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Transcript
Motion of a Charged Particle in a Magnetic Field AP Physics C Montwood High School R. Casao • The magnetic force acting on a charged particle moving in a magnetic field is always perpendicular to the velocity of the particle. • From this property, the work done by the magnetic force is 0 J since the displacement of the charge is always perpendicular to the magnetic force. W = F·d·cos q • Therefore, a static magnetic field changes the direction of the velocity but does not change the speed or the kinetic energy of the charged particle. • Consider a positively charged particle moving in a uniform external magnetic field with its initial velocity vector perpendicular to the magnetic field. • The magnetic field B is into the page (as indicated by the x’s). • The figure shows that the charged particle moves in a circle whose plane is perpendicular to the magnetic field. • The circular path results because the magnetic force Fmag is at right angles to the velocity v and the magnetic field B and has a constant magnitude equal to q·v·B. • The force deflects the particle and the directions of v and B change continuously. • The force Fmag is a centripetal force, which changes only the direction of the velocity while the speed remains constant. • The direction of the rotation is given using the right hand rule – right hand for positive charges and left hand for negative charges (Casao’s rule). • Since Fmag= Fcentripetal: 2 mv qvB r • This reduces to: mv qB r • Solving for the radius of curvature: • The radius of curvature is proportional to the momentum of the particle and inversely proportional to the magnetic field B. • The angular frequency w of the rotating charged particle is: v qB ω r m mv r q B • The period T of the circular motion (time for one revolution) is equal to the circumference of the circle divided by the speed of the particle: d 2π r 2π 2π m T v v ω q B • The angular frequency and the period of the circular motion do not depend on the speed of the particle or the radius of the orbit. • The angular frequency w is also called the cyclotron frequency since charged particles circulate at this frequency in a particle accelerator called a cyclotron. • If a charged particle moves in a uniform magnetic field B with its velocity at some angle q to B, its path is a helix. • For the field B in the x-direction, there is no component of force in the x direction, therefore ax = 0 m/s2 and the x component of v is constant. • The magnetic force q·(v x B) causes the components of vx and vy to change in time and the resulting motion is a helix having its axis parallel to the magnetic field B. • The projection of the path onto the yz plane (viewed along the x axis) is a circle. • The distance between successive rotations in the helical path is called the pitch, p. • The pitch is parallel to the magnetic field B. • The perpendicular velocity influences how much time it takes to complete the circular path. • The parallel velocity determines the pitch. p v parallel t p v parallel t 2πm p v parallel qB • The motion of a charged particle in a nonuniform magnetic field is complex. • If a magnetic field is strong at the ends and weak in the middle, the particles oscillate back and forth between the end points. • Such a field can be produced by two current loops at the ends of the “bottle” to produce a strong magnetic field to pinch off the ends. • A charged particle starting at one end will spiral along the field lines until it reaches the other end, where it reverses directions and spirals back. This configuration is known as a “magnetic bottle” because charged particles can be trapped in it. – This concept has been used to confine plasmas (hot gases consisting of electrons and protons). – The magnetic bottle may pay a role in achieving a controlled nuclear fusion process. – The problem is that if a large number of particles are trapped in the magnetic bottle, collisions between the particles cause them to “leak” from the system. • The Van Allen radiation belts consist of charged particles (e& p+) surrounding the earth. • The charged particles are trapped by the earth’s nonuniform magnetic field and spiral around the earth’s field lines from pole to pole. • Most of the charged particles come from the sun. • When the charged particles are in the atmosphere over the poles, they can collide with other atoms, causing them to emit visible light, the Aurora Borealis and Aurora Australis.