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Lecture 19: MON 02 MAR

Physics 2102 Jonathan Dowling. Lecture 19: MON 02 MAR. Aurora Borealis. Magnetic fields Ch.28.6-7. “I’ll be back…. Second Exam Review: 6-7PM WED 04 MAR Nicholson 130 Second Exam (Chapters 24–28): 6–7PM THU 05 MAR Lockett 6. v. F. B into blackboard. Circular Motion:.

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Lecture 19: MON 02 MAR

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  1. Physics 2102 Jonathan Dowling Lecture 19: MON 02 MAR Aurora Borealis Magnetic fields Ch.28.6-7 “I’ll be back….

  2. Second Exam Review: 6-7PM WED 04 MAR Nicholson 130 Second Exam (Chapters 24–28): 6–7PM THU 05 MAR Lockett 6

  3. v F B into blackboard. Circular Motion: Since magnetic force is perpendicular to motion, the movement of charges is circular. r In general, path is a helix (component of v parallel to field is unchanged).

  4. . electron Radius of Circlcular Orbit . C r Angular Frequency: Independent of v Period of Orbit: Independent of v Orbital Frequency: Independent of v

  5. A B v v Example Two charged ions A and B traveling with a constant velocity venter a box in which there is a uniform magnetic field directed out of the page. The subsequent paths are as shown. What can you conclude? • (a) Both ions are negatively charged. • (b) Ion A has a larger mass than B. • (c) Ion A has a larger charge than B. • (d) None of the above. Same charge q, speed v, and same B for both masses. So: ion with larger mass/charge ratio (m/q) moves in circle of larger radius. But that’s all we know! Don’t know m or q separately.

  6. Fermilab, Batavia, IL (1km) Linear accelerator (long). Examples of Circular Motion in Magnetic Fields Aurora borealis (northern lights) Synchrotron Suppose you wish to accelerate charged particles as fast as you can.

  7. L Magnetic Force on a Wire. Note: If wire is not straight, compute force on differential elements and integrate:

  8. L R R L Example Wire with current i. Magnetic field out of page. What is net force on wire? By symmetry, F2will only have a vertical component, Notice that the force is the same as that for a straight wire, and this would be true no matter what the shape of the central segment!.

  9. L B I Example 4: The Rail Gun rails • Conducting projectile of length 2cm, mass 10g carries constant current 100A between two rails. • Magnetic field B = 100T points outward. • Assuming the projectile starts from rest at t = 0, what is its speed after a time t = 1s? projectile • Force on projectile: F= iLB (from F = iL x B) • Acceleration: a = F/m = iLB/m (from F = ma) • v = at = iLBt/m (from v = v0 + at) • = (100A)(0.02m)(100T)(1s)/(0.01kg) = 2000m/s • = 4,473mph = MACH 8!

  10. Rail guns in the “Eraser” movie "Rail guns are hyper-velocity weapons that shoot aluminum or clay rounds at just below the speed of light. In our film, we've taken existing stealth technology one step further and given them an X-ray scope sighting system," notes director Russell. "These guns represent a whole new technology in weaponry that is still in its infancy, though a large-scale version exists in limited numbers on battleships and tanks. They have incredible range. They can pierce three-foot thick cement walls and then knock a canary off a tin can with absolute accuracy. In our film, one contractor has finally developed an assault-sized rail gun. We researched this quite a bit, and the technology is really just around the corner, which is one of the exciting parts of the story." Warner Bros., production notes, 1996. http://movies.warnerbros.com/eraser/cmp/prodnotes.html#tech Also: INSULTINGLY STUPID MOVIE PHYSICS: http://www.intuitor.com/moviephysics/

  11. Electromagnetic Slingshot These Devices Can Launch 1000kg Projectiles At Mach 100 at a Rate of 1000 Projectiles Per Second. Using KE = 1/2mv2 This corresponds to an output about 1012 Watts = TeraWatt. Uses: Put Supplies on Mars. Destroy NYC in about 10 minutes.

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