# Deflection of Alpha & Beta Radiation in an Electric & Magnetic Field

To explain how alpha and beta particles are deflected, the following two figures have been provided in my textbook but without any explanation for the intensity of the deflections shown: I have two theories regarding why the alpha particle was deflected more than the beta particle:

The alpha particle has a greater overall charge and hence feels a greater pull towards the (-) plate

The beta particle is travelling much too fast for it to be "properly" deflected & attracted to the positive plate

One of these ideas may be right, both or none of them -- I have no idea and Googling these does not give me a proper description. The second figure in the book shows the particles' deflection in a magnetic field, again with no explanation, leaving me to fumble around with my own ideas... Now, I guess the reason why the alpha particle didn't deflect quite as much here is due to its weight, but my reasoning breaks down for the beta particle — shouldn't it not have deflected as drastically due to its immense velocity?

Can anyone properly explain these ideas? I'm positive there is some deeper, more complex relationship I am missing out on.

• $\vec F=q\vec v \times \vec B$? Sep 19, 2021 at 13:16
• Deflection at same energy? Same velocity? Sep 19, 2021 at 14:06

For the first picture, you are right. The force on the $$\alpha^{++}$$ particle is twice that on the $$\beta^-$$ particle, but also the velocity of the $$\alpha^{++}$$ is much smaller, so it's easier to change direction.
In the second case, the centripetal force needed is much higher for the particle with larger mass, $$q\vec v\times\vec B=\frac{mv^2}{r}$$
so $$r$$ is much larger due to the large $$m$$, and double charge does not affect it significantly.