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1 Natural Sciences I lecture 12: more Electricity & Magnetism MAGNETISM Scientific curiosity about magnetism goes back to at least 600 B.C. to the Greeks' observations of the properties of lodestone, which included th following: N The two "poles" orient themselves more-or-less north and south in the Earth's magnetic field – hence the "N-S" designations we still use today. lodestone iron particles Before we go further, let's get ourselves oriented with respect to the Earth's magnetic field... o 22 E o o 20 E 18 W o 18 E o 10 W o 16 E o 6W o 14 E o 12 E o 10 E Magnetic declinations indicate the location of the magnetic north pole relative to true north o 8E o 6E o 4E 2E o 0 o 2 Earth's magnetic field magnetic north geographic (true) north The geographic and magnetic north poles are not located in exactly the same place. N We describe the orientation of the Earth's magnetic field at a given location in terms of a magnetic declination (p. 1) and a magnetic dip. The dip is horizontal at the magnetic equator and vertical at the north and south magnetic poles. S You are now familiar with the manner in which we represent the lines of force of an electric field. A Similar approach is used for magnetic fields... lines of force N S arrowheads indicate direction in which the north pole of a small magnet would point The magnetic field is analogous the electric field surrounding an electric charge and the gravity field surrounding a massive object. Like these other two force fields, a magnetic field results in "force at a distance" effects. An additional similarity with electric fields, of course, is that opposite poles attract and like poles repel – as is the case with electric charges. 3 Source of Magnetic Fields Despite the similarities noted on page 2, magnetic fields have properties that signal obvious differences from electric fields. Recall that an electric field is caused by a local excess of positive or negative charge (i.e, an overabundance or deficiency of electrons). A magnetic field, in contrast, is not caused by localization of north or south poles on an object. One way to demonstrate this fact is to cut up a bar magnet into progressively smaller pieces. No matter how hard you try, you will only produce shorter versions of the same magnet – i.e., you will not succeed in isolating a north or south pole (a magnetic monopole). S N S S N S N S N S N N S N S N S N S N S N S N S N S N S N magnetic poles come only in pairs (electric charges can be paired, but they don't have to be) Magnetism is now understood to be produced by electric currents – it is a secondary property of electricity. The first big breakthrough was made by a Danish physicist, Hans Christian Oersted, in 1820. Interestingly, his discovery was largely accidental, made during a lecture/demonstration to students gathered around a table. Here's a more elaborate representation of Oersted's unplanned experiment... compasses ( ) conventional current (+) When the wire (red) is connected to the battery, the compass needle aligns itself perpendicular to the wire, pointing in the direction given by the "right-hand rule" (thumb pointing in the direction of conventional current flow). 4 The right-hand rule is used to determine the direction of magnetic field lines around a conventional current (flow of positive charges) The left-hand rule is used to determine the direction of magnetic field lines around an electron current When the thumb is pointing in the direction of flow, the curled fingers reveal the orientation of the magnetic field. Current in a loop: The red arrows show the magnetic field lines at various points. Which "kind" of current (electron or conventional) is shown? (+) ( ) 5 What Oersted discovered is that electric current produces a magnetic field, suggesting for the first time that magnetism is a property of moving electric charges. A stationary charge produces only an electric field; charges in motion produce a magnetic field to accompany the electric field. Just as a gravitational field is a property of the space surrounding a mass and an electric field is a property of space surrounding a charge, a magnetic field is a property of space surrounding a moving charge (and the strength of the magnetic field is proportional to the speed at which the charge is moving). Digression on permanent magnets... Something seems amiss here. If magnetic fields result from charges in motion, what's the story with permanent magnets – such as bar magnets, horseshoe magnets, and those we use to post stuff on our refrigerators? There is no current flowing through these, so where does the magnetic field come from? The key to this apparent paradox is that all atoms consist of charged particles (electrons and protons), and the electrons of every atom are in motion about the nucleus. Since they are moving charges, these electrons create magnetic fields. (Note: This is a cartoon. It is not intended to be an accurate representation, but to convey an idea) In most materials, the tiny magnetic fields cancel one another, so the overall object exhibits no magnetic properties. In a few materials, however, the atoms are oriented in such a way that their individual magnetic fields can act in unison to impart magnetic properties to the overall structure. Examples of such materials are iron and some of its oxides, nickel, cobalt and rare earth elements. In a permanent magnet, the atoms are grouped in small regions called magnetic domains, and these domains are aligned with one another to create the overall magnetic field. (next page) 6 Permanent magnets (cont'd) UNMAGNETIZED IRON individual magnetic domains (~0.01 to 1 mm in size) orientations random MAGNETIZED IRON orientation of magnetic domains mostly aligned N When unmagnetized iron is placed in a magnetic field, the domains aligned with that field grow at the expense of those that are not aligned Digression on natural permanent magnets Permanent magnets are formed in nature in at least three ways (all involve the iron oxide magnetite – Fe3O4) All molten rocks contain some iron oxide. This forms crystals as the lava cools, some of which are magnetite. When the magnetite cools below its Curie o temperature (~500 C), it becomes magnetized in alignment with the Earth's magnetic field. corbis.com 7 Sedimentary rocks can also acquire permanent magnetism (called natural remanent magnetism), but by a quite different mechanism. Most sedimentary rocks are formed from rock and mineral particles settling that have settled to the bottom of a water body. Some of these particles are magnetite (Fe3O4), and if they settle in a "gentle" environment, they can orient themselves in the Earth's magnetic field. The resulting sedimentary rock thus records the location of the magnetic pole (which moves around over geologic time). N N S The rock that eventually forms from this sediment will contain innumerable tiny magnets, all pointing in the same direction Other natural permanent magnets: Navigational and "orientational" uses by organisms; for example migratory birds magnetotactic bacteria (apparently use internal magnetite grains to distinguish up from down) 8 ...back to ELECTRIC CURRENTS and MAGNETISM Current loops As we learned on page 3, electric current flowing in a wire creates a magnetic field around that wire. This leads to some interesting possibilities if we bend the wire or bring it close to other current carrying wires... First, a single loop: e e Because of the orientation of the magnetic field lines passing through the loop, the two sides of the loop will have different poles. Can you tell which pole the front side of the loop has? A current passing through a cylindrical coil (a solenoid) causes the coil to act like a bar magnet: (+) These suspended batteries orient themselves in the magnetic field of a bar magnet (Ampere showed this in 1820s) ( ) N S ( ) (+) S N 9 ( ) N The magnet pictured here is an electromagnet. Unlike a bar magnet, the magnetic field can be turned on and off by connecting or disconnecting the battery. Moreover, the strength of the field can be varied by changing the current or the number of loops in the solenoid. (+) S The discovery of the elec-tromagnet and description of its properties by Andre Ampere in the 1820's created the possibility of doing mechanical work with an electrical device. There are numerous applications of electromagnets that you can read about in your text (see p. 131ff). Some examples are discussed on page 12... The main lesson of the last few pages is that current flowing through a wire creates a magnetic field. If the wire is formed in a loop or coil, the resulting magnetic field has a poles like a bar magnet. Two other observations follow: the magnetic fields produced by two different current-carrying wires or current loops interact with one another (Ampere worked out the relationships) if a loop of wire is moved in a magnetic field, a current is induced in the wire – ELECTROMAGNETIC INDUCTION 10 ELECTROMAGNETIC INDUCTION Following quickly on the contributions of Oersted and Ampere to our understanding of magnetism came the further discovery that a coil of wire moving through a magnetic field develops a potential difference (a voltage) along its length. This is called an induced voltage, and it leads to flow of electrons (current). Current can also be induced in a coil by varying the strength of a magnetic field in which the coil sits. These induction phenomena were discovered simultaneously (and independently) in 1831 by Joseph Henry in the U.S. and Michael Faraday in England. Here's a simple demonstration... S amps 0 N S Moving the magnet through the coil induces a current through the coil. Changing the direction of motion changes the direction of current flow. amps 0 No current is detected if the magnet is not moving N 11 Here's the device Faraday first used to demonstrate electromagnetic induction: switch amps 0 primary coil (insulated wire) iron ring secondary coil (insulated wire) Immediately upon closing the switch, Faraday noticed a brief flicker of current in the secondary coil, but none thereafter. As the magnetic field was being established in the iron ring (i.e., when the field lines were moving), a current was induced in the secondary coil. However, once the magnetic field in the ring became stable (very quickly!), there was no induced current. This observation led Faraday to the realization that the magnetic field has to be moving in order for induction to work. Question of the day: What would have happened if Faraday had used an electrical outlet rather than a battery?? (OK, so he didn't have any outlets handy) Demonstrations like the preceding ones can be used to show that electromagnetic induction occurs when the coils of wire move across magnetic field lines (or vice versa: it doesn't matter which one moves as long as there is relative motion between the magnetic field lines and the coil). The induced voltage depends upon: the number of loops (the more loops, the higher the voltage) the rate of relative motion the strength of the magnetic field 12 Two EXAMPLES of devices that use electromagnetic induction... A simple dynamo (AC generator) As it rotates, the wire loop (green) crosses the magnetic field lines in alternating directions. The resulting current in the loop therefore flows first in one direction, then the other. magnet field lines S rotation R N output (voltage or direction of current flow) rotation angle 13 The transformer As noted in the last lecture, Thomas Edison was an advocate of DC power for commercial and domestic use, but ultimately AC power was adopted instead. A principal reason was that AC power can readily be stepped up and down in voltage for different applications (it can also be transmitted over long distances more efficiently). Although he didn't use it in this way, the device built by Faraday (page 11) demonstrates the great advantage of AC power over DC power: it is a crude transformer. With direct current flowing in the primary coil, however, it works as a transformer only for a fleeting moment when the current is first turned on. This is the only time when the magnetic field created in the iron ring is changing. If the current in the primary coil were AC, then a stable alternating current would be established in the secondary coil as well... Step-down transformer SECONDARY PRIMARY 120 V 10 loops 1 loop 12 V voltsprimary voltssecondary = loopsprimary loopssecondary power input = power output Step-up transformer watts input = watts output (amps volts)in = (amps volts)out 1 loop 1200 V 120 V 10 loops 14 Calculations with transformers – Examples 1. A step-up transformer has 10 loops on its primary coil and 40 loops on its secondary coil. If the primary coil is supplied with AC current at 120 V, what is the voltage on the secondary coil? NP = 10 loops NS = 40 loops VP = 120 V VS = ? VS VP = NS NP VS = VS = VPNS NP (120 V) (40 loops) 10 loops = 480 V 2. The primary coil of the step-up transformer in example 1 is supplied with 10 amps of AC current at 120 V. What current flows in the secondary coil? VP = 120 V IP = 10 A VPIP = VSIS IS = VPIP VS IS = (120 V) (10 A) 480V VS = 480 V IS = ? = 2.5 A The same amount of electrical energy can be carried at a lower current (reducing loss due to resistance) when a higher voltage is used (power = amps x volts). This is why very high voltage transmission lines (50,000 or more volts) are used to carry electrical energy from generating facilities to consumers. Such voltages are way too high for normal uses, so they are stepped way down with transformers. 15 Back to Earth... source of the Earth's magnetic field magnetic field lines outer core (molten iron + nickel) CONVECTING inner core (solid iron + nickel) 16 The Earth’s magnetic field is now understood to work like a “self-exciting” dynamo... dynamo magnetic field lines metal disk (e.g., copper) S permanent magnet The magnetic field induces the radial current in the disk. amps N 0 “self-exciting” dynamo magnetic field lines electromagnet (coil) amps 0 The magnetic field induces the radial current in the disk; the current flowing through the coil sustains the magnetic field.