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Lecture_12

# Lecture_12 - The magnetosphere is not perfectly spherical...

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The magnetosphere is not perfectly spherical: It gets defected by the solar wind and protects the earth from the sun’s radiation. 21.2 The Magnetic Force Charges feel forces in electric fields . Magnets feel forces in magnetic fields . And,….until 1820, everyone thought electricity and magnetism had absolutely nothing to do with each other. But, it turns out that electrical charges WILL also feel a force in magnetic fields, under certain conditions. First…. 1. The charge must be moving, i.e. it has a nonzero velocity. There is no magnetic force on a stationary charge! 2. The charge’s velocity must have a component that is perpendicular to the magnetic field. And…

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Thus, if a charge is moving in a magnetic field, but it moves along the same direction as the field (parallel to it), there is no force. Magnetic Field v q F = 0 Magnetic Field v q F ≠ 0 So, if there is a force on a moving charged particle in a magnetic field, how do we calculate that force? B v q F ρ ρ ρ × = θ sin qvB = θ sin qvB F = B is the magnetic field. θ is the angle between B and v . Units on B ? [ ] [ ] T Tesla m C s N Velocity Charge Force = = = × = B 1 T is a pretty big field. We also use another unit of mag. field, the gauss : 1 gauss = 1 × 10 -4 T. Earth’s mag. Field is ~0.5 gauss θ
Direction of the force on a charged particle in a mag. field: Use Right Hand Rule 1 (RHR-1) Point the fi ngers of your right hand in the direction of the magnetic fi eld. Point your thumb in the direction of the charge’s velocity. The force is directed away from your palm. This is the procedure to follow when the charge is positive. If the charge is negative, do everything exactly the same, but then reverse the direction of the force at the end. Examples Β + q v Force? Force is into the page Β - q v Force? Force is out of the page

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21.3 Motion of a charged particle in electric and magnetic fields The force on a charged particle in an electric field is directed along the field, either parallel or antiparallel: qE F = + + + + + - - - - - E +q v The positively charged particle feels a force upward due to E .
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Lecture_12 - The magnetosphere is not perfectly spherical...

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