away from our pesky atmosphere. Splitting the broadly blended colors we perceive into their component wavelengths.

That last one’s the easiest to accomplish in a student lab setting, and it’s a broadly useful scientific tool across many disciplines. Turns out that certain particular constraints caused by quantum physics make all sorts of other observations possible. Who knew?

Pictured above is a low-pressure sodium lamp, just like the ones that once illuminated nighttime streets around the world with their flattening orange glow. Looks orange to our eyes, but it’s primarily a mix of red, orange, and yellow wavelengths. If you measure those carefully enough, you can discern a certain “fingerprint” on a spectrum of light that would tell you if sodium is or isn’t present in what you’re observing.

Same applies to hydrogen and helium. Nitrogen and oxygen. Argon and neon. Carbon dioxide. Water. Every atom and molecule – including different ionized states, which is a particularly useful bit of information for astronomers – has its own unique spectrum of light it emits. You just need to look at it in the right way.

Even if it’s millions of light-years distant.

Fan carts

Posted on by Brian Garthwaite
Fan cart on track.
An object at rest.

Newton proposed three laws of motion, and it’s the second one that makes for the most interesting labs. Maybe there’s some way to make inertia both fun and educational, but let’s leave the first law for lecture. Equal and opposite reactions are pretty great, but that’s ideal for big demos. Read: rocket launches.

Force, though. Force lets students do stuff and observe what happens. Doing stuff and getting results is how you make physics more interesting.

One tool in our Newton’s-second-law arsenal is the fan cart. An assemblage of a cart with low-friction wheels and a simple DC motor holding a plastic fan. The fan mount pivots, providing variable direction of force. Runs on AA batteries, and is held together mostly with hot glue.

Two very good reasons for the hot glue: 1) When one of these invariably plummets to the floor, the less-than-rigid connections absorb a good deal of the impact when it all falls apart. Usually it falls into pieces, but nothing’s really broken. 2) After one has taken a tumble, it’s mere minutes to get it re-glued and running once more.

The reason for the fan is that it provides a close approximation of constant force, F. If F is constant, and mass (m) is constant, then by F = ma, acceleration (a) is constant, too. Give a running cart a little backwards push – an additional force – and study how its position and velocity change over time. Simple? Sure, and that’s helpful when tying together various concepts.

Relationships between force, mass, and acceleration according to Newton’s second law. Two-dimensional vectors come into play when rotating the fan. Our motion detectors read position, so it’s an illustration of integrals and derivatives underpinning the acceleration, velocity, and position of a moving cart.

The importance of catching a speeding object before it bangs into the end of the track and crashes to the floor.