Specific charge of the electron (A-level only) Flashcards – AQA A-level Physics (7408)

What energy equation relates the accelerating potential difference $V$ to an electron’s speed $v$, assuming negligible initial kinetic energy?

$eV = \frac{1}{2}m_e v^2$

What equation relates magnetic force to centripetal force for an electron moving perpendicular to a uniform magnetic field of flux density $B$?

$evB = \frac{m_e v^2}{r}$, where $r$ is the path radius.

How is the magnitude of the electron’s specific charge calculated from accelerating potential difference $V$, magnetic flux density $B$ and beam radius $r$?

$\frac{e}{m_e} = \frac{2V}{B^2r^2}$

Why does the electron beam travel through low-pressure gas in a specific-charge experiment?

The gas makes the beam’s path visible.

How is the magnetic field produced in the electron-beam specific-charge experiment?

By coils.

In the specific-charge expression $e/m_e$, what does $e$ represent, given that the electron’s charge is negative?

The magnitude of the electron’s charge.

What did Thomson’s cathode-ray measurement indicate about the constituents of atoms?

Atoms contain negatively charged electrons.

What did Thomson’s electron measurement show about the divisibility of atoms?

Atoms are divisible, not indivisible.

How does the magnitude of the electron’s specific charge compare with that of a hydrogen ion?

It is approximately 1,800 times greater. The electron is negatively charged; the hydrogen ion is positively charged.

What can a charge-to-mass ratio alone not establish separately?

The particle’s charge and mass.

If an electron and a hydrogen ion have charges of equal magnitude, how do their masses compare?

The electron’s mass is approximately $1/1800$ of the hydrogen ion’s mass.