Download Bending radiation To understand why alpha particles and beta

P2 5.3b – Student worksheet
Bending radiation
To understand why alpha particles and beta particles bend in electric and magnetic fields you need
to revisit exactly what these particles are.
Deflection in electric fields
An alpha particle is made up of two protons and two neutrons. This means that it has the same
mass as the nucleus of a helium atom. The only difference between a helium atom and an alpha
particle is that the alpha particle has no electrons around it and is moving quite fast. Because it has
no electrons around it, there in nothing to balance the positive charges of the proton and that is
why it reacts to an electric or magnetic field.
Figure 1
A beta particle is a single electron but moving incredibly fast. Again, it is charged but this time has
only one charge and it is negative.
Figure 2
A gamma ray has no mass or charge so it is unaffected by either a magnetic or electric field.
If you fire an electron between two metal plates where one is positive and one is negative, the
negative charge will bend away from the negative plate and towards the positive plate (Figure 3).
So the beta particle bends in the electric field between the two metal plates.
A similar thing happens to the alpha particle, but there are some differences. The alpha particle is
positively charged so it bends in the opposite direction. The charge of the alpha particle is twice as
big as an electron but the alpha particle is between 7000 and 8000 times heavier than an electron
so it takes a lot more to pull it off track. However, it is moving much more slowly so there is more
time to bend it off track. The net result is that an alpha particle tends to bend less than a beta
particle and in the opposite direction.
Figure 3
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P2 5.3b – Student worksheet
Deflection in magnetic fields
When an alpha or a beta particle moves through a magnetic field there is a similar effect of
bending. The directions are again opposite and again the beta particle bends more than the alpha
particle.
Figure 4
Questions
1
Explain why gamma rays do not get deflected in electric or magnetic fields. (1 mark)
2
In which direction will a beta particle be forced in an electric field? (1 mark)
3
Why do alpha particles get deflected in the opposite direction to beta particles and are
deflected less? (2 marks)
4
What property of a beta particle tends to make it deflect more in an electric field? (1 mark)
5
Write a series of bullet points to summarise the information on this page. (5 marks)
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© Pearson Education Ltd 2011. Copying permitted for purchasing institution only. This material is not copyright free.