14 7 At the end of the 19th century, J.J. Thompson used electric and magnetic fields to deflect beams of charged particles. A photograph of his apparatus is shown. electron source deflecting system evacuated tube screen © Science Museum London Electrons were accelerated through a potential difference to produce a beam of high-energy electrons. The beam was then deflected in perpendicular directions by the magnetic and electric fields. The final position of the beam on the screen was determined by the charge and mass of the electrons. (a) Explain how electrons from the source become a beam of high-energy electrons. 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(b) An electron is travelling left to right and enters a region of uniform magnetic field as shown below. The direction of the magnetic field is perpendicular to the direction of travel of the electron. – uniform magnetic field (i) The magnetic field deflects the electron in the direction up the page. Explain the direction of the magnetic field that would produce this deflection. (2) .................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... . .................................................................................................................................................................................................................................................. 15 Turn over (ii) Explain why the electron would travel in a circular path if no other forces acted on it. (2) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (c) In a modern version of Thompson’s experiment, a uniform electric field of electric field strength E is applied so that the electric and magnetic forces on the electrons are equal and in opposite directions. (i) Show that for electrons to be undeflected their velocity must be given by v E B = where B is the magnetic flux density of the magnetic field. 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(ii) The beam is produced by accelerating electrons through a potential difference of 250 V. The electric field strength is 1.4 × 104 V m−1. The magnetic flux density is 1.5 × 10−3 T. Calculate the value of the specific charge e/m for the electron using this data. (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. e/m = ....................................................... 16 (d) In his original experiments, Thompson determined the specific charge of a range of particles. His results indicated that the specific charge of an electron is about 2000 times bigger than that for a hydrogen ion. Deduce what conclusion can be made from this information. (1) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (Total for Question 7 = 12 marks) 17 Turn over BLANK PAGE