## What is a Magnetic Field?

Everyone knows just how fun magnets can be. As a child, who among us didn’t love to see if we could make our silverware stick together? And how about those little magnetic rocks that we could arrange to form just about any shape because they stuck together? Well, magnetism is not just an endless source of fun or good for scientific experiments; it’s also one of basic physical laws upon which the universe is based.

The attraction known as magnetism occurs when a magnetic field is present, which is a field of force produced by a magnetic object or particle. It can also be produced by a changing electric field and is detected by the force it exerts on other magnetic materials. Hence why the area of study dealing with magnets is known as electromagnetism.

## Definition:

Magnetic fields can be defined in a number of ways, depending on the context. However, in general terms, it is an invisible field that exerts magnetic force on substances which are sensitive to magnetism. Magnets also exert forces and torques on each other through the magnetic fields they create.

They can be generated within the vicinity of a magnet, by an electric current, or a changing electrical field. They are dipolar in nature, which means that they have both a north and south magnetic pole. The Standard International (SI) unit used to measure magnetic fields is the Tesla, while smaller magnetic fields are measured in terms of Gauss (1 Tesla = 10,000 Guass).

Mathematically, a magnetic field is defined in terms of the amount of force it exerted on a moving charge. The measurement of this force is consistent with the Lorentz Force Law, which can be expressed as F= qvB, where F is the magnetic force, q is the charge, v is the velocity, and the magnetic field is B. This relationship is a vector product, where F is perpendicular (->) to all other values.

## Field Lines:

Magnetic fields may be represented by continuous lines of force (or magnetic flux) that emerge from north-seeking magnetic poles and enter south-seeking poles. The density of the lines indicate the magnitude of the field, being more concentrated at the poles (where the field is strong) and fanning out and weakening the farther they get from the poles.

A uniform magnetic field is represented by equally-spaced, parallel straight lines. These lines are continuous, forming closed loops that run from north to south, and looping around again. The direction of the magnetic field at any point is parallel to the direction of nearby field lines, and the local density of field lines can be made proportional to its strength.

Magnetic field lines resemble a fluid flow, in that they are streamlined and continuous, and more (or fewer lines) appear depending on how closely a field is observed. Field lines are useful as a representation of magnetic fields, allowing for many laws of magnetism (and electromagnetism) to be simplified and expressed in mathematical terms.

A simple way to observe a magnetic field is to place iron filings around an iron magnet. The arrangements of these filings will then correspond to the field lines, forming streaks that connect at the poles. They also appear during polar auroras, in which visible streaks of light line up with the local direction of the Earth’s magnetic field.

## History of Study:

The study of magnetic fields began in 1269 when French scholar Petrus Peregrinus de Maricourt mapped out the magnetic field of a spherical magnet using iron needles. The places where these lines crossed he named “poles” (in reference to Earth’s poles), which he would go on to claim that all magnets possessed.

During the 16th century, English physicist and natural philosopher William Gilbert of Colchester replicated Peregrinus’ experiment. In 1600, he published his findings in a treaties (De Magnete) in which he stated that the Earth is a magnet. His work was intrinsic to establishing magnetism as a science.

In 1750, English clergyman and philosopher John Michell stated that magnetic poles attract and repel each other. The force with which they do this, he observed, is inversely proportional to the square of the distance, otherwise known as the inverse square law.

In 1785, French physicist Charles-Augustin de Coulomb experimentally verified Earths’ magnetic field. This was followed by 19th century French mathematician and geometer Simeon Denis Poisson created the first model of the magnetic field, which he presented in 1824.

By the 19th century, further revelations refined and challenged previously-held notions. For example, in 1819, Danish physicist and chemist Hans Christian Orsted discovered that an electric current creates a magnetic field around it. In 1825, André-Marie Ampère proposed a model of magnetism where this force was due to perpetually flowing loops of current, instead of the dipoles of magnetic charge.

In 1831, English scientist Michael Faraday showed that a changing magnetic field generates an encircling electric field. In effect, he discovered electromagnetic induction, which was characterized by Faraday’s law of induction (aka. Faraday’s Law).

Between 1861 and 1865, Scottish scientist James Clerk Maxwell published his theories on electricity and magnetism – known as the Maxwell’s Equations. These equations not only pointed to the interrelationship between electricity and magnetism, but showed how light itself is an electromagnetic wave.

The field of electrodynamics was extended further during the late 19th and 20th centuries. For instance, Albert Einstein (who proposed the Law of Special Relativity in 1905), showed that electric and magnetic fields are part of the same phenomena viewed from different reference frames. The emergence of quantum mechanics also led to the development of quantum electrodynamics (QED).

## Examples:

A classic example of a magnetic field is the field created by an iron magnet. As previously mentioned, the magnetic field can be illustrated by surrounding it with iron filings, which will be attracted to its field lines and form in a looping formation around the poles.

Larger examples of magnetic fields include the Earth’s magnetic field, which resembles the field produced by a simple bar magnet. This field is believed to be the result of movement in the Earth’s core, which is divided between a solid inner core and molten outer core which rotates in the opposite direction of Earth. This creates a dynamo effect, which is believed to power Earth’s magnetic field (aka. magnetosphere).

Such a field is called a dipole field because it has two poles – north and south, located at either end of the magnet – where the strength of the field is at its maximum. At the midpoint between the poles the strength is half of its polar value, and extends tens of thousands of kilometers into space, forming the Earth’s magnetosphere.

Other celestial bodies have been shown to have magnetic fields of their own. This includes the gas and ice giants of the Solar System – Jupiter, Saturn, Uranus and Neptune. Jupiter’s magnetic field is 14 times as powerful as that of Earth, making it the strongest magnetic field of any planetary body. Jupiter’s moon Ganymede also has a magnetic field, and is the only moon in the Solar System known to have one.

Mars is believed to have once had a magnetic field similar to Earth’s, which was also the result of a dynamo effect in its interior. However, due to either a massive collision, or rapid cooling in its interior, Mars lost its magnetic field billions of years ago. It is because of this that Mars is believed to have lost most of its atmosphere, and the ability to maintain liquid water on its surface.

When it comes down to it, electromagnetism is a fundamental part of our Universe, right up there with nuclear forces and gravity. Understanding how it works, and where magnetic fields occur, is not only key to understanding how the Universe came to be, but may also help us to find life beyond Earth someday.

We have written many articles about the magnetic field for Universe Today. Here’s What is Earth’s Magnetic Field, Is Earth’s Magnetic Field Ready to Flip?, How Do Magnets Work?, Mapping The Milky Way’s Magnetic Fields – The Faraday Sky, Magnetic Fields in Spiral Galaxies – Explained at Last?, Astronomy Without A Telescope – Cosmic Magnetic Fields.

If you’d like more info on Earth’s magnetic field, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Sources:

## What Are The Uses Of Electromagnets?

Electromagnetism is one of the fundamental forces of the universe, responsible for everything from electric and magnetic fields to light. Originally, scientists believed that magnetism and electricity were separate forces. But by the late 19th century, this view changed, as research demonstrated conclusively that positive and negative electrical charges were governed by one force (i.e. magnetism).

Since that time, scientists have sought to test and measure electromagnetic fields, and to recreate them. Towards this end, they created electromagnets, a device that uses electrical current to induce a magnetic field. And since their initial invention as a scientific instrument, electromagnets have gone on to become a regular feature of electronic devices and industrial processes.

## Who Discovered Electricity?

Electricity is a form of energy and it occurs in nature, so it was not “invented.” As to who discovered it, many misconceptions abound. Some give credit to Benjamin Franklin for discovering electricity, but his experiments only helped establish the connection between lightning and electricity, nothing more.

The truth about the discovery of electricity is a bit more complex than a man flying his kite. It actually goes back more than two thousand years.

In about 600 BC, the Ancient Greeks discovered that rubbing fur on amber (fossilized tree resin) caused an attraction between the two – and so what the Greeks discovered was actually static electricity. Additionally, researchers and archeologists in the 1930’s discovered pots with sheets of copper inside that they believe may have been ancient batteries meant to produce light at ancient Roman sites. Similar devices were found in archeological digs near Baghdad meaning ancient Persians may have also used an early form of batteries.

But by the 17th century, many electricity-related discoveries had been made, such as the invention of an early electrostatic generator, the differentiation between positive and negative currents, and the classification of materials as conductors or insulators.

In the year 1600, English physician William Gilbert used the Latin word “electricus” to describe the force that certain substances exert when rubbed against each other. A few years later another English scientist, Thomas Browne, wrote several books and he used the word “electricity” to describe his investigations based on Gilbert’s work.

In 1752, Ben Franklin conducted his experiment with a kite, a key, and a storm. This simply proved that lightning and tiny electric sparks were the same thing.

Italian physicist Alessandro Volta discovered that particular chemical reactions could produce electricity, and in 1800 he constructed the voltaic pile (an early electric battery) that produced a steady electric current, and so he was the first person to create a steady flow of electrical charge. Volta also created the first transmission of electricity by linking positively-charged and negatively-charged connectors and driving an electrical charge, or voltage, through them.

In 1831 electricity became viable for use in technology when Michael Faraday created the electric dynamo (a crude power generator), which solved the problem of generating electric current in an ongoing and practical way. Faraday’s rather crude invention used a magnet that was moved inside a coil of copper wire, creating a tiny electric current that flowed through the wire. This opened the door to American Thomas Edison and British scientist Joseph Swan who each invented the incandescent filament light bulb in their respective countries in about 1878. Previously, light bulbs had been invented by others, but the incandescent bulb was the first practical bulb that would light for hours on end.

Swan and Edison later set up a joint company to produce the first practical filament lamp, and Edison used his direct-current system (DC) to provide power to illuminate the first New York electric street lamps in September 1882.

Later in the 1800’s and early 1900’s Serbian American engineer, inventor, and all around electrical wizard Nikola Tesla became an important contributor to the birth of commercial electricity. He worked with Edison and later had many revolutionary developments in electromagnetism, and had competing patents with Marconi for the invention of radio. He is well known for his work with alternating current (AC), AC motors, and the polyphase distribution system.

Later, American inventor and industrialist George Westinghouse purchased and developed Tesla’s patented motor for generating alternating current, and the work of Westinghouse, Tesla and others gradually convinced American society that the future of electricity lay with AC rather than DC.

Others who worked to bring the use of electricity to where it is today include Scottish inventor James Watt, Andre Ampere, a French mathematician, and German mathematician and physicist George Ohm.

And so, it was not just one person who discovered electricity. While the concept of electricity was known for thousands of years, when it came time to develop it commercially and scientifically, there were several great minds working on the problem at the same time.

We have written many articles about electricity for Universe Today. Here’s a separate article about static electricity, and here’s an interesting story about how astronomy was part of how electricity was brought to the World’s Fair in Chicago in 1933.

For more detailed information about the discovery of electricity, see our sources, below.

We’ve also recorded an entire episode of Astronomy Cast all about Electromagnetism. Listen here, Episode 103: Electromagnetism.

## Flying Space Toasters: Electrified Exoplanets Really Feel the Heat

Overheated and overinflated, hot Jupiters are some of the strangest extrasolar planets to be discovered by the Kepler mission… and they may be even more exotic than anyone ever thought. A new model proposed by Florida Gulf Coast University astronomer Dr. Derek Buzasi suggests that these worlds are intensely affected by electric currents that link them to their host stars. In Dr. Buzasi’s model, electric currents arising from interactions between the planet’s magnetic field and their star’s stellar wind flow through the interior of the planet, puffing it up and heating it like an electric toaster.

In effect, hot Jupiters are behaving like giant resistors within exoplanetary systems.

Many of the planets found by the Kepler mission are of a type known as “hot Jupiters.” While about the same size as Jupiter in our own solar system, these exoplanets are located much closer to their host stars than Mercury is to the Sun — meaning that their atmospheres are heated to several thousands of degrees.

One problem scientists have had in understanding hot Jupiters is that many are inflated to sizes larger than expected for planets so close to their stars. Explanations for the “puffiness” of these exoplanets have generally involved some kind of extra heating process — but no model successfully explains the observation that more magnetically active stars tend to have puffier hot Jupiters orbiting around them.

“This kind of electric heating doesn’t happen very effectively on planets in our solar system because their outer atmospheres are cold and don’t conduct electricity very well,” says Dr. Buzasi. “But heat up the atmosphere by moving the planet closer to its star and now very large currents can flow, which delivers extra heat to the deep interior of the planet — just where we need it.”

More magnetically active stars have more energetic winds, and would provide larger currents — and thus more heat — to their planets.

The currents start in the magnetosphere, the area where the stellar wind meets the planetary magnetic field, and enter the planet near its north and south poles. This so-called “global electric circuit” (GEC) exists on Earth as well, but the currents involved are only a few thousand amps at 100,000 volts or less.

On the hot Jupiters, though, currents can amount to billions of amps at voltages of millions of volts — a “significant current,” according to Dr. Buzasi.

“It is believed that these hot Jupiter planets formed farther out and migrated inwards later, but we don’t yet fully understand the details of the migration mechanism,” Dr. Buzasi says. “The better we can model how these planets are built, the better we can understand how solar systems form. That in turn, would help astronomers understand why our solar system is different from most, and how it got that way.”

Other electrical heating processes have previously been suggested by other researchers as well, once hints of magnetic fields in exoplanets were discovered in 2003 and models of atmospheric wind drag — generating frictional heating — as a result of moving through these fields were made in 2010.

(And before anyone attempts to suggest this process supports the alternative “electric universe” (EU) theory… um, no.)

“No, nothing EU-like at all in my model,” Dr. Buzasi told Universe Today in an email. “I just look at how the field aligned currents that we see in the terrestrial magnetosphere/ionosphere act in a hot Jupiter environment, and it turns out that a significant fraction of the resulting circuit closes inside the planet (in the outer 10% of the radius, mostly) where it deposits a meaningful amount of heat.”

This work will be presented at the 222nd meeting of the American Astronomical Society on June 4, 2013.

## What is Conductance?

Electricity is an amazing, and potentially very dangerous, thing. In addition to powering our appliances, heating our homes, starting our cars and providing us with unnatural lighting during the evenings, it is also one of the fundamental forces upon which the Universe is based. Knowing what governs it is crucial to using it for our benefit, as well as understanding how the Universe works.

For those of us looking to understand it – perhaps for the sake of becoming an electrical engineer, a skilled do-it-yourselfer,  or just satisfying scientific curiosity – some basic concepts need to be kept in mind. For example, we need to understand a little thing known as conductance, and quality that is related to resistance; which taken together govern the flow of electrical current.

## Definition:

Conductance is the measure of how easily electricity flows along a certain path through an electrical element, and since electricity is so often explained in terms of opposites, conductance is considered the opposite of resistance. In terms of resistance and conductance, the reciprocal relationship between the two can be expressed through the following equation: R = 1/G, G=1/R; where R equals resistance and G equals conduction.

Another way to represent this is: W=1/S, S=1/W, where W (the Greek letter omega) represents resistance and S represents Siemens, ergo the measure of conductance. In addition, Siemens can be measured by comparing them to their equivalent of one ampere (A) per volt (V).

In other words, when a current of one ampere (1A) passes through a component across which a voltage of one volt (1V) exists, then the conductance of that component is one Siemens (1S). This can be expressed through the equation: G = I/E, where G represents conductance and E is the voltage across the component (expressed in volts).

The temperature of the material is definitely a factor, but assuming a constant temperature, the conductance of a material can be calculated.

## Measurement:

The SI (International System) derived unit of conductance is known as the Siemens, named after the German inventor and industrialist Ernst Werner von Siemens. Since conductance is the opposite of resistance, it is usually expressed as the reciprocal of one ohm – a unit of electrical resistance named after George Simon Ohm – or one mho (ohm spelt backwards).

Recently, this term was re-designated to Siemens, expressed by the notational symbol S. The factors that affect the magnitude of resistance are exactly the same for conductance, but they affect conductance in the opposite manner. Therefore, conductance is directly proportional to area, and inversely proportional to the length of the material.

We have written many articles about conductance for Universe Today. Here’s What are Electrons?, Who Discovered Electricity?, What is Static Electricity?, What is Electromagnetic Induction?, and What are the Uses of Electromagnets?

We’ve also recorded an entire episode of Astronomy Cast all about Electromagnetism. Listen here, Episode 103: Electromagnetism.

Sources:

## What are Electrons

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If you have heard of electrons you know that they have something to do with electricity and atoms. If so you are mostly right in describing what are electrons. Electrons are the subatomic particles that orbit the nucleus of an atom. They are generally negative in charge and are much smaller than the nucleus of the atom. If you wanted a proper size comparison the size of the earth in comparison to the sun would be a pretty close visualization.

Electrons are known to fall into orbits or energy levels. These orbits are not visible paths like the orbit of a planet or celestial body. The reason is that atoms are notoriously small and the best microscopes can only view so much of atoms at that scale. Even if we could view electrons they would move too fast for the human eye. As a matter of fact scientists still can’t calculate the exact position of electrons. They can only estimate their locations. That is why the modern model of the atoms has an electron cloud surrounding the nucleus of an atom instead of a defined system of electrons in concentric orbits.

Electrons are also important for the bonding of individual atoms together. With out this bonding force between atoms matter would not be able to interact in the many reactions and forms we see every day. This interaction between the outer electron layers of an atom is call atomic bonding. It can occur in two forms. One is covalent bonding where atoms share electrons in their outer orbits. The other is ionic bonding where an atom gives up electrons to another atom. In either case bonding must meet specific rules. We won’t go into great detail, but each electron orbit or electron energy level can only hold so many electrons. Atoms can only bond if there is room to share or receive extra electrons on the outermost orbit of the atom.

Electrons are also important to electricity. Electricity is basically the exchange of electrons in a stream called a current through a conducting medium. In most cases the medium is an acid, metal, or similar conductor. In the case of static electricity, a stream of electrons travels through the medium of air.

The understanding of the electron has allowed for a better understanding of some of the most important forces in our universe such as the electromagnetic force. Understanding its workings has allowed scientist to work out concepts such potential difference and the relationship between electrical and magnetic fields.

We have written many articles about electrons for Universe Today. Here’s an article about the atom diagram, and here’s an article about the electron cloud model.

If you’d like more info on Electrons, check out the Discussion about Electrons, and here’s a link to the History of the Electron Article.

We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.

Sources:
Wikipedia
Windows to Universe

## Electromagnetism

The short version: electromagnetism is one of the four fundamental forces (the strong force, the weak force, and gravitation are the other three), responsible for all magnetic, electrical, and electromagnetic phenomena.

The long version is a little more complicated.

Start with history … phenomena we today call electrical have been known for millennia (e.g. static electricity), as have their magnetic counterparts (e.g. lodestone). The 17th and 18th centuries saw considerable scientific study of each, as separate forces, with Ørsted and Ampère uniting the two into electromagnetism, around 1820. Maxwell consolidated (in 1864) everything known about electromagnetism into what today we call Maxwell’s equations … and predicted electromagnetic waves (or radiation), a prediction verified by Hertz, two decades later. However, Maxwell’s equations opened a can of worms (to do with the aether, and the speed of light) … which lead to Einstein and special relativity. In parallel, a series of discoveries lead to photons (the quanta of electromagnetic radiation) and quantum mechanics, and these in turn to the recognition that the spectacular success of classical electromagnetism (i.e. Maxwell’s equations) actually depends on quantum field theory (with all its counter-intuitives).

Fast forward to the 1940s, and Quantum Electrodynamics (QED), which has electrically charged particles interacting via exchanges of photons (real or virtual), and describes all electromagnetic phenomena. QED is the most successful theory in physics, period (it has been tested, and found accurate, to one part in 1012!).

Here’s a fun fact: QED incorporates special relativity … and an electric charge (with no magnetic field) becomes an electric current (with an associated magnetic field), in relativity, simply by switching to a frame of reference moving with respect to the (stationary) electrical charge.

So, in its classical form, electromagnetism is an instantaneous ‘action at a distance’ type of force; in its quantum form; it’s an exchange of virtual photons, at the speed of light.

Now for more complication.

In 1979 Sheldon L. Glashow, Abdus Salam, and Steven Weinberg shared the Nobel Prize for Physics, for their contributions to the unification of electromagnetism and the weak force … which goes under the name electroweak interaction. So electromagnetism is just one manifestation of something more general, just as electricity and magnetism are two manifestations of one underlying thing, electromagnetism.

Want to learn more? Try Stargazers’ Electromagnetism, Math Pages on Maxwell’s equations, Richard Feynman’s excellent non-technical book on QED, and the 1979 Nobel Press Release on the electroweak interaction.

To get a handle on how diverse the roles of electromagnetism are, in astronomy, check out these Universe Today articles (just some of the many): Stellar Jets are Born Knotted,
Magnetic “Ropes” Connect the Northern Lights to the Solar Wind, and Spitzer Spies Ghostly Magnetar.

Astronomy Cast has an episode devoted to electromagnetism, called Electromagnetism. Some others you may also find interesting, on this topic, are The Search for the Theory of Everything, and The Important Numbers in the Universe.

Sources:
Wikipedia
University of Oregon
NASA