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UVA PHYS 632 - Lecture 8 Magnetic Fields Chp. 29

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Lecture 8 Magnetic Fields Chp. 29Magnetic FieldsMagnetism’s sociabilitiesCompass and DeclinometerPermanent MagnetsPermanent Magnets (continued)Magnetic field linesMagnetic field lines do not stop at surface. They are continuous. They make complete loops. Field lines for a bar magnet are the same as for a current loopPowerPoint PresentationIn analogy with the electric force on a point charge, the corresponding equation for a force on a moving point charge in a magnetic field is:Use right hand rule to find the direction of F. F=q v x BMotion of a point positive charge in a magnetic fieldApply Newton’s 2nd Law to circular motionExample: If a proton moves in a circle of radius 21 cm perpendicular to a B field of 0.4 T, what is the speed of the proton and the frequency of motion?Slide 15Suppose we have an electron Which picture is correct?Example of the force on a fast moving proton due to the earth’s magnetic field. (Already we know we can neglect gravity, but can we neglect magnetism?)Force on a current-carrying wireShow force on a wire in a magnetic fieldMagnetic bottle. The charge is trapped inside and spirals back and forthTorques on current loopsTorque on a current loopGalvanometerMagnetic dipole moment mDemo: show torque on current loop (galvanometer)Warm-up set 8Slide 27Cathode Ray TubeLecture 8 Magnetic Fields Chp. 29•Cartoon Magnesia, Bar Magnet with N/S Poles, Right Hand Rule•Topics–Magnetism is likable, Compass and diclinometer, Permanent magnets–Magnetic field lines, Force on a moving charge, Right hand rule,–Non-uniform magnetic field–Force on a current carrying wire, Torque on a current loop•Demos–Natural magnetic rock–Compass and diclinometer–Iron fillings and bar magnets–Compass needle array–Pair of gray magnets–Magnetic blocks from Fermi LaB–CRT illustrating electron beam bent bent by a bar magnet–Gimbal mounted bar magnet–Wire jumping out of a magnetron magnetMagnetic Fields•Magnetism has been around as long as there has been an Earth with an iron magnetic core.•Thousands of years ago the Chinese built compasses for navigation in the shape of a spoon with rounded bottoms on which they balanced (Rather curious shape for people who eat with chopsticks).•Certain natural rocks are ferromagnetic – having been magnetized by cooling of the Earth’s core.•Show a sample of natural magnetic rockMagnetism’s sociabilities•Magnetism has always has something of a mystic aura about it. It is usually spoken of in a favorable light.•Animal magnetism, magnetic personality, and now you can wear magnetic collars, bracelets, magnetic beds all designed to make you healthier – even grow hair.•We do not have the same feeling about electricity. If you live near electric power lines, the first thing you want to do is to sue the electric company.Compass and Declinometer•In 1600 William Gilbert used a compass needle to show how it oriented itself in the direction of the north geographic pole of the Earth, which happens to be the south magnetic pole of the Earth’s permanent magnetic field.•Show compass and declinometer. Each has a slightly magnetized needle that is free to rotate. The compass lines up with the component of the magnetic field line parallel to the surface of the Earth. The declinometer lines up with the actual magnetic field line itself. It says that the angle between the field lines and the surface is 71 degrees as measured from the south.•Note that this looks a lot like the UVa electroscope. Recall it lined up with the electric field of the VDG. Show model of Earth field lines assuming a uniformly magnetized sphere•Basically there are two types of magnets: permanent magnets and electromagnetsPermanent Magnets•Show Fe fillings in oil and bar magnet. Show bar magnet surrounded by compass needle array. Show overhead of two bar magnets.•Bar magnet is a model of a ferromagnetic material that can be permanently magnetized. Other ferromagnetic materials are cobalt and nickel.•The origin of magnetism in materials is due to the orbiting motion of the charged electron around the nucleus and the spinning motion of the charges electron on its own axis. In most materials the contribution from all electrons cancel out.•In ferromagnetic atoms they don’t cancel out. There are whole sections of the iron called domains where the magnetism does add up from individual electrons. Then there are other sections or domains where contributions from different domains can cancel. However, by putting the iron in a weak magnetic field you can align the domains more or less permanently and produce a permanent bar magnet as you see here.•Show strong grey magnets and geometrically small, but high density rare earth alloy that produces a whopping magnetic field.Permanent Magnets (continued)•In ferromagnetic atoms they don’t cancel out. There are whole sections of the iron called domains where the magnetism does add up from individual electrons. Then there are other sections or domains where contributions from different domains can cancel. However, by putting the iron in a weak magnetic field you can align the domains more or less permanently and produce a permanent bar magnet as you see here.•Show strong grey magnets and geometrically small, but high density rare earth alloy that produces a the atomic level – Magneic atoms have an atomic dipole – not a monopole as whopping magnetic field.+avElectron orbiting nucleusMagnetic dipoleseElectron spinning on its axisMagnetic dipole-Magnetic field linesSimilarities to electric lines•A line drawn tangent to a field line is the direction of the field at that point.•The density of field lines still represent the strength of the field.Differences•The magnetic field lines do not terminate on anything. They form complete loops. There is no magnetic charge on which top end as there aws in the electric case. This means if you cut a bar magnet in half you get two smaller bar magnets ad infinitum all the way down to the atomic level – Magneic atoms have an atomic dipole – not a monopole as is the case for electric charge.•They are not perpendicular to the surface of the ferromagnetic material.B = Magnetic flux =  B•dAE = Electric flux =  E•dAMagnetic field lines do not stop at surface. They are continuous.They make complete loops.Field lines for a bar magnet are the same as for a current loop•B = F/(qv) definition of a magnetic field•The units of B are


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