1 / 23

Magnetic Force

Magnetic Force. Physics 102 Professor Lee Carkner Lecture 18. PAL #17 Magnetic Field. Direction electron is fired into magnetic field that points north if it is deflected up Force equation: F = qvB sin q q = sin -1 [(1.7X10 -14 )/((1.6X10 -19 )(3X10 5 )(0.5))] q =

dani
Télécharger la présentation

Magnetic Force

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Magnetic Force Physics 102 Professor Lee Carkner Lecture 18

  2. PAL #17 Magnetic Field • Direction electron is fired into magnetic field that points north if it is deflected up • Force equation: F = qvB sin q • q = sin-1 [(1.7X10-14)/((1.6X10-19)(3X105)(0.5))] • q = • v vector points 45 west of north, which is pointed northwest, so electron was fired from southeast

  3. Electron in B Field North From right hand rule: B is north and force is up so v is from west (reversed to east for electron) B v q West East South

  4. A beam of electrons is pointing right at you. What direction would a magnetic field have to have to produce the maximum deflection in the right direction? • Right • Left • Up • Down • Right at you

  5. A beam of electrons is pointing right at you. What direction would a magnetic field have to have to produce the maximum deflection in the up direction? • Right • Left • Up • Down • Right at you

  6. A beam of electrons is pointing right at you. What direction would a magnetic field have to have to produce no deflection? • Right • Left • Up • Down • Right at you

  7. Electric and Magnetic Force • How do the electric and magnetic forces differ? • Dependences • Magnetic force depends on v and q, as well as B and q • Vector • Force vector does change for a magnetic field, since as the particle is deflected the v vector changes

  8. Particle Motion • A particle moving freely in a magnetic field will have one of three paths, depending on q • Straight line • Circle • Helix • This assumes a uniform field that the particle does not escape from

  9. Circular Motion

  10. Circular Motion • If the particle moves at right angles to the field the force vector will cause the path to bend • The particle will move in a circle • How big is the circle? • Magnetic force is F = • Centripetal force is F = • We can combine to get r = mv/qB • Radius of orbit of charged particle in a uniform magnetic field

  11. Circle Properties • Circle radius is inversely proportional to q and B • r is directly proportional to v and m • Can use this idea to make mass spectrometer • Send mixed atoms through the B field they will come out separated by mass

  12. Today’s PAL • How long would it take an electron to complete one circular orbit around a 1 G magnetic field?

  13. Helical Motion • If the initial velocity is not completely perpendicular to the field, instead of a circle you get a spiral or helix • Charged particles will spiral around magnetic field lines • For example, if the lines begin and end at a pole • Examples: • Gyrosynchrotron radio emission from planets and stars

  14. Helical Motion

  15. Solar Wind Particles in Earth’s Magnetic Field

  16. Magnetic Field and Current • Since a current is moving charge, a magnet will produce a force on a wire with a current flowing through it • So qv = IL, thus: F = BIL sin q • We can use the right hand rule to get the direction of the force • Use the direction of the current instead of v

  17. Force on a Wire

  18. Force on a Loop of Wire • Consider a loop of wire placed so that it is lined up with a magnetic field • Two sides will have forces at right angles to the loop, but in opposite directions • The loop will experience a torque

  19. Loop of Current

  20. Torque on Loop • For a loop of width w and height h, force is F = BIL sin q for each long side • Since q = 90 and L = h, • The torque is the force times the moment arm (distance to the center), which is w/2 • Total torque = • but hw is the area of the loop, A • If the field is at angle q to the loop then

  21. Torque on Loop

  22. General Loops • If there are multiple loops (N), the torque is the sum of each t = IBAN sin q • A loop placed along a magnetic field will try to align such that the field goes straight through it • Can harness the spin to do work • Called a motor

  23. Next Time • Read 20.7-20.8 • Homework: Ch 20, P 4, 17, 38, 49 • Exam #2 Friday

More Related