The Charge to Mass Ratio of Electrons
Measurement of e/m for Electrons
Friday, Section 006
TA: Yilikal Ayino
John Greavu with Kevin Haar & Gregory Overdorf
February 28, 2014 PreLab
At Cambridge University in 1897, J.J. Thomson studied the mysterious “corpuscles” of the rays emanating from the cathode of a cathode ray tube. Using knowledge of the Lorentz force, Thomson measured the charge-to-mass ratio of these corpuscles, which we now know as electrons.
A slightly modified version of Thomson’s experiment using (essentially) a cathode ray tube will be explained here. A cathode ray tube (CRT) is an evacuated spherical glass chamber containing an electron gun and accelerating electrode plates. The CRT is filled with a small amount of a noble gas, which when ionized by a passing electron, glows along its path. The electron gun is an indirectly heated barium oxide-coated cathode with a Wehnelt cylinder at negative potential for focusing the emanating rays into a nearby anode with a hole in it. The electrons are accelerated from the cathode, focused into a helix, through the hole in the anode (at positive potential—creating an electric field), thus emerging with kinetic energy, K, equal to the product of the electron’s charge, e, and the voltage, V (potential difference). Assuming they are emitted with negligible energy,
where m is the mass of the electron, in this case. After emerging from the hole in the anode, the electrons enter a region with no electric fields, therefore then traveling at a constant velocity. The beam’s path is illuminated due to the ionization of the gas.
A magnetic field is applied to the beam using Helmholtz coils. Helmholtz coils are two vertically orientated, identical circular coils of wire (solenoids) that are placed parallel to each other, symmetric along a shared axis, one on each side of the domain of the experiment. There are equal currents flowing in the same direction in each coil. The distance, d, between the coils is equal to the radius, RC, of the coils. As a
Friday, Section 006
TA: Yilikal Ayino
John Greavu with Kevin Haar & Gregory Overdorf
February 28, 2014 PreLab
At Cambridge University in 1897, J.J. Thomson studied the mysterious “corpuscles” of the rays emanating from the cathode of a cathode ray tube. Using knowledge of the Lorentz force, Thomson measured the charge-to-mass ratio of these corpuscles, which we now know as electrons.
A slightly modified version of Thomson’s experiment using (essentially) a cathode ray tube will be explained here. A cathode ray tube (CRT) is an evacuated spherical glass chamber containing an electron gun and accelerating electrode plates. The CRT is filled with a small amount of a noble gas, which when ionized by a passing electron, glows along its path. The electron gun is an indirectly heated barium oxide-coated cathode with a Wehnelt cylinder at negative potential for focusing the emanating rays into a nearby anode with a hole in it. The electrons are accelerated from the cathode, focused into a helix, through the hole in the anode (at positive potential—creating an electric field), thus emerging with kinetic energy, K, equal to the product of the electron’s charge, e, and the voltage, V (potential difference). Assuming they are emitted with negligible energy,
where m is the mass of the electron, in this case. After emerging from the hole in the anode, the electrons enter a region with no electric fields, therefore then traveling at a constant velocity. The beam’s path is illuminated due to the ionization of the gas.
A magnetic field is applied to the beam using Helmholtz coils. Helmholtz coils are two vertically orientated, identical circular coils of wire (solenoids) that are placed parallel to each other, symmetric along a shared axis, one on each side of the domain of the experiment. There are equal currents flowing in the same direction in each coil. The distance, d, between the coils is equal to the radius, RC, of the coils. As a