harged Particle Motion in Magnetic Fields 19 The charge-to-mass ratio (e/m) for

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harged Particle Motion in Magnetic Fields 19 The charge-to-mass ratio (e/m) for the electron will be desermined from measurements of the trajectory of electrons in a magnetic field. Introduction In his investigations of cathode rays, J.J.Thomson showed conclusively that they consisted of negatively charged particles which he associated with electrons. In addition, by measuring the deflection of the rays in magnetic and electric fields, Thomson made the first determination of the ratio of the charge of the electron to its mass, e/m. Using a value for "e" deduced from other studies he was then able to show that its mass must be some 2000 times less than that of the lightest atom, hydrogen. In this experiment we shall measure the charge-to-mass ratio (e/m) of the electron using a magnetic deflection technique and a specially designed electron vacuum tube. Electrons are ejected by thermionic emission from a cathode which is indirectly heated by a hot wire filament (see Fig. I). A potential difference V applied between the anode and cathode of the electron gun causes the electrons to accelerate toward the anode, thus acquiring an amount of kinetic energy given by 1) After acceleration, the electron beam emerges through a small aperture into the tube region beyond the anode. (The grid electrode serves to focus the beam through this aperture.) To provide a visible trace of the electron trajectory in the glass tube, the tube is evacuated during its manufacture and then filled with a small amount of an inert gas to a pressure of about 102 torr. When the electrons collide with the gas atoms some of the atoms become ionized. As the ions quickly recombine with other stray electrons, light is emitted having wavelengths characteristic of the inert gas atom. Since recombination and emission occur very near the point where initial ionization takes place, the path of the electrons is made clearly visible as they travel through the gas. electrons. If e is the angle between the direction of the electron's velocity vector and the magnetio tield direction, then the magnitude of the force is given by the Lorentz relation: If a magnetic field B exists within the tube then this will exert a force on the moving 2) IF s Bev sin ? will cause the electrons to be deflected and in general to follow the path a. Ir, however, the magnetic field is aligned so as to be perpendicular to the direction of of an open This

Explanation / Answer

Objective:- To Dtermine charge to mas ratio of an electron

Introduction:- This experiment measures e/m, the charge to mass ratio of the electron. This ratio was first measured by J. J. Thomson in 1897. He won a Nobel prize for his study of electrons. In the present experiment a beam of electrons is accele rated through a known potential, so the velocity of the electrons is known. A pair of Helmholtz coils produces a uniform and measurable magnetic field at right angles to the electron beam. This magnetic field deflects the electron beam in a circular path. By measuring the accelerating potential, the current to the Helmholtz coils, and the radius of the circular path of the electron beam, the ratio e/m is calculated.

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