Electronic Engineering Technology



Electricity and MagnetismAll About Circuits: Chapter 14: MAGNETISM AND ELECTROMAGNETISMPermanent magnetsElectromagnetismMagnetic units of measurementPermeability and saturationElectromagnetic inductionMutual inductanceNew Question Set on Magnetism, Inductance and CapacitanceIntroductionWhat is a permanent magnet?A material which has the intrinsic property of creating a magnetic fieldE.g. neodymium magnetField always has a north and a southLike poles repel. Opposite poles attractDifferent types of magnetic material:Ferromagnets (strongly attracted by magnetic field): can become permanent magents or will interacte strongly with magnetic fieldsAt the atomic level, the spin and orbital angular momentum of each of the electrons creates a magnetic dipole moment (in other words a tiny magnet). If all of the magnetic dipole moments are oriented in the same direction, then ferromagnets become permanent magnetsExamples of Ferromagnets:Iron, Cobalt, Nickel, neodymiumParamagnets (weakly attracted by magnetic field): The magnetic dipole moments will align to an externally applied field, but if the field is removed, the alignment is lostExamples of Paramagnets:Tungsten, cesium, aluminumDiamagnets (weakly repelled by magnetic field): The magnetic dipole moments align to oppose an external magnetic fieldWater is a good diamagnete.g. Levitating frog in a 16 Tesla magnetic field: For more info: What is an electromagnet?A magnet whose magnet field is created by passing strong current through a wire. Usually the wire is coiled around a ferromagnet to increase the field strengthElectromagnetism = interface between electric world and mechanical worldElectricity ? Magnetism ? Physical WorldHow?A changing electric field creates a magnetic fieldCurrent = movement of electric charges and is therefore a changing electric field. Any time there is current, a magnetic field will be created.A changing magnetic field creates an electric fieldE.g. a spinning magnet = a changing magnetic field which will create an electric field. A generator works by spinning magnets to create a voltage (electric field.Other examples of electromagnetism at work:? DC and AC motors? Generators? Rail guns (maglev trains, theoretical space vehicle launch, weapons)? speakers and microphones? transformers? magnetic switches (reed switches, relays)? Magnetic storage system (harddrives, tapes)? Magnetic resonance imaging? Magnetoencephalography (MEG) ? SQUID (Superconducting Quantum Interference Device)(e.g. Strange Days movie)Again, the basic Idea behind Electromagnetism:Electricity ? MagnetismSome demos:Drop magnet down the copper rodPhET Bar MagnetPhET Electromagnet simulation: Electromagnetic Acceleration Overview (Plasmaboy): (also see )Magnetic Fields (00623)Magnetic poles are designated by North (N) and South (S). The end of a magnet that points to the Earth’s north pole is called the North pole of the magnet.Opposite poles attract and like poles repelIf this is true, what is the magnetic polarity of the North pole of the earthMagnetic Field QuantitiesGiven a magnet (permanent or electric) the strength of the magnetic field at some point away from the magnet depends on two thingsHow strong the magnet isWhat kind of material the magnetic field is in and passes through (00625)Putting an iron bar in the vicinity of the permanent magnet will actually increase the magnetic field strength in and around the iron. The strength of a magnetic field is usually indicated in a diagram by how close together the lines representing the magnetic field are drawn. The lines are called lines of flux and the closer together the lines are, the greater the flux density. Lines of flux strive to be as short as possible, therefore the lines of flux that pass from one magnet to another tend to pull the magnets together (i.e., pull opposite poles together)In many ways, a magnetic circuit is like an electric circuit:This electric circuit has a “force” that is pushing electric charges. This force is the voltage. The opposition to the force is the electrical resistance in the circuit and the result of the force is an electrical currentThere’s a cause (voltage) an effect (current) and an opposition (resistance)(00678)This magnetic circuit also has a force/cause (the magnet) creating an effect (the magnetic flux) and is encountering an opposition (in a magnetic field, this opposition is called reluctance) (00678)Comparison of Electrical and Magnetic PropertiesElectrical QuantityUnitMagnetic QuantityUnitsVoltage (V)VoltsMagnetomotive Force (MMF)Amp-turnsCurrent (I)AmpsMagnetic Flux (Φ)WebersResistance (R)OhmsReluctance (R)Amp-turns/WeberR=(ρl/A)R =(1/μ)(l/A)Current DensityAmps/m^2Magnetic Flux Density (B)Tesla (Weber/m^2)Electric FieldV/mMagnetic Field Intensity (H)Amp-turns/mEach of the rows shows an electrical quantity and the analogous magnetic property. Why do you think the units for MMF are Amp-turns?The amount of flux created by the MMF depends on the distance from the wire as well as the material the flux is passing throughNote the last two rows in the table above, they talk about Flux per unit area and mmf per unit length:MMF is amp-turns due to a wire, but a more useful measurement is field intensity which is MMF per unit lengthField Intensity is denoted with the letter HH=MMFL MMF=HLFlux tells us how much magnetic field is created, but a more useful measurement is flux density which is Flux per using areaFlux density is denoted with the letter BB=ΦA Φ=BAWhile the two electrical and the magnetic quantities are analogous, they are not identical:For example: We can express a linear relationship between voltage, current (the slope of this line is 1/resistance):(00682)But the relationship between MMF, Magnetic Flux and Reluctance is not linear (to create a graph like the one below, you would increase the mmf, by turning up the current in an electromagnet and measuring the flux) (00682)Like with the V-I graph, the inverse of the slope of this graph will give us reluctance, but you can see that reluctance will not be constant. When will reluctance be greatest and when will it be lowest?The above graph doesn’t actually tell the whole picture of the relationship between flux and MMF. Not only is the relationship non-linear, but it also exhibits hysteresis…this means that the relationship changes depending on if MMF is increasing or decreasing:Just as we have a relationship between MMF and Φ,we have a relationship between H and BMMF=?RHL=BARμ is called the permeability of a material.Represents how magnetized a material (which can include air or a vacuum) becomes in the presence of an applied magnetic field.Looking at a graph similar to the Flux as a function of MMF graph above, what can you say about the permeability of the material represented in the graph: (00472)Some more questions about the graphThis graph demonstrates the magnetic phenomena of saturation and hysteresis. Using your understanding of these phenomenon, describe what would happen if an electromagnet fully energized with DC is suddenly turned off.Do electric circuits have a saturation point?A Changing Electric Field Creates a Magnetic FieldPHET Example: A current is moving electrical charge. If an electrical charge is moving, then its electric field is moving. Therefore a current carrying wire will create a magnetic field. In fact, the definition of current comes from the fact that current creates a magnetic field. 1 Amp is defined as the current required to produce an attractive magnetic force of 2 × 10–7 newtons per meter of length between two straight, parallel, infinitely long conductors with negligible cross sectional area placed one meter apart in a vacuum. It’s a strange definition and I don’t know why the Coulomb isn’t used as a base SI unit and then define 1 Amp based on the Coulom.The orientation of the magnetic field generated by the current depends on the direction of current:(00175)XXX add something like figures 6.4 and 6.5 from Boylestads Intro to E&MIf you imagine wrapping the fingers of your right hand around the wire, while pointing your thumb in the direction of conventional current flow. Your fingers will curl around the wire in the direction of the magnetic field being generated.If you coil up a wire as shown in the picture on the right hand side, then the magnetic field from each wire adds to the whole magnetic field and you end up with a magnetic field that is stronger than if you had just one wire. The strength of the field from the coiled wire is equal to the strength of a single wire multiplied by the number of coils (turns) you have in the wire.For this picture, an electrical wire is wrapped around part of an steel torus. The torus keeps the magnetic field concentrated inside of it. What direction does the magnetic field go? If the field intensity from the wire before it is wrapped is 1 Amp-turn/m, what is the flux density of the field in the torus assuming the μ of steel is 8.74x10^-4 H/m?(00679)Visualizing the magnetic field generated by loops of wire can be somewhat tricky. Try to determine which direction the field is inside one loop of wire, then all of the other loops will generate the same field. The field on the outside of the loops will be in the opposite direction.Let’s quantify some of these values in an example: (00679)total length of the iron core toroid is 0.2m, and cross-sectional area is 4×10-4m2current through wires is 10AThe wire is wrapped around the toroid 20 timesμ for the toroid is 4.1×10-4Wb/AmFind the field intensity (H) and the flux density (B) in this toroid.Using the concepts of flux, mmf, flux density and field intensity, we can see how magnetic circuits are analogous to electrical circuits in another way:(00678)In this magnetic circuit, let’s actually quantify some of these values. This circuit is somewhat more realistic: something could be placed in the air gap to expose it to a magnetic field. In an MRI (magnetic resonance imaging machine) that something could be you.In the above circuit, assume that the “cause” is an electromagnet that has 1000 loops around the coreThe total length of the circuit is 1 meter and the air gap is 10 cm.The cross sectional area of the core is 0.1m2μ for the toroid is 4.1×10-4Wb/Amμ for the gap is 4π×10-7Wb/AmDetermine how much current will be necessary to create 1Tesla of flux densityTwo points to keep in mind:Flux and B are constant throughout circuit (magnetic equivalent to KCL)The mmf from the electromagnet equals the mmf in the core plus the mmf in the air gap (magnetic equivalent to KVL)It takes a lot of mmf to create flux in an air gap (high reluctance)It takes a lot of voltage to create a current through something with high resistance.In the above example, the actual amount of flux is:ApplicationsCurrent Clamp: the current in a wire can be measured indirectly by measuring the magnetic field around the wire. A current clamp is a measuring instrument that does just that. If you cannot break a wire to insert an ammeter, you can use a current clamp to measure the magnetic field around the wire and convert that in to a current.()Motor: AC and DC motors work when current flows in the motor creating a magnetic field. A pre-existing magnetic field (often from permanent magnets) attracts or repels the magnetic field created by the current and since one of the magnetic fields is stationary and the other is on a rotor that can spin, the motor turns. We will cover motors in much more detail later.Speakers: Speakers consist of a stationary permanent magnet and an electromagnet connected to a diaphragm. When current moves through the electromagnet in one direction, it will be attracted to the permanent magnet and move the diaphragm towards the permanent magnet. When current moves through the electromagnet in the other direction it will pushed away from the permanent magnet. The current alternates direction many times per second causing the diaphragm to move in and out. The diaphragm movement is what causes sound A Changing Magnetic Field Creates and Electric FieldPHET. Faraday’s Electromagnetic Lab: The strength of the electric field created is measured in volts (electric field strength is actually measured in V/m) and is proportionally to the rate of change of the flux times the number of turns of wire:V=NdΦdtWhere the amount of flux that is measured must be perpendicular to the direction that current would be produced. For example, look at these pictures and determine which one will produce the greatest ΔΦ (i.e., change in flux):(00174)Going back to Faraday’s Law which is the equation relating the voltage produced by a changing magnetic field:V=NdΦdtIf you substitute, Φ=BA into the equation, you get:V=NdBAdtSo you can have a change in the flux density, or a change in the area through which the lines of flux go to you will induce a voltage. If you go back to Faraday’s EM Lab (PHET), you have the option of changing the area of the loop of wire…if that is all that you change, you can also induce a current. Numerical examples:A coil of wire with 1000 loops surrounds a permanent magnet with a flux of 10mWb. If the magnet is quickly withdrawn from the loop, decreasing the flux to 4mWb in 0.1s, what is the induced voltage?V=NdΦdt=10004mWb0.1s=40VHow many loops of wire would you need to induce a voltage of 50V when the flux through the loops is changing at a rate of 0.01Wb/s?N=Vd?dt=500.01=5000 turnsLenz’s LawWhen a changing external magnetic field induces a current in a conductor, the direction of the current is such that the magnetic field created by it will oppose the change in the external magnetic fieldThere doesn’t even need to be a circuit like shown above. If the object is a conductor of any shape, current will be created that will create a magnetic field in opposition to the changing external magnetic field ................
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