Pendula of different materials are suspended here from a horizontal rod. As a large magnet is moved in next to them, we see an unsurprising response. The steel pendulum leaps over to the magnet; the wooden pendulum is entirely unaffected. There is a third pendulum, however, that appears to be slightly, but not very strongly, attracted to the magnet. The bob on this pendulum is a lump of dysprosium, as seen in our new demonstration, J5-15.

Dysprosium is a rare earth element that historically has been noteworthy primarily for being difficult and expensive to refine, and for having very few real world applications. It has been joked about as being one of the most useless of elements. This is not really true, though. Late 20^th century chemical processes have made extracting it somewhat more practical, and it does have a few uses when in combination with other materials – it is a minor component used in making neodymium magnets, compounds of dysprosium are used in high powered floodlights (remember gas discharge lighting from last month’s blog post?), and it even has uses in making industrial radiation detectors. Mostly, though, these are often forgotten about because dysprosium appears as a small component of another chemical or system.

 Dysprosium on its own does have one interesting property, though: it is a paramagnetic material.

 Paramagnetic materials are those which are only weakly attracted to magnetic fields. The reason for this is complex, and relates to the quantum behaviour of electrons.

Every atom in a substance has its own tiny magnetic field, determined largely by the electrons in its outer shell. In a ferromagnetic material, like iron, these tiny fields tend to spontaneously align with each other, giving the larger object a permanent, measureable magnetic field. In a paramagnetic material, however, they mostly do not interact with each other, and are oriented randomly in the material… until an external magnetic field affects them and temporarily causes them to orient themselves in line with it. Thus, a paramagnetic material will act like a weak magnet when in a magnetic field, but won’t retain that magnetism when the external field goes away.

 Interestingly, materials can switch from being ferromagnetic and paramagnetic depending on temperature. The temperature at which they make this transition is called the Curie temperature, after Pierre Curie, who performed important early research on paramagnetism in the 19^th century.

 Learn more about paramagnetism 

We have introduced several new demonstrations recently, in a variety of topic areas. Be sure you take some time to visit our website occasionally and see what's new! Here are brief introductions to two of them.

Electricity and Magnetism: Forces Between Current-Carrying Coils

Check out K1-07 Interacting Coils, developed by student (now alumna) Sarah Monk! This is a new way of illustrating the forces between parallel current-carrying coils. This attractive new tabletop demonstration will be easier to use in many classes. Challenge your students to predict how changing the direction of the current will change the motion of the coil.

This demonstration was itself developed in conjunction with classwork, as part of a final project for PHYS411 last semester. There are many different ways to explore incorporating demonstrations and hands-on learning activities into your classes – not only using demonstrations to illustrate concepts, but advanced students can learn a great deal by developing systems and devices themselves!

Buoyancy, Density, and Pressure

At the suggestion of Mark Eichenlaub, we have also added F2-27 Buoyancy Paradox: Two Spheres. Two identical beakers of water sit on the pans of a pan balance. In one, a ping-pong ball is tethered to the bottom of the beaker so the ball floats submerged in the water. In the other, a steel ball if the same size hangs from an overhead hook, submerged at the same height. Invite your students to predict whether the pans will remain balanced, or will show one as heavier!

This demonstration was first developed for use in a presentation for AAPT's US Physics Team. Every year AAPT trains high school students for the International Physics Olympiad, and UMD Physics hosts their training camp in the early summer, with demonstrations and problem solving exercises.

Physics demonstrations can enhance learning and student engagement in a wide variety of contexts. What experiments could you use to expand student interaction in your classes?

Welcome back to the Physics Demonstrations Blog! We’re back from the winter deep freeze and here to share more fun ideas about physics. This week, a brief look into the quantum world with electron beams!

The electron is a fundamental particle, one of the earliest discovered in modern physics, which has a small but finite rest mass and carries a negative electrical charge.
However, quantum mechanics has taught us that, under certain circumstances, the electron, which we conventionally think of as a particle, can also behave like a wave.

A stream of electrons traveling through space means a stream of charges is traveling through space – this is, in effect, what an electrical current is. And an electrical current can be deflected by a magnet – so, too, can a beam of electrons. If you hold a magnet near a beam of electrons, like the one in this cathode ray tube, the beam will deflect in a direction perpendicular to the magnetic field.

Likewise, if you run two electrical currents parallel to each other, the electric field of each will deflect the other slightly, which can be seen in either two wires running parallel, or by running a wire parallel to the electron beam.

This beam is made up of electrons; the glow in the screen behind it is made when a few individual electrons passing by interact with the phosphors in the screen.

But what happens if we take that beam and pass it through a narrow opening? Conventionally, if you throw a bunch of particles at a hole, either they pass through in a straight line, or they miss and bounce off. But instead, look what happens when we do it with electrons:

The electrons are not passing through in a straight line, but instead are diffracting, forming a pattern of rings. This is an interference pattern formed by the interaction of the peaks of a wave. We can see this frequently when light passes through a narrow opening, forming such a pattern.

But here, it is being formed by electrons – very small particles, but particles nonetheless. Thus, here we see an electron behaving as a wave.

Today we introduce another new demonstration*, this time on electrostatics. Remember that soda can from last month? It has now dried out and has returned for another adventure!  

Some materials, when rubbed together, build up an electric charge through contact; this phenomenon is known as triboelectricity. In the case of the materials we have here, after the rubber has extensive contact with the wool, the rubber is charged with an excess of electrons, giving it a net negative charge. If you touched the rod with your bare hand, you might feel a faint shock as that excess charge jumped to you. (Perfectly safe, just a little surprising!)

In this case, though, we're going to be careful not to discharge the rod too soon. Instead, we hold the rod carefully near and parallel to the soda can, being careful not to touch the rod to either the can or the table. As we do, we see an unusual phenomenon: the can will slowly start rolling towards the rod! We can even slowly pull the can across the table as it seemingly chases the rod about. How can this be?

What we are seeing is a property known as electrostatic induction. Left to its own devices, the can has no net charge - it has an equal number of positive and negative charges, distributed evenly throughout the material of the can. But because the aluminum can is an electrical conductor, some of those charges can be free to move around if acted upon by an outside force. When the negatively charged rod is brought near the can, the electrostatic force (or Coulomb force) that this negative charge exerts affects the atoms in the can. Some of those free electrons, also carrying negative charge, are repelled and move to the far side of the can. This leaves the side of the can nearest the rod with a net positive charge, which is now attracted to the rod! And so the whole can, being quite lightweight, can slowly start moving towards the rod. 

Try this out in your own classes with demonstration J1-14: Electrostatic Induction - Attracting A Can.

 (*New to us; credit for this development goes to the Physics Instructional Resource Association)