## Tuesday, October 25, 2011

### Magnetism Explained: 5 Electromagnetism

Magnetism + Electricity = Electromagnetism

Magnetism and electricity cannot be separated from one another. Whenever you have a moving charge, you have magnetism and whenever you have a changing magnetic field, you have electricity. For example, let's move a copper (electrically conductive) wire through the magnetic field of a stationary magnet. You will get eddy currents of electricity in the wire because of the magnetic force acting on the electrons inside it. Now, let's hold the wire stationary and move a magnet across it. Here, exactly the same eddy currents will be produced but classical electromagnetic theory says they are the result of an applied electric force, rather than a magnetic force! Albert Einstein called this the "moving magnet and conductor problem" in his 1905 paper on special relativity. The current in the wire experiences a magnetic force in the frame of reference of the magnet and an electric force in the frame of reference of the wire itself. This consistent and predictable alteration from one measurement to another is called a Lorentz transformation. It simply means that varying measurements of magnetic and electric fields can be converted into each other's frame of reference. This transformation holds up equally well for space and time under the theory of special relativity, which is explored in my article, "Time." In our case, the same phenomenon is either electric or magnetic in nature depending on your frame of reference. Electricity and magnetism are different aspects of the same force, the electromagnetic force. Maxwell's equations predict this transformation and it is through them that the unified concept of electromagnetism was created.

Electromagnetism = Light

The electromagnetic force is behind almost all the phenomena we encounter in daily life, with the exception of gravity. It holds electrons and protons together inside atoms (as virtual photons) and it holds atoms together within matter. It governs all the processes involved in chemistry, and that is why it should come as no surprise that the ways in which some atoms arrange themselves and interact electrically with each other has so much to do with distinguishing one kind of magnetism from another, as we have seen in the previous two articles, "Ferromagnetism" and "Other Kinds of Magnetic Ordering."

Michael Faraday and James Maxwell gave us our first understanding of how magnetism and electricity work together as one unified force. Maxwell used his electric and magnetic equations to derive the waveform of electromagnetism. His wave equation predicted the speed of light, leading him to conclude that light itself is an electromagnetic wave. A light wave, according to his formulae, consists of a spatially shifting electric field causing changes in a magnetic field. Each field causes the other one to shift as they move forward in the same direction, and this is how electromagnetic (EM) fields propagate. Photons are the force carriers of propagating EM fields. This animation shows how it works, with the electric field in the vertical plane, Z, and the magnetic field in the horizontal plane, X.

Λ (lambda) represents one wavelength, E represents the electric oscillation and B represents the magnetic oscillation. This particular EM wave is polarized - each of the red and blue fields is perfectly aligned along one plane. All EM radiation, including visible light, propagates as a transverse oscillating wave of electric and magnetic fields. The energy of this propagation travels at the speed of light, as photons. All EM radiation behaves like this. The only part that changes, making red light different from blue light, for example, is the wavelength. When wavelength changes, frequency changes as well, because the two are inversely proportional to each other. Blue visible light  has more energy than red light because its wavelength, about 400 nm, is shorter than that for red light, which is about 700 nm. Shorter wavelength EM waves  have more energy than longer waves. You can think of an EM wave as a skipping rope. If you try to make your skipping rope have more waves in it, you need to apply more energy with your hand. This 5-minute NASA video introduces you to EM waves, and shows you how EM radiation fits into our modern lives:

How Is An EM Wave Created?

We know that when electrons move, they create a magnetic field. When electrons move back and forth, or oscillate, their electric and magnetic fields change together and this creates an EM wave. An EM wave consists of one or more photons, which are spontaneously emitted from atoms with sufficiently high kinetic energy. Here is an example of how it works: An atom absorbs energy when a photon collides with it. If the photon has enough energy, the atom it strikes will become excited. The atom's potential energy increases, as a result, and one of its electrons moves from ground state to an excited state. The atom, like any physical system, wants to maintain the lowest possible energy state, so the extra energy that the electron absorbed is released as a photon (with exactly the same energy). The atom emits the photon and as it does so, the excited electron moves back down to its ground state. The atom returns to its original energy state. For interest's sake, if the incoming photon has extremely high energy, say a gamma ray for example, the electron will move up to state of maximum potential energy and then leave the atom altogether. The atom loses this electron and becomes ionized. If you are interested in examples of phenomena that rely on photon absorption and emission, take a look at my articles called "Radiation: What is Happening in Japan?" and "The Northern Lights." Every kind of atom, every element, has its own ionization energy, above which it becomes ionized.

This brief 2 - minute animation will help you visualize what is going on when an atom emits a photon:

A photon is absorbed when it hits opaque matter. This collision must have enough energy to cause an electron within an atom of the opaque matter to oscillate (this is a transfer of kinetic energy) with enough energy to emit a new photon. Otherwise, the matter is simply warmed up. Its electrons, and therefore its atoms, have increased kinetic energy in general but they are not energetic enough to be in an excited state. If you heat matter, electrons in this matter will eventually begin to emit low energy photons such as infrared photons. You can sense this kind of EM radiation, called thermal radiation, when you stand near a warm stove, for example. Getting back to our matter, if you continue to heat it, the electrons will emit higher and higher-energy photons.

If you want to know more about the nature of light, please see my article called, "The Nature of Light."

Electromagnetic Waves Within Magnetic and Electric Fields

All EM waves can, and do, interfere with each other all the time. This is why we have to deal with radio interference and dropped cell phone calls. However, because EM waves are electric/magnetic in nature, you might be wondering how EM waves act when they travel through magnetic and electric fields, which they, of course, do all the time. Light and other kinds of EM radiation from the Sun, for example, must go through Earth's magnetic field, before it can strike Earth's surface. It is important to keep in mind that electric and magnetic fields can and do bend the paths of charged particles. This is how the cathode rays in old-fashioned TV's work, for example. EM radiation is indeed an electromagnetic phenomenon but, and here is the bullet point, the force carriers themselves, photons, are not charged particles. For this reason they do not interact with magnetic and electric fields, with one very small "but."

There are a few very small and rare interactions that do occur but these kinds of effects not usually apparent to us in our daily lives - if light were bent by EM fields, we would never get clear TV reception nor would we get reliable radio reception. Both TV and radio transmitters create powerful EM fields around them that would bend any EM radiation coming from them (here, I'm using old-fashioned TV's again not digital ones).  But occasional effects do happen and the physics involved can get complicated. One explanation for how it can happen looks at EM radiation at the quantum level. A phenomenon called Delbruck scattering can occur where powerful EM fields can break down a photon into an electron and a positron, both of which carry a charge. When this happens, the path of an EM wave can be altered - usually a type of scattering occurs when the positron and electron, once formed, rapidly annihilate each other and two new lower-energy photons created in the annihilation carry on, in altered directions. This may be part of the answer, but it is not the whole story, as very powerful EM Fields are required. What happens in the far tamer EM fields around Earth?

EM waves do not tend to be affected by fields that are not rapidly changing. Earth's magnetic field is relatively constant and almost all EM radiation passes right through it unaffected. If EM waves pass through static magnetic or electric fields (fields that don't change over time) in a linear medium, they are not affected at all. The vacuum of space is a static linear field. Static simply means non-changing but what does linear mean? In nonlinear fields, interactions can occur, and those effects can be unpredictable. To understand what a nonlinear field is we must acquaint ourselves with systems theory. According to this theory, a field is linear when it satisfies the superposition principle. This means that the net response of a linear system to two or more different stimuli is simply the sum of those responses, which would have been caused by each stimulus alone. If two or more stimuli are applied to a nonlinear system, its net response is not the sum of the stimuli. Other interactions occur between, for example, the stimuli themselves, and this can change the net response, often with unpredictable results (this underlies chaos in chaos theory. In reality, almost all systems are nonlinear in nature, and engineers have to deal with them all the time. For example, the wing of this plane creates a tip vortex, made visible by using coloured smoke, which results in powerful turbulence. Fluid turbulence is an excellent example of a chaotic nonlinear system. This particular phenomenon happens to develop through a strange attractor. This is why planes must maintain a set distance away from each other when they land - the turbulence of landing planes is unpredictable so pilots cannot adjust for it.

Climate and weather are other common examples of complex nonlinear systems. They have a lot of different variables and those variables interact with each other in chaotic and unpredictable ways.

In the case of EM waves, an example of an interaction with a nonlinear field is the Faraday effect. Here, light interacts with a magnetic field in a medium. The Faraday effect causes the plane of polarization of light to rotate. This diagram explains how it works. B is the magnetic field and E represents the polarized electric oscillation of an EM wave.

You can see a large Faraday effect rotation in magnetic garnets, which are transparent to some of the light going through them. Consider a beam of polarized light as a superposition of equal amounts of right and left circularly polarized beams. The magnetic field (inside the garnet) causes each component to experience a slightly different refractive index and that causes the rotation, as one circularly polarized beam slows down and emerges very slightly out of phase with the other component.

This effect happens to radio waves passing through Earth's ionosphere. The ionosphere contains charged ions (most of the effect comes from free electrons), which create a magnetic field. This field, along with Earth's intrinsic magnetic field, rotates the polarization of radio waves. The amount of rotation varies with electron density in the ionosphere and this, in turn, is significantly affected by sunspot activity on the Sun. And, as we just learned, the effect itself is unpredictable. This effect diminishes greatly at higher frequencies, so radar and satellite communications (between 1 and 15 GHz) are not generally affected. (You may have heard of radar jamming, radar noise and clutter, etc. - these are all the effects  of interference). However, interference in VHF televisions (30 - 300 MHz) caused by the Faraday effect can be a serious problem in Japan. HF ham radio operators can also experience signal fading due to ionic interference when they transmit across the poles where the magnetic field is intense. This is an unpredictable Faraday effect that is further complicated by other phenomena such as absorption, multipathing, etc.

Kerr Effect

Along with the Faraday effect, the Kerr effect can affect EM waves. While the Faraday effect tends to cause problems mostly within radio frequencies, the Kerr effect is generally described in terms of visible light. Like the Faraday effect, the Kerr effect is a response to a nonlinear field, in this case an electric field, which can change the refractive index of a material. Here, the effect is not quite as random in nature as the Faraday effect because the change in refractive index is directly proportional to the square of the electric field. All materials show a Kerr effect, but some liquids, such as nitrobenzene, display it particularly strongly. When an electric field is applied to this liquid, its (anisotropic) molecules easily align with the field, changing opaque molecules into transparent ones. This allows someone to control the amount of light passing through a transparent cell filled with this fluid, simply by applying a transient electric field to it. It's a very rapid and reversible effect, limited only by how fast the electric field can be changed. You may have heard of special high-speed cameras capable of taking still images with exposure times as short as 10 billionths of a second. These cameras, called rapatronic cameras, make use of a Kerr cell shutter to film such events as nuclear tests. Click here to see some famous and eerie rapatronic images of atmospheric nuclear tests done in the 1950's in Nevada. The Kerr effect can also come from the electric field produced by the EM radiation itself. Here, light, through the Kerr effect, can change the refractive index of the media through which it is passing. It is responsible for nonlinear optical effects such as self-focusing and modulational instability. This kind of Kerr effect only becomes significant with very intense beams of EM radiation such as lasers.

If you are unnerved by these seemingly mutually exclusive behaviours of EM radiation - it's a wave with electric and magnetic components, no - it's a charge-less particle, the photon, you are in good company. If you want to explore how EM radiation is both, take a look at my article called, "The Nature of Light," where I attempt to explain how both descriptions of light are true.

Geomagnetic Storms

You may have heard about how magnetic storms can potentially cause extensive damage to communications and navigation systems, satellites, the electric grid, and even pipelines. Is this not an example of magnetism effecting EM radiation? Not exactly - this is a great example of magnetic fields interacting with electric fields and the damage-causing mechanism involved is electromagnetic induction, Michael Faraday's famous discovery, and I will explain why.

As you learned in the introductory article in this series, Earth is surrounded by a gigantic magnetosphere. Magnetic storms are caused by periods of unusually intense solar wind. This usually happens when the Sun experiences a coronal mass ejection in which a slew of charged particles is released into space.  This creates a shock wave of solar wind that strikes Earth's magnetosphere and compresses it, while transferring energy from the solar wind magnetic field to Earth's magnetic field. Both interactions increase the movement of charged particles through our magnetosphere. These particles cause especially dramatic aurorae (Northern and Southern Lights), which in themselves are fairly benign. The damage is caused when these solar charged particles increase the electric current in the magnetosphere and ionosphere and push the boundary of the magnetosphere toward Earth's surface. At this point you get communications disruptions as charged particles interact with radio waves, etc. Even more damage happens when Earth's magnetic field lines get interrupted. This can happen with particularly violent magnetic storms. They separate and then snap back together, throwing a tremendous amount of radiation back toward Earth's surface. This is when you get extensive power outages and damage to hardware as electrical components are overloaded through the process of electromagnetic induction. The influx of charged particles induces changing magnetic fields. A changing magnetic field creates a current. Such induced currents are added to the current already within current-carrying wires and can overload them and blow up transformers. These induced currents can also corrode oil and water pipelines, creating a long-term problem. The current created by these charged particles creates a magnetic force that snaps the magnetosphere boundary back out away from Earth, like a rubber band. The largest magnetic storm ever recorded happened in 1859, when communications was restricted to telegraphs. Telegraph operators received shocks as induced electrical currents traveled up the telegraph wires. Paper caught fire. And many telegraph systems continued to receive and send signals even after the operators disconnected the batteries. Such a storm today would cause immeasurable damage.

Conclusion

Magnetism is not a mysterious property of specially treated iron. It is simply the net result of the motion of electrons inside atoms. When the spins of two nearby electrons line up parallel to each other, they create a tiny electrical current and a tiny magnetic field is created. Billions of atoms within molecular arrangements that allow some electron spins to line up with each other create magnetic domains and when enough of these domains line up parallel with each other in a material, you have a magnet. Different kinds of magnetism are simply the net magnetic results of different kinds of atomic arrangements within materials.

Subatomic particles display magnetism as well. Here, it is called the magnetic moment. It is built into electrons and atomic nuclei and it adds up to create atomic and molecular magnetic moments. Magnetism at this level can affect the chemistry of atoms as they combine into molecules but it is not powerful enough to affect the magnetism of macroscopic objects, which is based on the much more powerful magnetic effects of electron spins that are paired across atoms.

When an atom's electron attains sufficient kinetic energy, it vibrates fast enough to create a tiny changing electric charge, a current in essence, and this current spontaneously creates a changing magnetic field. This is the basic mechanism underlying all EM radiation, such as light, gamma rays, and radio waves. The modulation of electric and magnetic fields contains energy, and that energy travels at light speed, along a transverse EM wave, as photons.

1. So electromagnetism is behind almost all phenomena that we experience, except gravity? That makes no sense. Electromagnetism is an attractive/repellant force, gravity is an actractive force. Anyone see the similarities? Can't gravity merely be another aspect of electromagnetism wherein matter with weak electromagnetic properties attract each other weakly, but as more matter comes together their electromagnetic force grows bigger, if not necessarily stronger?

2. There are actually four fundamental forces in physics. Three of them are fairly well understood conceptually: electromagnetism, the strong force and the weak force. In these forces, a gauge boson, which can be a photon, gluon or W and Z bosons, respectively, carries both the energy and the momentum of the interaction. Electromagnetism - electricity and magnetism - are both unique in that these forces can be expressed as dipoles, and here is where repulsion/attraction comes in. You do not see this arise in the strong, weak or gravitational forces. Gravity so far has no known gauge boson associated with it, which means it does not yet fit into the hugely successful standard model of physics, although many theorists are looking for a theoretical graviton. Gravity is unique in that it is the only force that acts on all particles of mass, and therefore, it has something, at least indirectly, to do with the recently discovered Higgs Boson (this something hasn't been worked out yet). Like electromagnetism, gravity has infinite range but, unlike that force, it cannot be shielded against or absorbed. And, most importantly for your answer, if gravity were some form of electromagnetism, you might expect both repellent and attractive gravitational forces and there is no evidence for negative gravity (unless you want to get into the theories of dark energy, a possible repulsive force in universe and here too there is nothing - yet at least - to link dark energy to gravity). I hope this helps at least a little. I recommend that you google "fundamental forces" and start from there. You are right, though. There are strange similarities and differences between the fundamental forces that still puzzle even the experts.

3. Hi Martha, I still can't digest it, Why do electric field propagate at all? It's OK up to where we have short ranging electric and magnetic field around a coil or in an electric motor. But EMR for wide ranges is not justified for we have photons which are not charged and still have an electromagnetic field around them, What photon has to do with the EM?

1. Hi Nirbhay, I hope I understand this question correctly, and if I do, I think it's a really great one. I am glad to explain this as I didn't do so adequately in the original article.

Here we have a fundamental force – electromagnetism (EM) and its associated force-carrying boson - the photon. Why is electromagnetism, a force that can manifest itself as electrical force and magnetic force, be carried out by a particle that itself has no charge and no magnetic moment?

We know that electrons, for example are both electrically charged and have a spin and that makes them tiny magnets as well. They generate both electric forces and magnetic forces AND they can gain and lose energy in the form of photons.

The photon that is absorbed/emitted by an electron is a propagating EM wave (which also acts like a particle). This means that on a fundamental level the photon and the electron are intimately related to one another.

The key difference between the two is that the electron is a matter particle and the photon is a force particle. The photon is a force carrier not a charge carrier. In an electron, however, charge is fundamentally built in. Magnetism arises when this charge moves. The electron's intrinsic spin gives it a magnetic moment AND its movement within the atom creates another magnetic moment. Magnetism and electrical charge are two sides of the fundamental EM coin. BOTH magnetic and electric fields are built from virtual photons. These virtual photons are constantly emitted and reabsorbed. They don't fly off at the speed of light everywhere like "real" photons.

Both real and virtual photons are instruments of EM force. These forces are manifested as EM radiation as well as the phenomena of electricity and magnetism. Classically treated, EM waves are not affected by electrical or magnetic fields, and it seems they should be, considering that photons themselves ARE propagating electromagnetic waves. Now we know that photons, which have no charge, exert both electrical and magnetic forces (as virtual particles) on charged particles and particles that possess magnetic moments.

Just like the photon exhibits wave/particle duality, so do electric and magnetic fields. Maxwell's equations tell us how these fields interact. We can also model these interactions as an exchange of force through the exchange of particles using quantum electrodynamic theory (QED). This means that a charged particle like an electron is surrounded by an entourage of virtual photons carrying out its associated electric and magnetic field, and these virtual photons are behind what we observe as magnetic attraction/repulsion and charge interactions between electrons or electrons and protons.

2. Dear Martha, nice explanation to the bottom of physics. Can you please suggest me some books/references which can be used while teaching students at Bachelor/Masters level ?

3. Hi Ramesh,

Textbooks seem to be on the way out in schools and universities at least here in Canada. Online resources can be continuously updated and enriched with links to live lectures, interactive videos, animations and other multimedia resources. I think someone knowledgeable in their field would find it quite easy to build a course from online resources. Most, if not all, scientific papers, as you probably know, are accessible online now (some with a fee if you are not an academic member).

Some sites I frequent are:

Hyperphysics (http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html)
The Physics Class Room (http://www.physicsclassroom.com)
Wikipedia (I think it is very good in the sciences)