Отношение массы к заряду: физическая величина в химии и электродинамике.

Physical quantity of interest in chemistry and electrodynamics

The mass to charge ratio (m/Q) is a physical quantity relating the mass (quantity of matter) and the electric charge of a given particle, expressed in units of kilograms per coulomb (kg/C). It is most widely used in the electrodynamics of charged particles, e. g. in electron optics and ion optics. It appears in the scientific fields of electron microscopy, cathode ray tubes, accelerator physics, nuclear physics, Auger electron spectroscopy, cosmology and mass spectrometry. The importance of the mass to charge ratio, according to classical electrodynamics, is that two particles with the same mass to charge ratio move in the same path in a vacuum, when subjected to the same electric and magnetic fields. Some disciplines use the charge to mass ratio (Q/m) instead, which is the multiplicative inverse of the mass to charge ratio. The CODATA recommended value for an electron is

The units and notation above are used when dealing with the physics of mass spectrometry; however, the m/z notation is used for the independent variable in a mass spectrum. This notation eases data interpretation since it is numerically more related to the dalton. By doing this, he showed that the electron was in fact a particle with a mass and a charge, and that its mass to charge ratio was much smaller than that of the hydrogen ion H+. In 1898, Wilhelm Wien separated ions (canal rays) according to their mass to charge ratio with an ion optical device with superimposed electric and magnetic fields (Wien filter). In 1901 Walter Kaufman measured the increase of electromagnetic mass of fast electrons (Kaufmann–Bucherer–Neumann experiments), or relativistic mass increase in modern terms. In 1913, Thomson measured the mass to charge ratio of ions with an instrument he called a parabola spectrograph. Today, an instrument that measures the mass to charge ratio of charged particles is called a mass spectrometer.

In some experiments, the charge to mass ratio is the only quantity that can be measured directly. Often, the charge can be inferred from theoretical considerations, so the charge to mass ratio provides a way to calculate the mass of a particle. Often, the charge to mass ratio can be determined by observing the deflection of a charged particle in an external magnetic field. The cyclotron equation, combined with other information such as the kinetic energy of the particle, will give the charge to mass ratio. One application of this principle is the mass spectrometer. The same principle can be used to extract information in experiments involving the cloud chamber. The ratio of electrostatic to gravitational forces between two particles will be proportional to the product of their charge to mass ratios. It turns out that gravitational forces are negligible on the subatomic level, due to the extremely small masses of subatomic particles.

The electron charge to mass quotient, , is a quantity that may be measured in experimental physics. It bears significance because the electron mass me is difficult to measure directly, and is instead derived from measurements of the elementary charge e and It also has historical significance; the Q/m ratio of the electron was successfully calculated by J. J. Thomson in 1897—and more successfully by Dunnington, which involves the angular momentum and deflection due to a perpendicular magnetic field. Thomson's measurement convinced him that cathode rays were particles, which were later identified as electrons, and he is generally credited with their discovery. The CODATA recommended value is CODATA refers to this as the electron charge to mass quotient, but ratio is still commonly used. There are two other common ways of measuring the charge to mass ratio of an electron, apart from Thomson and Dunnington's methods. The magnetron method: Using a GRD7 Valve (Ferranti valve), electrons are expelled from a hot tungsten wire filament towards an anode. The electron is then deflected using a solenoid. From the current in the solenoid and the current in the Ferranti Valve, e/m can be calculated. Fine beam tube method: A heater heats a cathode, which emits electrons. The electrons are accelerated through a known potential, so the velocity of the electrons is known. The beam path can be seen when the electrons are accelerated through a helium (He) gas. The collisions between the electrons and the helium gas produce a visible trail. 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 (volts), the current (amps) to the Helmholtz coils, and the radius of the electron beam, e/m can be calculated.