what happens to the electric current if the number of loops of a wire doubles?
 | Introduction to Magnetism and Induced Currents |
Despite the increasing prevalence of CD-ROMs and the use of electronic storage in RAM, about data is nevertheless stored magnetically. This reading assignment reviews the basic concepts of magnetism, then introduces the three dissimilar effects which accept been utilized to read magnetic data.
Sources of Magnetic Fields
Discussion Question: What produces magnetic fields? Is there any departure between the fields of permanent magnets and the fields of electromagnets? Are the sources of the fields the same in these 2 cases, or are they different?
Our thinking virtually magnetic sources has changed considerably over the centuries. The simply course of magnetism known until the 19th century was ferromagnetism. Certain materials, when "magnetized", would attract sure other materials. The only materials attracted past a magnet were those that could become magnetized themselves. Since only certain materials exhibited magnetic properties, scientists ended 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 along the axis of the loop. The direction of the field within the loop tin be constitute by curling the fingers of the correct hand in the direction of the electric current through the loop; the thumb then points in the management of the magnetic field. With this discovery, magnetism appeared to occur in two different manners: ferromagnetism depending on the cloth, and electromagnetsim caused by currents.
 and (b) spin of an electron. | Every bit diminutive physics and chemistry began to explain the periodic tabular array with the aid of the Bohr model of the cantlet in the early 1900s, magnetic backdrop were assigned to the electrons in atoms. Electrons appeared to exhibit two types of motion in an cantlet: orbital and spin. Orbital movement referred to the movement of an electron effectually the nucleus of the atom. Since a charged particle was moving, a magnetic field was created. But electrons (and protons and other particles) also appeared to be spinning around their centers, creating yet some other magnetic field. The magnetic field due to the orbital motion and the magnetic field due to the spin could cancel or add together, only expressions for the exact coupling betwixt the ii are too complicated to become into here. Since electrons were moving and spinning inside atoms, ferromagnetism could now be explained by the move 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 and create an observable field. |
| That final sentence is slightly unrealistic. Solids contain incredably large numbers of electrons, and they will never all completely line up. Instead, a solid generally consists of magneticdomains. In a domain, the majority of electrons which tin (unpaired valance electrons) will have spins aligned. Side by side domains will generally not be oriented in identical directions. In magnetized materials, some domains will cancel, but the average domain orientation volition be in one direction, producing a net magnetic field. In unmagnetized materials, the domains are randomly oriented and cancel, so no observable field is created. The effigy to the right illustrates these concepts. The concept of magnetism existence entirely due to the motion of charges has been modified significantly in the 20th century, thanks to quantum mechanics. The Bohr model of the atom must be modified to include dubiety. We tin can never determine exactly the trajectory of an electron or say for certain where it will be establish. The dubiety principle requires that we instead say just where the electron is most likely to be plant. Until nosotros mensurate 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, simply . . . |  (b) are accounted for, a net magnetic field remains. |  (c) region is the cyberspace magnet- ization of the region, or domain. |
 (d) such domains, which are generally not aligned completely, just |  (east) domains in a magnetized solid don't completely cancel but exit a net field |  (f) the fields of nearby domains completely cancel, leaving no cyberspace field |
Our concept of spin must as well exist adjusted to fit with the discoveries of the 20th century. Electrons are idea to exist "point particles," which ways they accept no spatial extent. Which means they 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 physical rotation through infinite. Since electrons and other particles accept intrinsic spin, they create magnetic fields automatically. Subsequently considering quantum mechanics, nosotros are once more left with 2 types of magnetism: intrinsic magnetism due to the "spins" of electrons, and electromagnetism due to the motion of electrons.
Only 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 shell, filling it (the first beat contains but two states). The exclusion principle states that no ii electrons tin can accept the same verbal properties. For them both to occupy the aforementioned energy shell, their spins must be oppositely directed and abolish. Electrons in solids with partially-filled valence shells may, nevertheless, line up with the aforementioned spin as other electrons, thereby creating a non-nil internet magnetic field.
Magnetic Fields and Forces
Discussion Question: Think near what y'all have learned during your lifetime about electricity and magnetism. How are they alike? How are they different? Whatsoever charged object in an electric field experiences an electric force. Will whatever charged object in a magnetic field experience a magnetic strength? Does an object have to be charged to feel a magnetic force?
Magnets can exert a forcefulness at a distance, just similar electric charges. So it is advantageous to describe the effects of magnets in terms of a magnetic field, B i , much in the same mode that the furnishings of charges are described past the electrical field. Nosotros accept 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 = 1 Vs/gtwo). The tesla is a fairly big unit of magnetic field, and then we oftentimes listing magnetic field strengths in terms of Gauss (one Yard = 10-iv T). The magnetic field of the earth is about one-one-half gauss in strength.
 | Like an electric field, a magnetic field may be represented with field lines. These lines (and the magnetic field) bespeak from the northward 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 always closed - they never have a starting bespeak or stopping point. Whenever you lot accept a northward pole, you must accept a south pole too. Some other way to say this is that magnetic monopoles (single poles) practise not exist. Electric monopoles, on the other hand, be in abundance. Examples are an electron, a proton, or any other charged particle. |
| Even the magnetic field produced past a current-conveying wire must course complete loops. Above, y'all were told that a loop of current-carrying wire produces a magnetic field along the axis of the wire. The correct-hand rule gives the management of the field inside the loop of wire. The magnetic field turns back the other style outside of the loop. As shown in the figure on the right, this magnetic field from a loop of current-carrying wire looks like to the field from a permanent bar magnet. |  |
Anyone who has used a compass knows that a magnet experiences a force in a magnetic field. Just as for electric charges, opposite magnetic poles repel and similar poles concenter. Thus the magnetic field pointing from due north to south points in the direction of the forcefulness on a North POLE of a magnet. One interesting result of this is that the Earth'due south geographic north pole is its magnetic south pole. A compass needle's magnetic north pole will point toward the geographic north pole of the Earth. Since the north pole of a magnet is attracted to the s pole of another magnet, this means that the geographical north pole of the Earth is really a south magnetic pole.
Permanent magnets are not the but objects which feel the magnetic force. Electric charges tin can experience a magnetic force if two conditions are met:
- The charge must exist 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 strength is more accurately expressed in terms of a cantankerous-product:
F B = q v x B
The magnitude of a cross-product depends on sin q, giving the previous expression. For our purposes, the first expression is sufficient, provided y'all call up that the force is perpendicular to both the velocity and the magnetic field.
A current-carrying wire also experiences a forcefulness in a magnetic field, since current is nil more than moving charges. As for single charges, the current must exist moving in a management 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 current and 50 is the length of wire in the uniform magnetic field of strength B.
| one | To be verbal, the symbol B represents magnetic flux density, also called magnetic induction, non magnetic field. The truthful magnetic field is denoted by H. H and B differ only past a cloth-dependent constant. For most purposes, the departure is inconsequential, then we will refer to B as the magnetic field. If you accept further courses in magnetism, you will learn the distinction. |
Induced Currents, Induced EMF, and Faraday's Law
Discussion Question: Tin y'all create a current through a wire without connecting the wire directly to a voltage source like a battery? Practice all of your appliances have direct connections? What about your car engine?
If a coil of wire is placed in a changing magnetic field, a current volition be induced in the wire. This current flows because something is producing an electric field that forces the charges around the wire. (It cannot be the magnetic forcefulness since the charges are non initially moving). This "something" is called an electromotive force, or emf, fifty-fifty though it is not a forcefulness. Instead, emf is like the voltage provided by a bombardment. A changing magnetic field through a curlicue of wire therefore must induce an emf in the coil which in turn causes current to flow.
The constabulary describing induced emf is named subsequently the British scientist Michael Faraday, but Faraday'due south Constabulary should really exist chosen Henry'due south Law. Joseph Henry, an American from the Albany area, discovered that changing magnetic fields induced electric current before Faraday did. Unfortunately, he lived in the age before instantaneous electronic communication between Europe and America. Faraday got published and got famous before Henry could report his findings. Interestingly enough, Henry had to explain the results to Faraday when the two met a few years subsequently.
Briefly stated, Faraday's law says that a changing magnetic field produces an electric field. If charges are gratuitous to move, the electric field will crusade an emf and a current. For example, if a loop of wire is placed in a magnetic field and then that the field passes through the loop, a alter in the magnetic field volition induce a current in the loop of wire. A current is besides induced if the area of the loop changes, or if the area enclosing magnetic field changes. So it is the change in magnetic flux, divers as
that determines the induced current. A is the area vector; its magnitude is the area of the loop, and its management is perpendicular to the area of the loop, and q is the angle betwixt A and the magnetic field B. The last equality (removing the integral) is valid only if the field is uniform over the unabridged loop.
Faraday'southward Constabulary says that the emf induced (and therefore the current induced) in the loop is proportional to the rate of change in magnetic flux:
east is the emf, which is the piece of work done moving charges around the loop, divided past the charge. It is similar in concept to voltage, except that no accuse 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 weather are met; in practical applications, nevertheless, magnetic field will vary through a loop, and the field will not always exist perpendicular to the loop.
Since all applications of Faraday'due south Law to magnetic storage involve a coil of wire of fixed area, nosotros will also presume that (c) the area does not change in time. We then accept a simpler expression for the current induced in the gyre:
The induced current depends on both the area of the coil 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, so the induced emf volition exist proportional to the number of loops in a coil. Just doubling the number of loops doubles the length of wire used and and then doubles the resistance, so the induced current will not increase when loops are added.
Consecration and Magnetic Recording
To write magnetic data, current is sent through the curl in proportion to the desired signal. This current produces a magnetic field proportional to the current. The magnetic field aligns the spins in the ferromagnetic textile. As the material moves away from the coil, the magnetic field decreases, and the spins remain aligned until they enter another magnetic field (when they are erased).
Unlike electric storage, magnetic storage tin can be either analog or digital. The corporeality of spin alignment depends on the forcefulness of magnetic field, and then analog data tin exist recorded with a continually varying electric current producing a continually varying magnetic field. Digital data tin can exist recorded by alternating the direction of the current. To minimize information loss or errors, binary data is non determined solely by the management of magnitization in a domain. Instead, it is represented by the change in magnetic orientation between two domains. If one bit of magnetic field has the aforementioned direction every bit the one before it, that represents a 0 (no alter). If ane bit of magnetic field has the contrary direction equally the one before it, that represents a 1 (change). Then a 1 is written by changing the direction of electric current between the two domains comprising a bit, and a 0 is written by keeping the direction the same. Each bit starts with a change of orientation. This convention for recording information identifies errors, since i would never take three domains of the same orientation in a row. In improver, the orientation should change with every other domain. If the calculator thinks a scrap is consummate but the orientation does not alter, it knows that some fault 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 changing magnetic field caused by the material'south move induces a electric current in the coil of wire proportional to the change in field. If a 0 is represented, the magnetic field does non alter betwixt the ii domains of a bit, then no current is induced as the magnetic fabric passes the curl. For a 1, the magnetic field changes from one direction to the other; this change 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 express by the precision of the textile speed. The other limiting gene on inductive heads is the strength of the magnetic field. Every bit efforts to increase storage density keep, the size of a information element shrinks. Since fewer electrons are now independent in the region of one bit, the associated magnetic field is smaller. This smaller magnetic field produces less change and thus less induced current, requiring more loops to produce a measurable electric current. As mentioned above, more than loops means more than resistance which ways more than rut. Because of these limitations, new magnetic storage devices use the phenomenon of magnetoresistance to read magnetic data.
Summary
Facts Most the Force
(From Driving Force: The Natural Magic of Magnets, by James D. Livingston, (Havard University Press: Cambridge), 1996)
These ten facts nigh the force from Driving Force by Livingston summarize most of the information contained in this and the side by side reading. Of item interest to the workings of computers are steps 4, 6, and 8. 9 and 10 are also of import concepts to think. This reading assignment has only touched on the applications of magnets in information systems and other commonly-used technologies. If you lot are interested in learning more, the book by Livingston is an excellent place to start.
| 1. | If free to rotate, permanent magnets point approximately due north-south. |
| 2. | Like poles repel, unlike poles attract. |
| 3. | Permanent magnets concenter some things (like iron and steel) but not others (like woods or glass). |
| 4. | Magnetic forces deed at a altitude, and they can human action through nonmagnetic barriers (if non too thick). |
| v. | Things attracted to a permanent magnet get temporary magnets themselves. |
| vi. | A coil of wire with an electrical current flowing through it becomes a magnet. |
| 7. | Putting iron inside a current-conveying coil profoundly increases the strength of the electromagnet. |
| 8. | Changing magnetic fields induce electric currents in copper and other conductors. |
| 9. | A charged particle experiences no magnetic forcefulness when moving parallel to a magnetic field, but when information technology is moving perpendicular to the field it experiences a force perpendicular to both the field and the management of motion. |
| x. | A current-conveying 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 Press: Cambridge), 1996. An extremely good book about magnetism and their applications in our everyday activities. It's inexpensive, besides (most $12+shipping from Amazon, VarsityBooks, BigWords, or barnesandnoble.com).
Computing: The Technology of Data, by Tony Dodd. (Oxford Academy Printing: New York), 1995. Pages 70-71 include short description of capacitors in DRAM.
How Computers Work, by Ron White.
The Drawing Guide to Physics, by Larry Gonick and Art Huffman. (Harper Perennial: New York), 1991. This is a bang-up user-friendly treatment of the basic concepts in phsyics, including magnetism and induction.
Whatever introductory physics text, such every bit Fundamentals of Physics by 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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