Specific charge

In physics, the specific charge is a size of a particle . It is defined as the ratio of charge to mass . It is used to better compare the charge of individual elementary particles or of atomic nuclei . It is the reciprocal of the mass-to-charge ratio .

The size is important for the behavior of a particle in magnetic fields . If the magnetic flux density , the speed and the radius of the circular path on which the particle is moving is known, the ratio of charge and mass is clearly defined, since the Lorentz force and the centripetal force are clearly defined by these quantities . By equating the two forces: ${\displaystyle {\vec {B}}}$ ${\displaystyle {\vec {v}}}$ ${\displaystyle r}$ ${\displaystyle {\tfrac {Q}{m}}}$

${\displaystyle |{\vec {F}}_{\mathrm {L} }|=\left|Q\cdot {\vec {v}}\times {\vec {B}}\right|}$

${\displaystyle F_{\mathrm {Z} }=m{\frac {v^{2}}{r}}}$

surrendered

${\displaystyle {\frac {Q}{m}}={\frac {v}{Br}}}$.

example

According to the current measurement accuracy, the specific electron charge is:

${\displaystyle {\frac {-e}{m_{\mathrm {e} }}}=-1{,}758\,820\,010\,76(53)\,\cdot \,10^{11}\,\mathrm {\frac {C}{kg}} }$,

A mass spectrometer (e.g. a fine beam tube ) can be used to determine this value experimentally . Alternatively, it can be determined via the Zeeman effect using a Lummer-Gehrke plate or a Fabry-Perot interferometer . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.

Web links

• LP - determination of the specific electron charge e / m
• Virtual experiment to determine the specific electron charge (LMU Munich)

Individual evidence

1. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 1, 2019 .  Value for ${\displaystyle {\tfrac {-e}{m_{\mathrm {e} }}}}$