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Magnetic Fields - Chapter 26: The Force Exerted by a Magnetic Field

This lecture explores the motion of charged particles in a magnetic field, including the discovery of the electron and the application of crossed fields in the cyclotron and mass spectrometer. It also discusses the magnetic force on a current-carrying wire.

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Magnetic Fields - Chapter 26: The Force Exerted by a Magnetic Field

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  1. Magnetic Fields Chapter 26 26.2 The force exerted by a magnetic field Definition of B 26.3 Motion of a charged particle in a magnetic field Applications A circulating charged particle Crossed fields: discovery of the electron The cyclotron and mass spectrometer 26.4 Magnetic force on a current-carrying wire Last lecture This lecture

  2. Magnetic force and field The definition of B The sign of q matters!

  3. Charged particle moving in a plane perpendicular to a uniform magnetic field (into page). Find expression for radius, r

  4. CHECKPOINT: Here are three situations in which a charged particle with velocity v travels through a uniform magnetic field B. • In each situation, what is the direction of the magnetic force FBon the particle? • Left • Up • Into page • Right • Down • Out of page Answers: (a) +z (out) (b) –x (left, negative particle) (c) 0

  5. p e • CHECKPOINT: The figure shows the circular paths of two particles that travel at the same speed in a uniform B, here directed into the page. One particle is a proton; the other is an electron. • Which particle follows the smaller circle • A. p • B. e • Does that particle travel • clockwise or • anticlockwise? Answers: (a) electron (smaller mass) (b) clockwise

  6. Crossed magnetic and electric fields Net force: The forces balance if the speed of the particle is related to the field strengths by qvB = qE v = E/B (velocity selector)

  7. EXERCISE:Find an expression for q/m Measurement of q/m for electron J J Thomson 1897

  8. Sun-to-aurora TV analogy

  9. A small part of the sky overhead

  10. CHECKPOINT: the figure shows four directions for the velocity vector v of a positively charged particle moving through a uniform E (out of page) and uniform B. • Rank directions A(1), B(2) and C(3) according to the magnitude of the net force on the particle, greatest first. • Of all four directions, which might result in a net force of zero: • A(1), B(2), C(3) or D(4)? Answers: (a) 2 is largest, then 1 and 3 equal (v x B = 0) (b) 4 could be zero as FE and FB oppose

  11. Picture the problem: Velocity vector is in the y-direction. B is in the yz plane Force on proton must be towards West, ie in negative x-direction EXAMPLE: The magnetic field of the earth has magnitude 0.6 x 10-4 T and is directed downward and northward, making an angle of 70° with the horizontal. A proton is moving horizontally in the northward direction with speed v = 107 m/s. Calculate the magnetic force on the proton by expressing v and B in terms of components and unit vectors, with x-direction East, y-direction North and z-direction upwards).

  12. Circular motion of a charged particle in a magnetic field

  13. The Cyclotron It was invented in 1934 to accelerate particles, such as protons and deuterons, to high kinetic energies. S is source of charged particles at centre Potential difference across the gap between the Dees alternates with the cyclotron frequency of the particle, which is independent of the radius of the circle

  14. Schematic drawing of a cyclotron in cross section. Dees are housed in a vacuum chamber (important so there is no scattering from collisions with air molecules to lose energy). Dees are in uniform magnetic field provided by electromagnet. Potential difference V maintained in the gap between the dees, alternating in time with period T, the cyclotron period of the particle. Particle gains kinetic energy q V across gap each time it crosses V creates electric field in the gap, but no electric field within the dees, because the metal dees act as shields. Key point: fosc= f = qB/2m is independent of radius and velocity of particle

  15. The Cyclotron

  16. EXAMPLE: A cyclotron for accelerating protons has a magnetic field of 1.5 T and a maximum radius of 0.5 m. • What is the cyclotron freqency? • What is the kinetic energy of the protons when they emerge?

  17. 26.4 Magnetic force on a current-carrying wire Wire segment of length L carrying current I. If the wire is in a magnetic field, there will be a force on each charge carrier resulting in a force on the wire.

  18. Flexible wire passing between pole faces of a magnet. • no current in wire • upward current • downward current

  19. 26.4 Magnetic force on a current-carrying wire

  20. EXERCISE: A wire segment 3 mm long carries a current of 3 A in the +x direction. It lies in a magnetic field of magnitude 0.02 T that is in the xy plane and makes an angle of 30° with the +x direction, as shown. What is the magnetic force exerted on the wire segment?

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