Electromagnetism

Electromagnetism

Electricity and magnetism are closely related. You might have seen giant steel electromagnets working in a scrapyard. An electromagnet is a magnet that can be switched on and off with electricity. When the current flows, it works like a magnet; when the current stops, it goes back to being an ordinary, unmagnetized piece of steel. Scrapyard cranes pick up bits of metal junk by switching the magnet on. To release the junk, they switch the magnet off again.

Electromagnets show that electricity can make magnetism, but how do they work? When electricity flows through a wire, it creates an invisible pattern of magnetism all around it. If you put a compass needle near an electric cable, and switch the electricity on or off, you can see the needle move because of the magnetism the cable generates. The magnetism is caused by the changing electricity when you switch the current on or off.

This is how an electric motor works. An electric motor is a machine that turns electricity into mechanical energy. In other words, electric power makes the motor spin around—and the motor can drive machinery. In a clothes washing machine, an electric motor spins the drum; in an electric drill, an electric motor makes the drill bit spin at high speed and bite into the material you’re drilling. An electric motor is a cylinder packed with magnets around its edge. In the middle, there’s a core made of iron wire wrapped around many times. When electricity flows into the iron core, it creates magnetism. The magnetism created in the core pushes against the magnetism in the outer cylinder and makes the core of the motor spin around. Read more in our main article on electric motors.

Make an electromagnet

Parts needed for making your own electromagnet.

Picture: Why not make an electromagnet? All you need is a few common household items.

You can make a small electromagnet using a battery, some insulated (plastic-covered) copper wire, and a nail. Here are a couple of websites that tell you what to do step-by-step:

 

 

 

Electricity

What is electricity?

Electricity is a type of energy that can build up in one place or flow from one place to another. When electricity gathers in one place it is known as static electricity (the word static means something that does not move); electricity that moves from one place to another is called current electricity.

Static electricity

Static electricity often happens when you rub things together. If you rub a balloon against your pullover 20 or 30 times, you’ll find the balloon sticks to you. This happens because rubbing the balloon gives it an electric charge (a small amount of electricity). The charge makes it stick to your pullover like a magnet, because your pullover gains an opposite electric charge. So your pullover and the balloon attract one another like the opposite ends of two magnets.

Photo: Lightning in South Lakewood, Colorado. Photo by Dave Parsons courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).

Have you ever walked across a nylon rug or carpet and felt a slight tingling sensation? Then touched something metal, like a door knob or a faucet (tap), and felt a sharp pain in your hand? That is an example of an electric shock. When you walk across the rug, your feet are rubbing against it. Your body gradually builds up an electric charge, which is the tingling you can sense. When you touch metal, the charge runs instantly to Earth—and that’s the shock you feel.

Lightning is also caused by static electricity. As rain clouds move through the sky, they rub against the air around them. This makes them build up a huge electric charge. Eventually, when the charge is big enough, it leaps to Earth as a bolt of lightning. You can often feel the tingling in the air when a storm is brewing nearby. This is the electricity in the air around you. Read more about this in our article on capacitors.

How static electricity works

Electricity is caused by electrons, the tiny particles that “orbit” around the edges of atoms, from which everything is made. Each electron has a small negative charge. An atom normally has an equal number of electrons and protons (positively charged particles in its nucleus or center), so atoms have no overall electrical charge. A piece of rubber is made from large collections of atoms called molecules. Since the atoms have no electrical charge, the molecules have no charge either—and nor does the rubber.

Suppose you rub a balloon on your pullover over and over again. As you move the balloon back and forward, you give it energy. The energy from your hand makes the balloon move. As it rubs against the wool in your pullover, some of the electrons in the rubber molecules are knocked free and gather on your body. This leaves the balloon with slightly too few electrons. Since electrons are negatively charged, having too few electrons makes the balloon slightly positively charged. Your pullover meanwhile gains these extra electrons and becomes negatively charged. Your pullover is negatively charged, and the balloon is positively charged. Opposite charges attract, so your pullover sticks to the balloon.

That’s a very brief introduction to static electricity. You’ll find much more about it (and why it’s caused by something called triboelectricity) in our main article on static electricity.

Photo: A classic demonstration of static electricity you may have seen in your school. When this girl touches the metal ball of a Van de Graaff static electricity generator, she receives a huge static electric charge and her hair literally stands on end! Each strand of hair gets the same static charge and like charges repel, so her hairs push away from one another. Photo courtesy of Sandia National Laboratories/US Department of Energy.

Current electricity

When electrons move, they carry electrical energy from one place to another. This is called current electricity or an electric current. A lightning bolt is one example of an electric current, although it does not last very long. Electric currents are also involved in powering all the electrical appliances that you use, from washing machines to flashlights and from telephones to MP3 players. These electric currents last much longer.

Have you heard of the terms potential energy and kinetic energy? Potential energy means energy that is stored somehow for use in the future. A car at the top of a hill has potential energy, because it has the potential (or ability) to roll down the hill in future. When it’s rolling down the hill, its potential energy is gradually converted into kinetic energy (the energy something has because it’s moving). You can read more about this in our article on energy.

Static electricity and current electricity are like potential energy and kinetic energy. When electricity gathers in one place, it has the potential to do something in the future. Electricity stored in a battery is an example of electrical potential energy. You can use the energy in the battery to power a flashlight, for example. When you switch on a flashlight, the battery inside begins to supply electrical energy to the lamp, making it give off light. All the time the light is switched on, energy is flowing from the battery to the lamp. Over time, the energy stored in the battery is gradually turned into light (and heat) in the lamp. This is why the battery runs flat.

A dry cell Ever Ready battery

Picture: A battery like this stores electrical potential energy in a chemical form. When the battery is flat, it means you’ve used up all the stored energy inside by converting it into other forms.

Electric circuits

For an electric current to happen, there must be a circuit. A circuit is a closed path or loop around which an electric current flows. A circuit is usually made by linking electrical components together with pieces of wire cable. Thus, in a flashlight, there is a simple circuit with a switch, a lamp, and a battery linked together by a few short pieces of copper wire. When you turn the switch on, electricity flows around the circuit. If there is a break anywhere in the circuit, electricity cannot flow. If one of the wires is broken, for example, the lamp will not light. Similarly, if the switch is turned off, no electricity can flow. This is why a switch is sometimes called a circuit breaker.

You don’t always need wires to make a circuit, however. There is a circuit formed between a storm cloud and the Earth by the air in between. Normally air does not conduct electricity. However, if there is a big enough electrical charge in the cloud, it can create charged particles in the air called ions (atoms that have lost or gained some electrons). The ions work like an invisible cable linking the cloud above and the air below. Lightning flows through the air between the ions.

How electricity moves in a circuit

Materials such as copper metal that conduct electricity (allow it to flow freely) are called conductors. Materials that don’t allow electricity to pass through them so readily, such as rubber and plastic, are called insulators. What makes copper a conductor and rubber an insulator?

Illustration showing electrons flowing round a circuit between a battery and a lamp.

A current of electricity is a steady flow of electrons. When electrons move from one place to another, round a circuit, they carry electrical energy from place to place like marching ants carrying leaves. Instead of carrying leaves, electrons carry a tiny amount of electric charge.

Electricity can travel through something when its structure allows electrons to move through it easily. Metals like copper have “free” electrons that are not bound tightly to their parent atoms. These electrons flow freely throughout the structure of copper and this is what enables an electric current to flow. In rubber, the electrons are more tightly bound. There are no “free” electrons and, as a result, electricity does not really flow through rubber at all. Conductors that let electricity flow freely are said to have a high conductance and a low resistance; insulators that do not allow electricity to flow are the opposite: they have a low conductance and a high resistance.

For electricity to flow, there has to be something to push the electrons along. This is called an electromotive force (EMF). A battery or power outlet creates the electromotive force that makes a current of electrons flow. An electromotive force is better known as a voltage.

Direct current and alternating current

Electricity can move around a circuit in two different ways. In the big picture up above, you can see electrons racing around a loop like race cars on a track, always going in the same direction. This type of electricity is called direct current (DC) and most toys and small gadgets have circuits that work this way.

Electron flow in direct current and alternating circuits compared.

Artwork: Top: In a direct current (DC) circuit, electrons always flow in the same direction. Bottom: In an alternating current (AC) circuit, the electrons reverse direction many times each second.

The bigger appliances in your home use a different kind of electricity called alternating current (AC). Instead of always flowing the same way, the electrons constantly reverse direction—about 50–60 times every second. Although you might think that makes it impossible for energy to be carried round a circuit, it doesn’t! Take the flashlight bulb in the circuit above. With direct current, new electrons keep streaming through the filament (a thin piece of wire inside the bulb), making it heat up and give off light. With alternating current, the same old electrons whiz back and forth in the filament. You can think of them running on the spot, heating up the filament so it still makes bright light we can see. So both types of current can make the lamp work even though they flow in different ways. Most other electric appliances can also work using either direct or alternating current, though some circuits do need AC to be changed to DC (or vice versa) to work correctly.

Magnetism

Magnetism is one aspect of the combined electromagnetic force. It refers to physical phenomena arising from the force caused by magnets, objects that produce fields that attract or repel other objects.

A magnetic field exerts a force on particles in the field due to the Lorentz force, according to Georgia State University’s HyperPhysics website. The motion of electrically charged particles gives rise to magnetism. The force acting on an electrically charged particle in a magnetic field depends on the magnitude of the charge, the velocity of the particle, and the strength of the magnetic field.

All materials experience magnetism, some more strongly than others. Permanent magnets, made from materials such as iron, experience the strongest effects, known as ferromagnetism. With rare exception, this is the only form of magnetism strong enough to be felt by people.

Magnetic fields are generated by rotating electric charges, according to HyperPhysics. Electrons all have a property of angular momentum, or spin. Most electrons tend to form pairs in which one of them is “spin up” and the other is “spin down,” in accordance with the Pauli Exclusion Principle, which states that two electrons cannot occupy the same energy state at the same time. In this case, their magnetic fields are in opposite directions, so they cancel each other. However, some atoms contain one or more unpaired electrons whose spin can produce a directional magnetic field. The direction of their spin determines the direction of the magnetic field, according to the Non-Destructive Testing (NDT) Resource Center. When a significant majority of unpaired electrons are aligned with their spins in the same direction, they combine to produce a magnetic field that is strong enough to be felt on a macroscopic scale.

Magnetic field sources are dipolar, having a north and south magnetic pole. Opposite poles (N and S) attract, and like poles (N and N, or S and S) repel, according to Joseph Becker of San Jose State University. This creates a toroidal, or doughnut-shaped field, as the direction of the field propagates outward from the north pole and enters through the south pole.

The Earth itself is a giant magnet. The planet gets its magnetic field from circulating electric currents within the molten metallic core, according to HyperPhysics. A compass points north because the small magnetic needle in it is suspended so that it can spin freely inside its casing to align itself with the planet’s magnetic field. Paradoxically, what we call the Magnetic North Pole is actually a south magnetic pole because it attracts the north magnetic poles of compass needles.

If the alignment of unpaired electrons persists without the application of an external magnetic field or electric current, it produces a permanent magnet. Permanent magnets are the result of ferromagnetism. The prefix “ferro” refers to iron because permanent magnetism was first observed in a form of natural iron ore called magnetite, Fe3O4. Pieces of magnetite can be found scattered on or near the surface of the earth, and occasionally, one will be magnetized. These naturally occurring magnets are called lodestones. “We still are not certain as to their origin, but most scientists believe that lodestone is magnetite that has been hit by lightning,” according to the University of Arizona.

People soon learned that they could magnetize an iron needle by stroking it with a lodestone, causing a majority of the unpaired electrons in the needle to line up in one direction. According to NASA, around A.D. 1000, the Chinese discovered that a magnet floating in a bowl of water always lined up in the north-south direction. The magnetic compass thus became a tremendous aid to navigation, particularly during the day and at night when the stars were hidden by clouds.

Other metals besides iron have been found to have ferromagnetic properties. These include nickel, cobalt, and some rare earth metals such as samarium or neodymium which are used to make super-strong permanent magnets.

Magnetism takes many other forms, but except for ferromagnetism, they are usually too weak to be observed except by sensitive laboratory instruments or at very low temperatures. Diamagnetism was first discovered in 1778 by Anton Brugnams, who was using permanent magnets in his search for materials containing iron. According to Gerald Küstler, a widely published independent German researcher and inventor, in his paper, “Diamagnetic Levitation — Historical Milestones,” published in the Romanian Journal of Technical Sciences, Brugnams observed, “Only the dark and almost violet-colored bismuth displayed a particular phenomenon in the study; for when I laid a piece of it upon a round sheet of paper floating atop water, it was repelled by both poles of the magnet.”

Bismuth has been determined to have the strongest diamagnetism of all elements, but as Michael Faraday discovered in 1845, it is a property of all matter to be repelled by a magnetic field.

Diamagnetism is caused by the orbital motion of electrons creating tiny current loops, which produce weak magnetic fields, according to HyperPhysics. When an external magnetic field is applied to a material, these current loops tend to align in such a way as to oppose the applied field. This causes all materials to be repelled by a permanent magnet; however, the resulting force is usually too weak to be noticeable. There are, however, some notable exceptions.

Pyrolytic carbon, a substance similar to graphite, shows even stronger diamagnetism than bismuth, albeit only along one axis, and can actually be levitated above a super-strong rare earth magnet. Certain superconducting materials show even stronger diamagnetism below their critical temperature and so rare-earth magnets can be levitated above them. (In theory, because of their mutual repulsion, one can be levitated above the other.)

Paramagnetism occurs when a material becomes magnetic temporarily when placed in a magnetic field and reverts to its nonmagnetic state as soon as the external field is removed. When a magnetic field is applied, some of the unpaired electron spins align themselves with the field and overwhelm the opposite force produced by diamagnetism. However, the effect is only noticeable at very low temperatures, according to Daniel Marsh, a professor of physics at Missouri Southern State University.

Other, more complex, forms include antiferromagnetism, in which the magnetic fields of atoms or molecules align next to each other; and spin glass behavior, which involve both ferromagnetic and antiferromagnetic interactions. Additionally, ferrimagnetism can be thought of as a combination of ferromagnetism and antiferromagnetism due to many similarities shared among them, but it still has its own uniqueness, according to the University of California, Davis.

When a wire is moved in a magnetic field, the field induces a current in the wire. Conversely, a magnetic field is produced by an electric charge in motion. This is in accordance with Faraday’s Law of Induction, which is the basis for electromagnets, electric motors and generators. A charge moving in a straight line, as through a straight wire, generates a magnetic field that spirals around the wire. When that wire is formed into a loop, the field becomes a doughnut shape, or a torus. According to the Magnetic Recording Handbook (Springer, 1998) by Marvin Cameras, this magnetic field can be greatly enhanced by placing a ferromagnetic metal core inside the coil.

In some applications, direct current is used to produce a constant field in one direction that can be switched on and off with the current. This field can then deflect a movable iron lever causing an audible click. This is the basis for the telegraph, invented in the 1830s by Samuel F. B. Morse, which allowed for long-distance communication over wires using a binary code based on long- and short-duration pulses. The pulses were sent by skilled operators who would quickly turn the current on and off using a spring-loaded momentary-contact switch, or key. Another operator on the receiving end would then translate the audible clicks back into letters and words.

A coil around a magnet can also be made to move in a pattern of varying frequency and amplitude to induce a current in a coil. This is the basis for a number of devices, most notably, the microphone. Sound causes a diaphragm to move in an out with the varying pressure waves. If the diaphragm is connected to a movable magnetic coil around a magnetic core, it will produce a varying current that is analogous to the incident sound waves. This electrical signal can then be amplified, recorded or transmitted as desired. Tiny super-strong rare-earth magnets are now being used to make miniaturized microphones for cell phones, Marsh told Live Science.

When this modulated electrical signal is applied to a coil, it produces an oscillating magnetic field, which causes the coil to move in and out over a magnetic core in that same pattern. The coil is then attached to a movable speaker cone so it can reproduce audible sound waves in the air. The first practical application for the microphone and speaker was the telephone, patented by Alexander Graham Bell in 1876. Although this technology has been improved and refined, it is still the basis for recording and reproducing sound.

The applications of electromagnets are nearly countless. Faraday’s Law of Induction forms the basis for many aspects of our modern society including not only electric motors and generators, but electromagnets of all sizes. The same principle used by a giant crane to lift junk cars at a scrap yard is also used to align microscopic magnetic particles on a computer hard disk drive to store binary data, and new applications are being developed every day.

Staff Writer Tanya Lewis contributed to this report.

Additional resources

Magnetic Field of the Earth

Magnetic Field of the Earth

The Earth’s magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the Earth. The problem with that picture is that the Curie temperature of iron is about 770 C . The Earth’s core is hotter than that and therefore not magnetic. So how did the Earth get its magnetic field?

 

Magnetic fields surround electric currents, so we surmise that circulating electric currents in the Earth’s molten metallic core are the origin of the magnetic field. A current loop gives a field similar to that of the earth. The magnetic field magnitude measured at the surface of the Earth is about half a Gauss and dips toward the Earth in the northern hemisphere. The magnitude varies over the surface of the Earth in the range 0.3 to 0.6 Gauss.

The Earth’s magnetic field is attributed to a dynamo effect of circulating electric current, but it is not constant in direction. Rock specimens of different age in similar locations have different directions of permanent magnetization. Evidence for 171 magnetic field reversals during the past 71 million years has been reported.

Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part in generating the currents which are presumed to be the source of the magnetic field. Mariner 2 found that Venus does not have such a magnetic field although its core iron content must be similar to that of the Earth. Venus’s rotation period of 243 Earth days is just too slow to produce the dynamo effect.

Interaction of the terrestrial magnetic field with particles from the solar wind sets up the conditions for the aurora phenomena near the poles.

The north pole of a compass needle is a magnetic north pole. It is attracted to the geographic North Pole, which is a magnetic south pole (opposite magnetic poles attract).

The Dynamo Effect

The simple question “how does the Earth get its magnetic field?” does not have a simple answer. It does seem clear that the generation of the magnetic field is linked to the rotation of the earth, since Venus with a similar iron-core composition but a 243 Earth-day rotation period does not have a measurable magnetic field. It certainly seems plausible that it depends upon the rotation of the fluid metallic iron which makes up a large portion of the interior, and the rotating conductor model leads to the term “dynamo effect” or “geodynamo”, evoking the image of an electric generator.

Convection drives the outer-core fluid and it circulates relative to the earth. This means the electrically conducting material moves relative to the earth’s magnetic field. If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop could sustain the magnetic dipole type magnetic field of the earth. Large-scale computer models are approaching a realistic simulation of such a geodynamo.

Aurora

When energetic charged particles enter the earth’s atmosphere from the solar wind, they tend to be channeled toward the poles by the magnetic force which causes them to spiral around the magnetic field lines of the earth. They are energetic enough to ionize air molecules, so a considerable number of atoms and molecules are elevated to excited states. When they make the transition back to their ground states they emit light characteristic of the atoms and molecules. Red and green light emitted from oxygen atoms is a constituent of the light seen at the poles. Atmospheric nitrogen also plays a role. An example of the colors that might be visible can be found by observing the nitrogen spectrum. Near the north pole the light show is called the aurora borealis and near the south pole it is called aurora australis.

A polar satellite captured images of aurora over the South Pole of the Earth. UV photographs of Jupiter indicate that auroral phenomena occur in its polar regions. Images of Saturn aurora show a very active pulsating pattern.
This sketch of charged particles spiraling around magnetic field lines is conceptual only.

Weed and Fungus Control with Landscape Fabric and Soaker Hose

The best way to control weeds in the control is to start with prevention.

Before planting vegetables in the garden, you can roll out landscape fabric down the rows where the vegetable plants will be planted. The fabric should be porous, and will need to be stapled down with “sod staples” or pins. This technique works especially well when planting on “raised rows” or beds. Cut “X’s” in the fabric in order to plant your plants.

To make watering your garden easier, you can string out a soaker hose along each row, underneath the landscape fabric.  Leave the end of each hose sticking out at the end of the row. Whenever  you want to water, just attach your regular garden hose and turn on the water very low for about 30 minutes. This method also helps to prevent fungus because the soaker hose only waters the soil and roots without overhead sprinkling of the leaves.

Top