Fundamentals of Physics 8e 28 Magnetic Force D

Fundamentals of Physics 8/e 28 - Magnetic Force D. Halliday, R. Resnick, and J. Walker 國立彰化師大教學卓越計畫 理學院普通物理整合型教材 主持人：郭艷光院長

Outline n n n n n 28 -1 What Is Physics? 28 -2 What Produces a Magnetic Field? 28 -3 The Definition of 28 -4 Crossed Fields: Discovery of the Electron 28 -5 Crossed Fields: The Hall Effect 28 -6 A Circulating Charged Particle 28 -7 Cyclotrons and Synchrotrons 28 -8 Magnetic Force on a Current-Carrying Wire 28 -9 Torque on a Current Loop 28 -10 The Magnetic Dipole Moment 彰化師大教學卓越－普物教材－理學院長郭艷光主持 2

28 -1 What Is Physics? n As we have discussed, one major goal of physics is the study of how an electric field can produce an electric force on a charged object. n A closely related goal is the study of how a magnetic field can produce a magnetic force on a (moving) charged particle or on a magnetic object such as a magnet. 彰化師大教學卓越－普物教材－理學院長郭艷光主持 3

28 -2 What Produces a Magnetic Field? n. One way is to use moving electrically charged particles, such as a current in a wire, to make an electromagnet. n. The other way to produce a magnetic field is by means of elementary particles such as electrons because these particles have an intrinsic magnetic field around them. 彰化師大教學卓越－普物教材－理學院長郭艷光主持 4

28 -3 The Definition of Magnetic Field Lines At any point P the magnetic field vector is tangent to the magnetic field lines. n The magnitude of the magnetic field vector is proportional to the density of the magnetic field lines. n n P P magnetic field line Q magnetic field lines 彰化師大教學卓越－普物教材－理學院長郭艷光主持 7

28 -4 Crossed Fields: Discovery of the Electron Cathode Anode J. J. Thomson in 1897 used a version of this tube to investigate the nature of the particle beam that caused the fluorescent spot. 彰化師大教學卓越－普物教材－理學院長郭艷光主持 8

28 -6 A Circulating Charged Particle . electron C . r 彰化師大教學卓越－普物教材－理學院長郭艷光主持 10

28 -7 Cyclotrons and Synchrotrons n The key to the operation of the cyclotron is that the frequency f at which the proton circulates in the magnetic field (and that does not depend on its speed) must be equal to the fixed frequency fosc of the electrical oscillator, or f= fosc (resonance condition). n. At proton energies above 50 Me. V, the conventional cyclotron begins to fail because one of the assumptions of its design—that the frequency of revolution of a charged particle circulating in a magnetic field is independent of the particle’s speed—is true only for speeds that are much less than the speed of light. 彰化師大教學卓越－普物教材－理學院長郭艷光主持 12

28 -8 Magnetic Force on a Current-Carrying Wire i . 彰化師大教學卓越－普物教材－理學院長郭艷光主持 13