electromagnetism

• Coming Soon: Physics is Phun presents Induction and Deduction

Coming up in two weeks, University of Maryland Physics will present our first Physics is Phun program of the semester.

At Induction and Deduction, you are not just an audience; we are all detectives helping to solve a mystery. Our tools: Physics! Our guest presenter and lead detective: Dr. David Stewart of the Institute for Research in Electronics and Applied Physics.

Physics is Phun is our ongoing series of programs presenting exciting physics concepts and demonstrations at a high-school level. It is free and open to the public. We hope to see you there!

The program will be Thursday October 11 and Friday October 12 at 7:00 PM in UMD Physics Lecture Hall 1412.

More details are available at the UMD Physics Outreach page.

• Demo Highlight: Electromagnet With Bang

One of our most popular electromagnetism demonstrations is J6-01: Electromagnet With A Bang! We discussed this demonstration in a previous highlight article. But now, you can see it in action in this new video with Landry Horimbere.

A massive block of steel is suspended by an electromagnet, courtesy of a single D-cell flashlight battery. When the switch is flipped to open the circuit, the electromagnet turns off, and the block falls dramatically to the table.

The operation of an electromagnet is based on the discovery that an electrical current generates a magnetic field as it flows through a conductor. By grouping many conductors together in a coil, arranged so that their fields align, we can sum their individual electromagnetic fields into a much stronger one. Thus, we can create a strong electromagnet even from a relatively weak current.

You can also try this out at home and in the classroom with this updated magnet simulator from the PhET collection at the University of Colorado.

The simulator has both permanent magnet and electromagnet options. Flip to the electromagnet tab; you should see, as in the screencap above, a battery connected to a coil, with many magnetic field indicators all around. Controls in the margin let you adjust the number of loops in the coil, and a slider lets you vary the voltage. Both the large magnetic compass and the magnetic field meter can be dragged around the screen to measure at different points. You can also swap the battery out for an AC power supply. Try it out!

• Demo Highlight: Van de Graaff and Pie Pans

The Van de Graaff Generator was featured recently with a new animation of its inner workings. This week, we’re taking a look at the physics behind one of the most popular demonstrations that use it: demonstration J1-26, the levitating pie pans.

In this new video featuring Landry Horimbere, we can see the demonstration in action!

As the belt moves inside the Van de Graaff generator, it deposits an imbalance of charge on the combs inside the dome. These charges accumulate on the outside of the dome. When the conductive plates are placed on top of the dome, they accumulate the same charge.

Because like charges repel each other, the pie plates gradually try to push apart. When there is enough charge built up on the top plate to have a repulsive force greater than the force of gravity pulling it down, it lifts off and flies away.

In these animations by Don Lynch, you can see how the generator builds up charge…

and how the charges distribute through the pans to create the repulsion effect.

Also, the National High Magnetic Field Laboratory has shared this tutorial on Van de Graaffs: https://nationalmaglab.org/education/magnet-academy/watch-play/interactive/van-de-graaff-generator, another fun way to explore this electrifying demonstration!

• Demonstration Highlight: Charged Balloon

Welcome back! Today we’re checking out a classic demonstration and party trick: the triboelectrically charged balloon, demonstration J1-05.

Triboelectricity is the phenomenon whereby some combinations of materials become electrically charged from contact and separation. We sometimes refer to this as “charging by friction,” though this is not really accurate – simple contact causes the charge transfer, not movement, but rubbing the materials together often allows more surface contact and thus more charge transfer.

Once an object carries an imbalance of charge, such as this balloon picking up charge from some fabric or your hair, it will tend to be attracted to objects with a different charge, or even to neutrally charged objects that have enough charge mobility to let an imbalance form via induction.

This simulation, in the PhET Collection and the University of Colorado, shows graphically what happens. Touching the balloon to some fabric causes it to pick up an imbalance of charge. While close to the fabric, it tends to cling there, as the unlike charges attract. Once brought near the wall, though, we see an induced imbalance of charge form, as charges like that on the balloon are slightly repelled and charges unlike that on the balloon are slightly attracted, causing the balloon to cling to the wall in electrostatic attraction.

• Demonstration Highlight: Discharging capacitor

Perhaps our most popular and dramatic demonstration of capacitance is demo J4-32: Discharging a Capacitor with a Bang. You can see it in action in this video with physics student John Ball.

Energy is stored in the electric field inside the capacitor. When a circuit is completed by placing a conductor across the poles of the capacitor, that energy is quickly released. Unlike a battery, a capacitor stores energy directly in its internal electric field, which can allo0w it to discharge very quickly. In this case, it does so so rapidly that it actually breaks down the air around the electrodes, discharging with a loud bang and flash of light.

You can experiment with capacitance safely, and learn more about the physics underlying their use, in these simulated Capacitor Labs:

At oPhysics: https://www.ophysics.com/em5.html

• Demonstration Highlight: Eddy Current Pendulum

Today we’re looking at an exciting demonstration of electromagnetic induction: The Eddy Current Pendulum, seen in this video starring physics student Dan Horstman:

We have a very strong permanent magnet mounted at the bottom of an aluminum stand. We can install a variety of pendula to swing from the top of this stand. As the pendulum swings, the bob passes between the poles of the magnet. With a wooden bob, the pendulum swings freely, just like we would expected it to do without the magnet there; this is unsurprising.

A conductive copper pendulum bob, though, shows very different behaviour. While copper is not innately attracted to a magnet the way iron is, it is an electrical conductor. As the copper plate passes through the magnetic field, it experiences a changing magnetic flux. The laws of electromagnetism tell us that a conductor in this situation will have an induced electric current.

Loops of current, called eddy currents, form in the pendulum bob. These currents have their own magnetic fields, which interact with the magnetic field of the permanent magnet and slow the pendulum’s motion. The energy of the pendulum’s motion is gradually dissipated in this way, and the pendulum slows and stops.

This is a nice way to see electromagnetic induction in action. This effect also has many practical uses. Just like the magnetic field slows and stops the swing of the pendulum, eddy currents can be used to make brakes for vehicles! Automobiles, trains, and even roller coasters can use this process to slow their wheels without friction, reducing wear.

But there are other cases where you actively want to prevent eddy currents – if you’re trying to avoid losing energy! For example, conductive components of electrical transformers might be made with insulating gaps to make it harder for eddy currents to form, so you lose less of your electrical energy to heating up the transformer. Metal pendula with interruptions can model this behaviour as well.

• Demonstration Highlight: Electric Fields

Electric fields are an important topic in physics, and one that’s particularly challenging to demonstrate clearly in the classroom. You can find several demonstrations of this in section J3 of our catalog, wherein we qualitatively trace out electric field lines around a Van de Graaff Generator or a Wimshurst Machine, and have a variety of ways of gathering electrostatic potential and showing how it interacts with conductors in different configurations. One very popular one, demonstration J3-08, uses paper streamers around one or two Van de Graaff Generators to show how the fields bend and interact.

Understanding electric fields is important for everything from understanding the structure of matter to communications technology to the behavior of living cells. The electric field is a vector field of the electrostatic force (strength and direction) on a hypothetical charge placed at any given point. Thus, it is usually measured in Newtons per Coulomb or Volts per Meter.

We have an article in our Directory of Simulations with several ways of experimenting with electric fields virtually. Try out this one at oPhysics, by Tom Walsh. You can model our paired Van de Graaff Generators above as a pair of identical charges, and see the structure of the resulting electric field. Compare how this would change if one of the generators had the opposite charge, or if we used four generators instead of two.

Check out other simulators as well, and see what you can find!

• Demonstration Highlight: Electromagnet

Sometimes powerful things come in small packages, and this electromagnet is no exception! It features in two popular demonstrations in our collection, J6-01: Electromagnet with Bang and J6-04: Low-Power High-Force Electromagnet. These two demonstrations are frequently used, separately or together, in a variety of physics classes.They also featured in our popular Physics of Fantastic Worlds program!

This small electromagnet is powered by a single flashlight battery. But it is quite strong. In the first demonstration, we see a heavy block of steel being held up by the electromagnet. When we flip the switch to turn the electromagnet off, though, it falls to the table with a bang.

In the second, the electromagnet and a small steel plate are mounted on handles. If students grab the handles and touch the plate to the magnet, they cling together so tightly that even quite strong people cannot pull them apart. But flip the switch to turn off the electricity, and they fly apart!

But what is an electromagnet, and why does it work? Let’s find out.

The battery produces an electrical potential that causes a current to flow through the wire in the coil when the switch is closed. A current can only flow when the circuit is complete.

Maxwell’s Equations of Electromagnetism tell us that moving electrical charges, such as an electric current, create a magnetic field around it. This magnetic field acts just like the magnetic field of the permanent magnets we’re familiar with, like refrigerator magnets. The strength of the magnetic field is determined by the amount of current passing through an area.

(image credit: Wikimedia user Chetvornohttps://commons.wikimedia.org/wiki/File:Magnetic_field_of_wire_loop.svg)

Here we see a diagram of the magnetic field around a single loop of wire. We can see that the field wraps around the wire, so the direction of the force from the magnetic field will be different depending on where you are around the wire.We can see this field in motion in this animation from Penn State - click here!. See the animation "B Field Lines Due to a Current Loop."

The direction of the field also depends on which way the current flows; try this out in this simulator at JavaLab - click here!

• Imagine the field around that single loop in the illustrations above turned on its side, lined up with more like it.If you flipped one of the wire loops around, its field would be oriented the other way, leaving a slightly weaker point in the field; but if you flipped all of them at once, the field of the entire coil flips directions. Try this out with the simulator!

• You can also flip the battery aroundin the simulator to change the direction of the currentflowing through the wire.

Compare for yourself: what happens if you change the direction of the wire, or change the direction of the current, or both at once?

The force from a single wire is not very strong, especially with only a small electric current. You could make a stronger electromagnet by having a power source with a higher electrical potential to make a stronger current;but that might not be very practical, and would certainly be more expensive.

But if we have many loops of wire, and line them up so that the fields all are aligned, then the small magnetic force from each wire will add up to a much stronger force. This is how a strong electromagnetlike the one in the photographs aboveis built.

You can also try this at home; check out instructions to build your own small electromagnet on our outreach page, and try some experiments with it! Get the PDF here!

• Demonstration Highlight: Parallel Plate Capacitor

Today we’re visiting the ever-popular Parallel Plate Capacitor, in its simplest form found in our demonstration index as J4-01, or with a dielectric plate at J4-22. A capacitor stores energy in the electric field between its plates. The capacitance of a capacitor is technically the amount of charge stored per volt – in a sense, how capable it is of storing charge at a given potential. In a parallel plate capacitor, the capacitance goes up with greater surface area, and goes down with greater separation between the plates.

The parallel plate capacitor consists of two large aluminum plates with an air gap. The capacitor is charged with a potential of around 1000 Volts using a low-current DC power supply. The plates may then be separated and the voltage observed, demonstrating that for a fixed amount of charge, the voltage is proportional to the plate separation.

But if you insert a dielectric sheet into a charged capacitor, the voltage goes down, which means the total capacitance of the system has gone up! The capacitance of a system depends on the dielectric constant of the medium – for air, this is very nearly the same as pure vacuum, but some materials have a much greater dielectric constant. This plastic plate has a dielectric constant nearly 5 times that of air.

Now, try out these simulations to see if you observe the same behaviour!

The first, from the PhET collection at the University of Colorado, places a capacitor in a simple DC circuit. In the first simulation tab, you can adjust the input voltage and plate size, and measure the electric field, capacitance, and energy stored. The additional tabs show variations: you can add a dielectric to the capacitor, or place multiple capacitors in series and parallel.

The second simulation, at oPhysics, additionally lets you control many characteristics within an idealized circuit. Compare the results you get for different combinations of capacitor size and input voltage between the two simulations.

Our capacitor has 22cm diameter circular plates, rather than the square plates used in the simulations. In the simulations, try setting the plate area to be the same as ours and see how it responds to other voltages.

• Demonstration Highlight: Tesla Coil, Part 1

Welcome back! This week and next, we’re spending some time with the ever-popular Tesla Coil, demonstration K7-61 in our demonstration index. You can see it in action in this video with graduate student Haotian Wang

A Tesla coil is a particular type of transformer circuit, a tuned or resonant transformer, that can produce very high voltages. While transformers are used in many applications in technology to transfer energy and convert to different voltages, a resonant transformer incorporates an inductor-capacitor combination, or LC circuit (from the traditional notation on circuit diagrams for inductors and capacitors), to both step up the voltage and to temporarily store energy, releasing it in dramatic bursts.

This kind of induction device was historically used as part of early spark-gap radio systems. Today they are primarily used for research, education, and entertainment; although small Tesla coils also have applications for detecting minute changes in the conductivity of their surroundings, making them useful for testing vacuum seals in equipment.

• Demonstration Highlight: The Force on a Current in a Magnetic Field

Welcome back! This week we’re taking a look at demonstration K1-03, which shows the effect of the force that acts on an electrical current in a magnetic field.

A stiffened wire is suspended between the poles of a strong magnet. When we turn on a current through the wire, the wire leaps outwards. If we swap which end of the wire connects to which end of the battery, the direction the wire moves is reversed. What just happened?

The force in play here is known as the Lorentz Force, named for Dutch physicist Hendrik Lorentz, who formalized the equations for this force based on earlier work by James Clerk Maxwell, Oliver Heaviside, and many other scientists. Today Lorentz is most popularly associated with his later work on relativity, supporting the work of Albert Einstein; but this work partly grew out of his early studies of how charged particles interact with electric and magnetic forces.

Lorentz’s equation states (in part) that if an electrically charged particle is moving through a magnetic field, that particle experiences a force proportional to its velocity and to the strength of the magnetic field, but perpendicular to both. So as the electrons flow through the wire, passing through the field of the horseshoe magnet, the resulting force will push the electrons (and thus the wire carrying them) sideways, out from between the poles. If we change the direction of the electrons’ velocity, by swapping the direction of the current, then the direction of the force is reversed; the same happens if we reverse the direction of the magnetic field, by flipping over the magnet.

You can see this in action in an animated simulation from the National High Magnetic Field Laboratory, here: https://nationalmaglab.org/education/magnet-academy/watch-play/interactive/lorentz-force

Click on the switch graphic to complete the circuit, and you can see the motion of the wire. Buttons will let you change the direction of the magnetic field and the current; try one, then the other, then both, and see if what happens matches what you expect!

• Demonstration Highlight: The Tesla Coil, Part Two

We’re paying a second visit to the Tesla Coil today, exploring more about how it works.

Broadly speaking, we can wave our hands at the Tesla Coil and talk about inductance and resonance, but what does that really mean, and how does it lead to those lovely purple sparks?

Electromagnetic induction is the process by which a voltage is produced across an inductor in a changing magnetic field. In this case, we’re taking advantage of the studies of Maxwell and Faraday that showed the relationship between electricity and magnetism. An electrical current generates its own magnetic field; a changing electrical current thus produces a changing magnetic field, and so a changing electrical current in one conductor can induce a current in a nearby conductor. We can carefully choose these to create higher or lower induced voltages.

Electrical resonance occurs when a circuit is built to have a particular resonant frequency, at which the impedance (the way a circuit element resists an alternating current) of different components cancels out to let the circuit build up higher voltages or currents.

Our Tesla coil, circuit above, uses a 5000 volt transformer to charge a large oil capacitor. When the potential across the capacitor reaches the breakdown potential of the spark gap, breakdown across the gap occurs. The spark gap then becomes a conducting part of the RLC circuit, which resonates at a frequency of about 200 kilohertz. The large coil in the resonant circuit is the primary coil of the final transformer and the long coil of very fine wire is the secondary, producing about 200,000 volts at 200 kilohertz.

You can see what’s happening by examining a simulation of a similar circuit’s behaviour, like https://www.falstad.com/circuit/e-tesla.html . The initial transformer creates a high voltage, which eventually builds up enough to exceed the breakdown voltage of the air and make a spark across the spark gap. This then feeds into resonant circuits which build up very high electrical potential, which can create the discharge we see.

This uses Paul Falstad’s Circuit Simulator Applet, which you can explore further at https://www.falstad.com/circuit/index.html

Richie Burnett’s Operation of the Tesla Coil: http://www.richieburnett.co.uk/operation.html

Wikipedia: https://en.wikipedia.org/wiki/Tesla_coil

Kelley and Dunbar, “The Tesla Coil,” American Journal of Physics 20(32). https://doi.org/10.1119/1.1933098

• Demonstration Highlight: The Theremin

Demonstration J4-51: The Theremin is a fun and exciting way to illustrate electrical capacitance. You can see it in action in this new video with Angel Torres.

The theremin is an electronic musical instrument invented in the early 20th century by Russian scientist, engineer, and cellist Leon Theremin. As well as his musical work, Leon Theremin developed many other electronic devices in his career as an engineer, including an early motion detector and listening devices for espionage.

The two metal “antennas” on the sides of the theremin are not antennas in the usual sense. Each one functions as one plate of a capacitor. When you move your hand near the antenna, your hand serves as the other plate of that capacitor. Thus, each functions as a variable capacitor, where the capacitance, the ability of this air-filled capacitor to store electrical charge, varies as you move your hand and body near the antenna.

Each of these capacitors is part of a variable RLC (resistor-inductor-capacitor) oscillator circuit. One of these variable oscillators controls a second internal oscillator circuit; these together create the output frequency (or pitch) of the sound from the theremin.

The other variable oscillator, meanwhile controls the output amplitude. The resulting signal is fed through an amplifier circuit to a speaker. Together they form an electronic system that can create music, controlled by the motions of your body – without the player ever actually touching the device.

The theremin has been used in a wide range of music. Much of the early technique of playing it was developed by classical violinist and thereminist Clara Rockmore. The theremin can be heard in the work of orchestral composers like Dmitri Shostakovich and Percy Grainger, and in rock bands including the Rolling Stones and Led Zeppelin. And you can hear it on the sound tracks of movies ranging from Cecil B. deMille’s The TenCommandments, to the science fiction classic The Day the Earth Stood Still, to the 2006 animated film Monster House.  It’s a beautiful way to see and hear the fusion of art and science.

Our theremin seen here was built by the Moog Corporation, best known for their electronic synthesizers.

• Demonstration Highlight: Visible and Invisible Spectra

A recurring favourite optics demonstration in many of our classes is N1-05 Spectra: Visible and Invisible. This seemingly simple setup can show us some important truths about electromagnetic radiation.

A carbon arc lamp is used to create a bright, broad-spectrum white light. This is an example of what is known (confusingly) as blackbody radiation, the light that an object emits due to its temperature. Technically, a hot object radiates light across many frequencies, but what we think of as its “color” is made up of the frequency ranges with the greatest intensity, which depends on the object’s temperature. This we see here a bright blue-white light, with high emission across all the visible frequencies.

Lenses right next to the source focus this light onto a narrow slit, which then passes a narrow beam of light, focused by an additional lens, to a prism. The prism refracts the light at different angles depending on its frequency. So projected onto the wall we will see, rather than a spot of bright white light, a spectrum of all the colors making up the light.

But here’s what’s interesting about this carbon arc lamp. Not all of the light is in that visible range! We have a fluorescent screen, which glows in the visible light range when it absorbs higher-frequency ultraviolet light; using this, we can see that there are bright bands of ultraviolet light off beyond the blue end of the visible spectrum on the wall.

So does this mean there’s something off beyond the red end as well? To check this, we have a thermopile, a horn containing a series of sensors that sense when they get warm. Using this, connected to an audio oscillator that changes pitch when the thermopile senses heat, we can scan across the wall… and indeed, we can hear the pitch change when the horn is in the dark area past the red end of the spectrum. There is infrared light hiding here, frequencies too low for us to see!

You can vary the temperature of your source and see how that changes not only the intensity of light, but its color – or, more accurately, its distribution of color. A light source can radiate light across a broad range of frequencies, which may be centered within, above, or below the range we can see. Try it out for yourself, compare the spectra of a household lightbulb to the Sun or another star, and see if you can guess the temperature of the arc lamp we use here!

• Demonstrations Highlight: Lenz's Law

This week we’re taking a look at two related demonstrations of Lenz’s Law, K2-42 and K2-43.

We’ve seen before in our blog that a moving conductor in a magnetic field generates eddy currents, and that these eddy currents have their own magnetic fields that can interact with the original magnet.

Lenz’s Law formulates this more precisely: that if a conductor has an electrical current induced in it by a magnetic field, that current will be in a direction such that the magnetic field it creates opposes changes in the initial magnetic field.

Heinrich Emil Lenz was a 19th century physicist who spent much of his career teaching at the University of St Petersburg in Russia. Prior to this, he spent several years doing research at sea, studying meteorology and the properties of seawater. He is best remembered today, though, for his work on electromagnetism, including his law of electromagnetic forces published in 1834.

This is what allows things like magnetic braking, as discussed in the blog post linked above, to work – the continued movement of the conductor will change the magnetic field passing through it, so the induced current opposes this motion, slowing the swing of the pendulum. We can see this illustrated more clearly in a few more demonstrations.

Demonstration K2-43 has some simple conducting coils, one copper and one aluminum, hanging from strings. When you push a horseshoe magnet through the coils, the coil is dragged along with the magnet, as the magnetic field from the induced current is resisting the change in the field – and thus the coil moves to keep up! Conversely, in demonstration K2-42, if you drop a magnet through a conducting tube, it slows down as it falls, the eddy currents creating a magnetic field that drag back the falling magnet. You can try it out at home with a magnet and a loop of wire. Just be sure the loop forms a complete circuit; as you can see in K2-43, an incomplete loop won’t produce much current!

Or if you don’t have a magnet handy, try out this simple simulation from Michael Davidson of Florida State. As you move the bar magnet on the screen, you can see the current start to flow, and the magnetic field lines of the current appear next to the field lines of the bar magnet. When the magnet is stationary, there is no change in the magnetic field, and so no current is produced until you move it again.

• Happy birthday, Alexander Müller

Today marks the birthday of Swiss physicist K. Alexander Müller, who shared the 1987 Nobel Prize in Physics with Georg Bednorz for their discovery of the first high temperature superconductor.

Born on April 20th, 1927 in Basel, Switzerland, Alex Müller attended the Eidgenössische Technische Hochschule Zürich, the Federal Institute of Technology at Zurich, where he received his PhD in 1957. He worked at a variety of institutions throughout Switzerland, studying various aspects of what we now term Condensed Matter Physics.

In the 1980s, Müller and Bednorz were working together searching for high temperature superconductors. “High temperature,” in the context of superconductors, can be misleading to newcomers, as they are still very cold!

Superconductors are materials whose resistance drops to zero at low temperatures. These materials have many fascinating properties – they can transmit electricity with no loss, and they repel all magnetic flux. Generally, a superconductor has a criticaltemperature below which it exhibits superconducting properties; above this temperature it does not, behaving as ordinary materials do. For many superconducting materials, and all of those discovered in the first seventy years of them being studied, this temperature is around ten to twenty Kelvin, a temperature very difficult to achieve, maintain, or work with.

Müller and Bednorz, however, in 1986 discovered a ceramic compound material, lanthanum barium copper oxide, with a critical temperature of 35 Kelvin. Still very cold, but a definite improvement! More crucially, in addition to showing that higher critical temperatures were possible, they showed that superconductivity could be achieved in ceramics, driving other researchers to investigate similar compounds for this effect. Within a year, other such materials had been discovered, including the now popular yttrium barium copper oxide by Paul Chu of the University of Houston. This new material had a critical temperature of 92 Kelvin!

92 Kelvin is still almost -300 degrees Fahrenheit below zero, obviously much colder than any temperature found naturally on Earth! But it is much warmer than the early metal superconductors. And crucially, it crosses an important line: 77 Kelvin is the temperature of liquid nitrogen, a refrigerant that is much cheaper and easier to manufacture than the liquid helium used in earlier studies, and vastly easier to work with. Since these newest materials can exhibit superconducting behaviour at liquid nitrogen temperatures, it means we can use them in practical technology and experiment with them more easily… including in classroom demonstrations!

We currently have two demonstrations that use high temperature superconductors, both taking advantage of their effect of excluding magnetic flux. Demonstration I7-21: Superconductor – Magnet Levitation uses a yttrium barium copper oxide (YBCO) disc bathed in a liquid nitrogen bath. When a small permanent magnet is placed on top of the disc, the strong magnetic field is repelled from the superconductor, so strongly that the magnet itself levitates above the disc!

Taking the opposite approach, demonstration I7-23: Magnetic Track and Superconductor, built by our own Don Lynch, consists of an array of powerful neodymium magnets. A puck of high temperature superconducting material wrapped in a Teflon sheath is soaked ahead of time in liquid nitrogen, cooling it down such that it will hold its temperature for a few minutes. The puck is cooled while resting above a small block of magnets. When taken out of its bath and placed on the track, it again holds itself at the same height above the magnets of the track.

Resources:

The Nobel Prize in Physics 1987 at nobelprize.org

J. G. Bednorz and K. A. Müller (1986). "Possible highTc superconductivity in the Ba−La−Cu−O system". Z. Phys. B64 (1): 189–193.

Originally appearing in our demonstration catalog as J3-23 and now as the updated K8-46, the Faraday Cage and Radio Waves demonstration is a popular way of showing how a conductive surface interferes with the passage of electric fields, and thus can prevent the transmission of electromagnetic waves.

Now, our own Don Lynch has created an animation of the physics behind this demonstration; check it out below!

The Lecture-Demonstration Facility is introducing a series of teaching aid animations of popular demonstrations; watch for more coming soon!

• Highlight: Van de Graaff Generator Animation

The Van de Graaff Generator is our most popular way of demonstrating electrostatic phenomena in the classroom. Don Lynch has created a new animation to help illustrate how this beloved device works.

The Van de Graaff generator uses essentially similar triboelectric phenomena to the classic charging by friction demonstrations we often use at the beginning of any class on electricity, and that you have probably performed by accident on dry winter days in carpeted rooms. A grounded comb at the bottom exchanges charges with the moving belt, then this surplus of charge is deposited through the upper comb on the large conductive dome. The individual identical charges repel each other, so they become distributed across the entire outer surface of the dome.

You can find this animation, along with a growing collection of others, on our Teaching Aids page!

(And always remember: It’s Van de Graaff, with two F’s. With one F, it’s the band. Don’t mix them up!)

• Light Up the Night: Neon and "Neon" Lights

Lit up on a billboard so everyone sees them in neon

-In Neon, Elton John

For a century, neon lights have been synonymous – even metonymous – with marketing, entertainment, and nightlife. This month marks the 105th anniversary of neon lighting being patented in the US, so it seems like a good time to explore the physics behind the lights.

Gas discharge lighting uses the ionization of a gas to produce light. An electric field is created in the tube that strips some of the outer electrons from the atoms of the gas in the tube. These free electrons flow towards the positively charged anode on one end of the tube, while the now positively charged atoms flow towards the negatively charged cathode. They collide with other neutrally-charged atoms, exchanging charge and gaining energy from the collisions; when these now higher-energy atoms release this excess energy, they produce light. The color of this light is determined by the structure of the atoms; each produces a distinctive spectrum.

When first created as an experiment, these tubes were filled with a variety of gases, and often burned out quickly. French scientist Georges Claude discovered that using noble gases like neon and argon, which do not react with most other materials, greatly extended their lifespan; and also developed better ways of keeping the electrical power steady in the tube. This made them practical for lighting and signage, and he was able to patent the device;  a patent was approved in the US on January 19, 1914.

Lights like this have been used for many other purposes in the past, as well – many outdoor lamps use gas discharge lighting, though this is now being replaced in many places by more energy-efficient LED lighting. So soon we may only see gas discharge lighting in the laboratory and in artistic applications like these.

We often use straight tubes with a single gas in them in the classroom to demonstrate the spectrum of an element or compound. Many classes have used these when introducing the concept of spectroscopy or when discussing the structure of atoms; check out demonstration N2-05: Diffraction Spectra, which can be used with lightweight diffraction gratings to let every student see the spectrum of some common or popular elements. This has consistently been one of our more popular demonstrations. Less often used, though, are these classic discharge tubes (sometimes called Geissler tubes) found at P3-24. They can be a fun addition to any discussion of ionization and atomic structure.

Despite the common name, many of these signs today aren’t actually filled with neon, but argon; other gases can be used as well. In the pictures here, only one light has the distinctive red glow of neon. Colloquially, though, they tend to all be referred to as “neon lights,” if only because “gas discharge tube light” is hard to fit into a rock lyric.

• New Demonstration: The Paramagnetism of a Dysprosium Pendulum

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 20th 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 19th century.