MAGNETS: How Do They Work?

How do magnets work? Why do they attract and repel at long distances? Is it magic? No... it's quantum mechanics, and a bit more, as we explain in this, the longest MinutePhysics video ever.

 

 Magnetism seems like a pretty magical phenomenon. Rocks that attract or repel each other at a distance - that's really cool - and electric current in a wire interacts in the same way. What's even more amazing is how it works. We normally think of special relativity as having little bearing on our lives because everything happens at such low speeds that relativistic effects are negligible. But when you consider the large number of charges in a wire and the strength of the electric interaction, you can see that electromagnets function thanks to the special relativistic effect of length contraction. In a frame of reference moving with the charges, there is an electric field that creates a force on the charges. But in the lab frame, there is no electric field so it must be a magnetic field creating the force. Hence we see that a magnetic field is what an electric field becomes when an electrically charged object starts moving.

 

Bullet Block Experiment

A bullet is fired upward into a wooden block.  Does the block raise higher if the bullet hits its center of mass, or if the bullets hits elsewhere?

Experiment, prediction, results:


Explanation:

How To Make a Quantum Bit

We have looked at how a transistor works, the fundamental unit of classical computers, and how a quantum computer works in theory, taking advantage of quantum superposition to hold exponentially more information than classical computers. Now we look at the practical side of making a quantum bit, or qubit. How do you put it in a state where it is stable? How do you read and write information on it? These processes are described for a solid state qubit - a phosphorous atom in a silicon crystal substrate. Both the electron and the nucleus of the phosphorous atom can be used as qubits.

 

How Does a Transistor Work?

Transistors, semiconductors, doping.

 

The Truth About Electrons and Photons

Can we really touch things? Well if by touch we mean exchange a force-carrying particle with, then yes. The photon is the force-carrier of the electromagnetic interaction. But if the photon is also a particle of light then why aren't magnets glowing? Because the photons are virtual particles, which means they can't be directly detected (without changing the outcome we are trying to measure).

 

The Vacuum is NOT Empty

An atom is mostly empty space, but empty space is mostly not empty. The reason it looks empty is because electrons and photons don't interact with the stuff that is there, quark and gluon field fluctuations.

It actually takes energy to clear out space and make a true 'empty' vacuum. This seems incredibly counter-intuitive but we can make an analogy to a permanent magnet. When at low energies, like at room temperature, there is a magnetic field around the magnet due to the alignment of all the magnetic moments of the atoms. But if you add some energy to it by heating it, the particles gain thermal energy, which above the Curie temperature makes their magnetic moments randomly oriented and hence destroying the magnetic field. So in this case energy is needed to clear out the field, just as in the quantum vacuum.

 Special thanks to Professor Derek Leinweber, find out more about his research here.

 

Why the Sky ISN'T Blue

The sky appears blue because of Rayleigh scattering...but why doesn't it appear violet? If the hard disk of a laptop spins in the space station, does the laptop spins? What if you were able to pass the event horizon of a black hole and come back? Is there such a thing as randomness?

 And other questions...

 

Your Mass is NOT from Higgs Boson

The Higgs mechanism is meant to account for the mass of everything, right? Well no, only the fundamental particles, which means that electrons derive their mass entirely from the Higgs interaction but protons and neutrons, made of quarks, do not. In fact the quark masses are so small that they only make up about 1% of the mass of the proton (and a similar fraction of the neutron). The rest of the mass comes from the energy in the gluon field. Gluons are massless, but there is so much energy in the field that by E=mc^2 there is a significant amount of mass there. This is where most of your mass comes from and the mass of virtually everything around you.

 

World's Roundest Object!

The world's roundest object helps solve the longest running problem in measurement -- how to define the kilogram. A kilogram isn't what it used to be. Literally. The original name for it was the 'grave', proposed in 1793 but it fell victim to the French Revolution like its creator, Lavoisier. So begins the tale of the most unusual SI unit. The kilogram is the only base unit with a prefix in its name, and the only one still defined by a physical artifact, the international prototype kilogram or IPK.

But the problem with this definition has long been apparent. The IPK doesn't seem to maintain its mass compared to 40 similar cylinders minted at the same time. The goal is therefore to eliminate the kilogram's dependence on a physical object. Two main approaches are being considered to achieve this end: the Avogadro Project and the Watt Balance.

The Avogadro project aims to redefine Avogadro's constant (currently defined by the kilogram -- the number of atoms in 12 g of carbon-12) and reverse the relationship so that the kilogram is precisely specified by Avogadro's constant. This method required creating the most perfect sphere on Earth. It is made out of a single crystal of silicon 28 atoms. By carefully measuring the diameter, the volume can be precisely specified. Since the atom spacing of silicon is well known, the number of atoms in a sphere can be accurately calculated. This allows for a very precise determination of Avogadro's constant.

Special thanks to Katie Green, Dr. David Farrant, the CSIRO, and the National Measurment Institute for their help. Thanks also to Nessy Hill for filming and reviewing earlier drafts of this video.

Music by Kevin McLeod (incompetech.com) Decision, Danse Macabre, Scissors

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Single Photon Interference

What happens when single photons of light pass through a double slit and are detected by a photomultiplier tube? In 1801 Thomas Young seemed to settle a long-running debate about the nature of light with his double slit experiment. He demonstrated that light passing through two slits creates patterns like water waves, with the implication that it must be a wave phenomenon.

 However, experimental results in the early 1900s found that light energy is not smoothly distributed as in a classical wave, rather it comes in discrete packets, called quanta and later photons. These are indivisible particles of light. So what would happen if individual photons passed through a double slit? Would they make a pattern like waves or like particles?

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What Is Light? Young's Double Slit Experiment

Light is so common that we rarely think about what it really is. But just over two hundred years ago, a groundbreaking experiment answered the question that had occupied physicists for centuries. Is light made up of waves or particles?

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Heisenberg's Uncertainty Principle Experiment

Heisenberg's uncertainty principle tells us that it is impossible to simultaneously measure the position and momentum of a particle with infinite precision. In our everyday lives we virtually never come up against this limit, hence why it seems peculiar. In this experiment a laser is shone through a narrow slit onto a screen. As the slit is made narrower, the spot on the screen also becomes narrower. But at a certain point, the spot starts becoming wider. This is because the photons of light have been so localised at the slit that their horizontal momentum must become less well defined in order to satisfy Heisenberg's uncertainty principle.

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Space Junk & The Swiss 'Janitor Satellite'

Space junk is a real problem. NASA now tracks around 20,000 pieces of debris orbiting Earth, most of them larger than 10cm across. Since the average speed of a collision between orbiting objects is 10 km/s these pieces of trash can cause a lot of damage despite their small size. Scientists in Switzerland have a plan to clean up space junk - it involves creating a 'Janitor Satellite' that will seek out pieces of space junk to drag back into the atmosphere causing them to burn up. This video was created for the Aussie science show Catalyst on the ABC.

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Higgs Boson Update From CERN

"For a report on ABC's Catalyst program (http://www.abc.net.au/catalyst/), I visited the Large Hadron Collider in Switzerland to find out what is being done now that the Higgs Boson has been discovered.

 Although its mass has been measured around 125-126 GeV most of the other properties of the particle remain unknown. Its spin appears to be 0 or 2 but more results are required to nail this down. If it is the standard model Higgs, the spin should be 0, resulting in a fairly symmetric distribution of decay products in the detectors.

 We may know this year if it's not the standard model Higgs - this would be the case if it doesn't decay into specific particles with the expected frequency. However if it is the standard model Higgs, it may take many more years to be certain. The large hadron collider will be shut down in 2013 for upgrades so that higher energies up to 14 TeV can be tested. Right now the LHC is operating at 8 TeV. The next announcement is expected in December."

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Levitating Barbecue! Electromagnetic Induction

At the Palais de la Decouverte in Paris, they showed me this experiment where a 1kg aluminium plate is levitated above a large coil of wire that is being supplied with 800A of alternating current at 900Hz. This is by far the best demonstration of electromagnetic induction I have ever seen. Back in London, I visited the magnetic lab of Michael Faraday in the basement of the Royal Institution. It was here that he did his groundbreaking work on induction. People had previously observed that current in a wire causes a compass needle to deflect, but more exciting was the prospect of using a magnetic field to generate current. Faraday created his famous induction ring by winding two coils of insulated wire onto an iron ring. When he connected a battery to one coil, a small pulse of current was induced in the other. When the battery was disconnected, current was induced in the other direction. This led Faraday to the conclusion that current was induced in the second coil only when the magnetic field through it was changing. And if they hadn't been wrapped on the same ring, Faraday may have noticed that the two coils repel each other when the current is induced due to the interaction of their magnetic fields. This is the same thing that is happening with the aluminium plate, except we're using alternating current to create a continually changing magnetic field. This induces an alternating current in the plate, producing an opposing magnetic field which levitates the disk.

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Misconceptions About Falling Objects - ABC Catalyst

A basketball and a 5 kg ball: which one will hit the ground first?
This is a remake of this video.

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Solid Nitrogen, Vacuum Cooling and Dry Ice

What happens when you decrease the pressure around a liquid? It boils. Water boils at room temperature once the pressure is low enough. What is interesting is that this decreases the temperature of the liquid. The fastest molecules escape, leaving the slower ones behind. Using this trick with liquid nitrogen, it is possible to create solid nitrogen at a temperature of -210C. We then poured the solid and liquid nitrogen mixture onto a tray of water. The surface of the water became so cold that CO2 solidified out of the atmosphere on its surface. Then, since CO2 does not pass through the liquid phase at atmospheric pressure, it was propelled on the water surface by jets of gas as it sublimed. Huge thanks to the Palais de la Decouverte. Music by Kevin MacLeod of Incompetech.com (Mirage)

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Electric Flame

Is a flame really a plasma? Well it depends on your definition of plasma, but there are certainly ions in a flame, formed as molecules collide with each other at high speed, sometimes knocking electrons off of their atoms. Special thanks to the Palais de la Decouverte for helping me perform this experiment. Using tens of thousands of volts on two metal plates, we created a strong electric field around the plasma. This pulled positive ions in one direction and negative ions in the other direction elongating the flame horizontally and causing it to flicker like a "papillon" (butterfly). Then we showed that much longer sparks can be made through the flame than through air since the ions increase the conductivity.

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Misconceptions About Temperature

Made for ABC TV Catalyst  as an extended version of "Comparing Temperatures" video.

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How Does A Boomerang Work?

A boomerang can execute its unique roundtrip flight by making use of three fundamental physics principles: lift, relative velocity, and gyroscopic precession.

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