How Can Electric Currents and Magnets Exert Force
 | Introduction to Magnetism and Induced Currents |
Despite the increasing prevalence of CD-ROMs and the use of electronic storage in RAM, near data is still stored magnetically. This reading assignment reviews the basic concepts of magnetism, then introduces the three different furnishings which accept been utilized to read magnetic data.
Sources of Magnetic Fields
Discussion Question: What produces magnetic fields? Is there any difference between the fields of permanent magnets and the fields of electromagnets? Are the sources of the fields the aforementioned in these two cases, or are they different?
Our thinking most magnetic sources has inverse considerably over the centuries. The only form of magnetism known until the 19th century was ferromagnetism. Certain materials, when "magnetized", would concenter certain other materials. The only materials attracted past a magnet were those that could go magnetized themselves. Since but sure materials exhibited magnetic properties, scientists concluded that magnetism was an inherent property of materials.
Then, in the 19th century, scientists studying the relatively new field of electric currents discovered that moving charges produce magnetic effects. A current traveling through a loop of wire creates a magnetic field forth the axis of the loop. The direction of the field within the loop tin be found past crimper the fingers of the correct hand in the direction of the current through the loop; the thumb and then points in the direction of the magnetic field. With this discovery, magnetism appeared to occur in two different manners: ferromagnetism depending on the material, and electromagnetsim caused by currents.
 and (b) spin of an electron. | As diminutive physics and chemical science began to explicate the periodic table with the aid of the Bohr model of the atom in the early 1900s, magnetic properties were assigned to the electrons in atoms. Electrons appeared to exhibit two types of motion in an atom: orbital and spin. Orbital move referred to the movement of an electron around the nucleus of the atom. Since a charged particle was moving, a magnetic field was created. But electrons (and protons and other particles) likewise appeared to be spinning effectually their centers, creating yet another magnetic field. The magnetic field due to the orbital movement and the magnetic field due to the spin could cancel or add, only expressions for the exact coupling between the two are also complicated to go into hither. Since electrons were moving and spinning within atoms, ferromagnetism could at present be explained by the motility of charges within different materials. If all of the electrons in an object line up with their spins in the same direction, the spins will add together and create an observable field. |
| That last sentence is slightly unrealistic. Solids incorporate incredably large numbers of electrons, and they will never all completely line up. Instead, a solid mostly consists of magneticdomains. In a domain, the bulk of electrons which tin can (unpaired valance electrons) will have spins aligned. Adjacent domains will generally non be oriented in identical directions. In magnetized materials, some domains will cancel, but the boilerplate domain orientation will be in one management, producing a net magnetic field. In unmagnetized materials, the domains are randomly oriented and cancel, then no observable field is created. The figure to the right illustrates these concepts. The concept of magnetism being entirely due to the motion of charges has been modified significantly in the 20th century, thanks to breakthrough mechanics. The Bohr model of the atom must be modified to include doubtfulness. We can never make up one's mind exactly the trajectory of an electron or say for sure where information technology will be establish. The uncertainty principle requires that nosotros instead say only where the electron is almost likely to be institute. Until we measure the position of the electron, its wave function is spread out over all space, with a higher probability of finding the electron in the classical orbit described past Bohr. |  (a) Sample electron spins aligned, only . . . |  (b) are accounted for, a internet magnetic field remains. |  (c) region is the internet magnet- ization of the region, or domain. |
 (d) such domains, which are generally not aligned completely, only |  (e) domains in a magnetized solid don't completely cancel but leave a internet field |  (f) the fields of nearby domains completely abolish, leaving no internet field |
Our concept of spin must as well exist adjusted to fit with the discoveries of the 20th century. Electrons are idea to be "bespeak particles," which means they have no spatial extent. Which means they tin can't be physically spinning around their centers. While the word "spin" has survived, it now refers to an intrinsic holding of a particle rather than to any concrete rotation through space. Since electrons and other particles have intrinsic spin, they create magnetic fields automatically. After considering quantum mechanics, we are once again left with 2 types of magnetism: intrinsic magnetism due to the "spins" of electrons, and electromagnetism due to the movement of electrons.
Just as an aside, the reason that molecules such as He are non magnetized is the Pauli exclusion principle. The 2 electrons in helium atoms occupy the same energy beat out, filling it (the kickoff shell contains only 2 states). The exclusion principle states that no two electrons can have the same exact properties. For them both to occupy the same energy trounce, their spins must exist oppositely directed and abolish. Electrons in solids with partially-filled valence shells may, nevertheless, line upward with the aforementioned spin as other electrons, thereby creating a non-nothing net magnetic field.
Magnetic Fields and Forces
Discussion Question: Retrieve about what you have learned during your lifetime near electricity and magnetism. How are they alike? How are they different? Any charged object in an electric field experiences an electric force. Will any charged object in a magnetic field feel a magnetic force? Does an object accept to be charged to experience a magnetic force?
Magnets can exert a force at a altitude, just like electrical charges. And so information technology is advantageous to draw the furnishings of magnets in terms of a magnetic field, B 1 , much in the same manner that the effects of charges are described past the electric field. We take already invoked this concept of a magnetic field in the previous section. Magnetic fields permeate space and are strongest near a permanent magnet or electromagnet. IThe SI unit for B is the tesla (1 T = one Vs/m2). The tesla is a fairly large unit of magnetic field, so we oft list magnetic field strengths in terms of Gauss (1 G = 10-iv T). The magnetic field of the earth is about half gauss in strength.
 | Like an electrical field, a magnetic field may be represented with field lines. These lines (and the magnetic field) indicate from the north pole of a magnet to the south pole of a magnet, as shown in the figure to the left.. Dissimilar electric field lines, magnetic field lines are e'er closed - they never have a starting point or stopping betoken. Whenever you have a north pole, you lot must have a south pole as well. Another manner to say this is that magnetic monopoles (single poles) practise not exist. Electrical monopoles, on the other hand, be in abundance. Examples are an electron, a proton, or any other charged particle. |
| Even the magnetic field produced by a electric current-carrying wire must form complete loops. Higher up, you were told that a loop of current-carrying wire produces a magnetic field along the centrality of the wire. The right-hand rule gives the direction of the field inside the loop of wire. The magnetic field turns back the other mode outside of the loop. As shown in the figure on the correct, this magnetic field from a loop of electric current-conveying wire looks similar to the field from a permanent bar magnet. |  |
Anyone who has used a compass knows that a magnet experiences a forcefulness in a magnetic field. Just equally for electric charges, contrary magnetic poles repel and like poles attract. Thus the magnetic field pointing from n to due south points in the direction of the force on a NORTH POLE of a magnet. One interesting result of this is that the Earth's geographic north pole is its magnetic south pole. A compass needle's magnetic due north pole volition betoken toward the geographic north pole of the Globe. Since the north pole of a magnet is attracted to the south pole of another magnet, this means that the geographical due north pole of the Earth is really a south magnetic pole.
Permanent magnets are non the simply objects which experience the magnetic force. Electric charges can experience a magnetic force if two conditions are met:
- The charge must be moving through a magnetic field
- The velocity of the charge cannot be parallel (or antiparallel) to the direction of the magnetic field
FB = qvB sin q
The direction of the force is perpendicular to both the velocity and the magnetic field. The force is more than accurately expressed in terms of a cross-product:
F B = q v x B
The magnitude of a cross-production depends on sin q, giving the previous expression. For our purposes, the first expression is sufficient, provided you remember that the force is perpendicular to both the velocity and the magnetic field.
A current-conveying wire too experiences a strength in a magnetic field, since electric current is nothing more than moving charges. As for unmarried charges, the current must exist moving in a direction other than the management of the field. The magnitude of the magnetic force on a current-carrying wire is found from
FB = iLB sin q
where i is the electric current and L is the length of wire in the compatible magnetic field of strength B.
| 1 | To be exact, the symbol B represents magnetic flux density, too called magnetic induction, not magnetic field. The true magnetic field is denoted by H. H and B differ only by a material-dependent constant. For almost purposes, the difference is inconsequential, so nosotros will refer to B as the magnetic field. If you have farther courses in magnetism, you lot will learn the stardom. |
Induced Currents, Induced EMF, and Faraday'south Constabulary
Discussion Question: Tin can y'all create a current through a wire without connecting the wire directly to a voltage source like a battery? Exercise all of your appliances take straight connections? What about your car engine?
If a scroll of wire is placed in a changing magnetic field, a current will be induced in the wire. This current flows because something is producing an electric field that forces the charges effectually the wire. (It cannot be the magnetic force since the charges are non initially moving). This "something" is called an electromotive force, or emf, even though it is not a forcefulness. Instead, emf is similar the voltage provided by a bombardment. A changing magnetic field through a coil of wire therefore must induce an emf in the coil which in plow causes electric current to menstruum.
The law describing induced emf is named later the British scientist Michael Faraday, merely Faraday'south Law should really exist chosen Henry'due south Law. Joseph Henry, an American from the Albany surface area, discovered that changing magnetic fields induced current before Faraday did. Unfortunately, he lived in the age before instantaneous electronic communication between Europe and America. Faraday got published and got famous earlier Henry could report his findings. Interestingly enough, Henry had to explain the results to Faraday when the 2 met a few years later.
Briefly stated, Faraday'south law says that a irresolute magnetic field produces an electric field. If charges are complimentary to movement, the electric field volition cause an emf and a current. For example, if a loop of wire is placed in a magnetic field so that the field passes through the loop, a change in the magnetic field will induce a electric current in the loop of wire. A current is also induced if the area of the loop changes, or if the area enclosing magnetic field changes. Then it is the change in magnetic flux, defined every bit
that determines the induced current. A is the area vector; its magnitude is the surface area of the loop, and its direction is perpendicular to the area of the loop, and q is the bending between A and the magnetic field B. The last equality (removing the integral) is valid only if the field is compatible over the entire loop.
Faraday'southward Police force says that the emf induced (and therefore the electric current induced) in the loop is proportional to the charge per unit of modify in magnetic flux:
e is the emf, which is the work done moving charges around the loop, divided by the charge. Information technology is like in concept to voltage, except that no charge separation is necessary. The magnetic flux F B equals the magnetic field B times the area A of the loop with magnetic field through it if (a) the magnetic field is perpendicular to the plane of the loop, and (b) the magnetic field is uniform throughout the loop. For our purposes, we will assume these 2 atmospheric condition are met; in applied applications, however, magnetic field volition vary through a loop, and the field will not always be perpendicular to the loop.
Since all applications of Faraday's Constabulary to magnetic storage involve a scroll of wire of fixed area, nosotros will also assume that (c) the surface area does not change in time. We so accept a simpler expression for the current induced in the coil:
The induced electric current depends on both the area of the whorl and the change in magnetic field. In a coil of wires, each loop contributes an expanse A to the right-hand side of the equation, and then the induced emf will exist proportional to the number of loops in a curlicue. Only doubling the number of loops doubles the length of wire used so doubles the resistance, and then the induced current will not increment when loops are added.
Induction and Magnetic Recording
To write magnetic data, current is sent through the curlicue in proportion to the desired bespeak. This current produces a magnetic field proportional to the electric current. The magnetic field aligns the spins in the ferromagnetic cloth. Every bit the cloth moves abroad from the coil, the magnetic field decreases, and the spins remain aligned until they enter some other magnetic field (when they are erased).
Dissimilar electric storage, magnetic storage can be either analog or digital. The amount of spin alignment depends on the strength of magnetic field, so analog data can be recorded with a continually varying current producing a continually varying magnetic field. Digital data tin can be recorded by alternating the direction of the current. To minimize data loss or errors, binary data is not determined solely by the management of magnitization in a domain. Instead, information technology is represented past the change in magnetic orientation betwixt ii domains. If one bit of magnetic field has the same direction every bit the i before it, that represents a 0 (no modify). If 1 chip of magnetic field has the reverse management as the one before it, that represents a one (alter). Then a 1 is written by changing the management of current between the 2 domains comprising a bit, and a 0 is written by keeping the management the same. Each bit starts with a modify of orientation. This convention for recording data identifies errors, since i would never have iii domains of the same orientation in a row. In addition, the orientation should change with every other domain. If the calculator thinks a bit is complete but the orientation does non change, it knows that some error has occurred. Some examples of domains, bits, and strings are shown below.
To read magnetic data, the ferromagnetic textile is moved past the coil of wire. The irresolute magnetic field caused past the material's motility induces a current in the scroll of wire proportional to the change in field. If a 0 is represented, the magnetic field does not alter between the two domains of a bit, so no current is induced as the magnetic material passes the coil. For a one, the magnetic field changes from ane management to the other; this alter induces a current in the coil.
Inductive reading of magnetic data is subject to many limitations. Since the change in magnetic field will be greater if the ferromagnetic material is moved faster, the induced current is dependent on the speed of the material. Thus the sensitivity of inductive read heads is limited by the precision of the textile speed. The other limiting factor on inductive heads is the strength of the magnetic field. As efforts to increase storage density go on, the size of a data element shrinks. Since fewer electrons are now contained in the region of one scrap, the associated magnetic field is smaller. This smaller magnetic field produces less change and thus less induced current, requiring more than loops to produce a measurable current. Equally mentioned above, more loops means more than resistance which means more than oestrus. Because of these limitations, new magnetic storage devices use the phenomenon of magnetoresistance to read magnetic data.
Summary
Facts About the Force
(From Driving Force: The Natural Magic of Magnets, by James D. Livingston, (Havard University Press: Cambridge), 1996)
These x facts near the forcefulness from Driving Force by Livingston summarize most of the data contained in this and the side by side reading. Of particular interest to the workings of computers are steps 4, vi, and 8. nine and 10 are also important concepts to remember. This reading assignment has just touched on the applications of magnets in information systems and other ordinarily-used technologies. If you are interested in learning more, the book past Livingston is an excellent identify to beginning.
| 1. | If costless to rotate, permanent magnets point approximately due north-due south. |
| 2. | Like poles repel, unlike poles attract. |
| 3. | Permanent magnets concenter some things (like iron and steel) only non others (similar wood or drinking glass). |
| iv. | Magnetic forces act at a distance, and they can act through nonmagnetic barriers (if non too thick). |
| v. | Things attracted to a permanent magnet become temporary magnets themselves. |
| 6. | A whorl of wire with an electrical electric current flowing through it becomes a magnet. |
| 7. | Putting iron inside a current-conveying coil greatly increases the strength of the electromagnet. |
| viii. | Changing magnetic fields induce electric currents in copper and other conductors. |
| 9. | A charged particle experiences no magnetic force when moving parallel to a magnetic field, simply when it is moving perpendicular to the field it experiences a force perpendicular to both the field and the direction of motility. |
| 10. | A current-carrying wire in a perpendicular magnetic field experiences a force in a direction perpendicular to both the wire and the field. |
Suggested Additional Reading
Driving Force: The Natural Magic of Magnets, by James D. Livingston, (Havard University Printing: Cambridge), 1996. An extremely good volume about magnetism and their applications in our everyday activities. It'due south cheap, too (about $12+shipping from Amazon, VarsityBooks, BigWords, or barnesandnoble.com).
Computing: The Technology of Information, past Tony Dodd. (Oxford University Printing: New York), 1995. Pages 70-71 include short clarification of capacitors in DRAM.
How Computers Piece of work, by Ron White.
The Cartoon Guide to Physics, by Larry Gonick and Art Huffman. (Harper Perennial: New York), 1991. This is a swell convenient treatment of the basic concepts in phsyics, including magnetism and induction.
Whatsoever introductory physics text, such equally Fundamentals of Physics past Halliday, Resnick and Walker.
Copyright © 2001-2002 Doris Jeanne Wagner. All Rights Reserved.
Source: https://www.rpi.edu/dept/phys/ScIT/InformationStorage/faraday/magnetism_a.html
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