Specific Charge Experiments Flashcards
what is specific charge of an electron?
- charge per unit mass of the electron
- e / me
how can the specific charge of an electron be determined with a magnetic field?
- when an electron moves in a magnetic field, it experiences a force perpendicular to its motion
- radius of electron beams circular path can be measured to calculate specific charge
derive how we can calculate electrons SC from magnetic field only
Fc = Fm
v = Ber / m
1/2mv^2 = eV
q / m = 2V / B^2 r^ 2
how can we measure the specific charge of an electron from magnetic and electric fields?
- balancing magnetic and electric forces on a beam of electrons
- a HelmHoltz coil generates a magnetic field
- oppositely charged plates generate an electric field
how can we vary the electric field for the second experiment until Fe = Fm?
- change the potential difference across the plates
derive the equation for the second experiment of sc of an electron
Fe = Fb so:
qV / d = Bqv, so v = V / Bd
switching the electric field off and v = rBq / m
so then e / me = V / B^2 d r
how can we measure the specific charge of an electron using an electric field only?
- using constant acceleration equations for beams of electrons passing through an electric field
what are the equations used for suvat?
t = distance / velocity of electrons
acceleration can be found with electric force
s = 1/2 at^2, as all values can be measured
what are the significance of these experiments?
- Thomson found out the specific charge of an electron was around 1800 times larger than that of the hydrogen ion
- either the electron had a smaller mass or a much larger magnitude of charge
- this was a mystery and needed to be found out
