Why do beta particles travel farther in air than alphas?

The greater the rate of ionisation, the shorter the path length

Wrong Track: If beta particles travel farther in air, their rate of ionisation of air molecules must be greater than for alpha particles.

Right Lines: If beta particles travel farther in air, their rate of ionisation of air molecules must be less than for alpha particles. The longer path in air is linked with a lower rate of ionisation.

Why is the rate of ionisation of air greater for alpha particles?

Thinking about the learning

The thinking that causes difficulty here follows on directly from the previous challenge. Relationships of the form more of this results in more of that are common in physics. For example, a larger battery potential difference drives a bigger electric current around a fixed circuit. In this case, however, there is an inverse relationship between path length and rate of ionisation.

Thinking about the teaching

The key point to address here is why the rate of ionisation is greater for alpha than for beta particles. In other words, why is it that with alpha particles the initial energy of the particle is shifted within a relatively short path? The idea to get across here is that the alpha particles, in comparison with beta particles, are massive and carry double the charge. This means that they are much more likely to interact with air molecules as they pass through air causing ionisation.

The ionisation process is often described in terms of the alpha particle knocking an electron out of the atom, which gives rise to an image of some kind of physical collision. In reality, the collision is electrostatic in nature as the positively charged alpha particle removes the negatively charged electron through a process of electrostatic attraction.