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Unformatted text preview: HB 10-20-08 Charge to Mass Ratio of Electron Lab 11 1 Charge to Mass Ratio of Electron Lab 11 Equipment ELWE e/m tube, ELWE Helmholtz coils, ELWE 4 voltage power supply, Safety Glasses, Fluke multimeter, leads, one night vision flashlight per student, desk lamp with red bulb Reading Your textbook. Electrical Safety at the beginning of this manual. SAFETY 1. SAFETY GLASSES MUST BE WORN. The glass tube could implode and eye protec- tion is essential. Treat the glass tube carefully. 2. The accelerating voltage for the electrons is lethal. When the power supply is on, do not touch leads and terminals. 1 Background The orbit of a charged particle in a uniform and constant magnetic field is a circle when the initial velocity of the particle is perpendicular to the magnetic field. The radius of the orbit depends on the charge to mass ratio of the particle, q/m , the speed of the particle v , and the strength of the magnetic field B . When the strength of the magnetic field and the initial speed of the particle are known, a measurement of the radius of the orbit determines q/m . This principle was used by J.J. Thomson to measure the charge to mass ratio of the electron, e/m , in 1897. The reason for a circular orbit can be understood by the fact that a charged particle experiences a force at right angles both to the instantaneous velocity and to the direction of the magnetic field. The particle therefore moves under the influence of a force whose magnitude is constant but whose direction is always at right angles to the velocity. Like a ball whirled on a string, the orbit is a circle. The measurement of q/m in modern physics provides a way to identify atoms and molecules by a device called a mass spectrometer. Similarly the measurement of q/m by devices such as cloud chambers and bubble chambers identifies sub-atomic particles, such as the electron and the muon. In this experiment e/m of the electron is determined from the relationship between the electric potential used to accelerate the electron to a given speed, the strength of the magnetic field that influences the electrons motion, and the radius of the circular path which the electron follows. The charge to mass ratio of a particle is often referred to as the specific charge. 2 Apparatus The equipment consists of an evacuated glass bulb in which the electrons in a beam execute circular orbits. See Figs. 1 and 2. A pair of wire coils provide a reasonably uniform magnetic field in the region of the bulb. Inside the glass bulb is an electron gun mounted so that the initial direction of the electron beam is horizontal. The electron gun consists of the following metallic elements. See Fig. 2 which is not very complete. Look at the electron gun yourself to verify this statement! Be sure your safety glasses are on as you put your eyes near the glass bulb....
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