SM_PDF_chapter22 - Magnetic Forces and Magnetic Fields...

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607 Magnetic Forces and Magnetic Fields CHAPTER OUTLINE 22.1 Historical Overview 22.2 The Magnetic Field 22.3 Motion of a Charged Particle in a Uniform Magnetic Field 22.4 Applications Involving Charged Particles Moving in a Magnetic Field 22.5 Magnetic Force on a Current-Carrying Conductor 22.6 Torque on a Current Loop in a Uniform Magnetic Field 22.7 The Biot-Savart Law 22.8 The Magnetic Force Between Two Parallel Conductors 22.9 Ampère’s Law 22.10 The Magnetic Field of a Solenoid 22.11 Magnetism in Matter 22.12 Context Connection The Attractive Model for Magnetic Levitation ANSWERS TO QUESTIONS Q22.1 If they are projected in the same direction into the same magnetic field, the charges are of opposite sign. Q22.2 Similarities: Both can alter the velocity of a charged particle moving through the field. Both exert forces directly proportional to the charge of the particle feeling the force. Positive and negative charges feel forces in opposite directions. Differences: The direction of the electric force is parallel or antiparallel to the direction of the electric field, but the direction of the magnetic force is perpendicular to the magnetic field and to the velocity of the charged particle. Electric forces can accelerate a charged particle from rest or stop a moving particle, but magnetic forces cannot. Q22.3 (a) The q r r vB × force on each electron is down. Since electrons are negative, r r × must be up. With r v to the right, r B must be into the page, away from you. (b) Reversing the current in the coils would reverse the direction of r B , making it toward you. Then r r × is in the direction right × toward you = down , and q r r × will make the electron beam curve up. Q22.4 If the current is in a direction parallel or antiparallel to the magnetic field, then there is no force. Q22.5 Yes. If the magnetic field is perpendicular to the plane of the loop, then it exerts no torque on the loop. Q22.6 If you can hook a spring balance to the particle and measure the force on it in a known electric field, then q F E = will tell you its charge. You cannot hook a spring balance to an electron. Measuring the acceleration of small particles by observing their deflection in known electric and magnetic fields can tell you the charge-to-mass ratio, but not separately the charge or mass. Both an acceleration produced by an electric field and an acceleration caused by a magnetic field depend on the properties of the particle only by being proportional to the ratio q m .
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608 Magnetic Forces and Magnetic Fields Q22.7 The Earth’s magnetic field exerts force on a charged incoming cosmic ray, tending to make it spiral around a magnetic field line. If the particle energy is low enough, the spiral will be tight enough that the particle will first hit some matter as it follows a field line down into the atmosphere or to the surface at a high geographic latitude.
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This note was uploaded on 10/18/2008 for the course PHYS 3Q2341234 taught by Professor Dafsf during the Spring '08 term at UCLA.

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SM_PDF_chapter22 - Magnetic Forces and Magnetic Fields...

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