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SECTION 29.5 • Applications Involving Charged Particles Moving in a Magnetic Field 913 The Cyclotron A cyclotron is a device that can accelerate charged particles to very high speeds. The energetic particles produced are used to bombard atomic nuclei and thereby produce Both electric and magnetic forces have a key role in the operation of a cyclotron. A schematic drawing of a cyclotron is shown in Figure 29.27a. The charges move inside 1 and D 2 , referred to as dees, because of their shape like the letter D. A high-frequency alternating potential difference is applied to the dees, 2 is at a lower electric potential than D 1 by an amount 0V, the ion accelerates across the gap to D 2 and its kinetic energy increases by an amount q 0V. It then moves around D 2 in a semicircular path of greater radius (because its speed has increased). After a time interval T/2, it again arrives at the gap between the dees. By We can obtain an expression for the kinetic energy of the ion when it exits the cyclotron in terms of the radius R of the dees. From Equation 29.13 we know that (29.19) When the energy of the ions in a cyclotron exceeds about 20 MeV, relativistic effects come into play. (Such effects are discussed in Chapter 39.) We observe that T K " 1 2 mv
2 " q
2 B
2 R
2 2m B P D 1 D 2 (a) North pole of magnet Particle exits here Alternating ∆V ▲ PITFALL PREVENTION 29.1 The Cyclotron Is Not State-of-the-Art The cyclotron is important histori- Figure 29.27 (a) A cyclotron consists of an ion source at P, two dees D 1 and D 2 across which an alternating potential difference is applied, and a uniform magnetic field. (The south pole of the magnet is not shown.) The red dashed curved lines represent the path of the particles. (b) The first cyclotron, invented by E. O. Lawrence and M. S. Livingston in 1934. Courtesy of Lawrence Berkeley Laboratory/University of California (b) |