Answer the following question:
What is so special about the combination e/m? Why do we not simply talk of e and m separately?
e/m or the specific charge is a important ratio as it is found in basic equations of electro-magnetism, for instance,
evB = mv2/r …(1)
→ (e/m)vB = v2/r
v = Br(e/m)
eV = 1/2mv2 …(2)
→ v = 
We see that the ratio (e/m) has an impact on the dynamics of electron in magnetic field hence e/m is given more importance that e or m.
37