Radioactivity and the Nucleus: Question 4

Worked solution

Step 1: Identify the charge of each type of radiation

• Alpha particles are helium nuclei, carrying a charge of $+2e$.
• Beta particles are fast-moving electrons, carrying a charge of $-1e$.
• Gamma rays are electromagnetic radiation, carrying no charge.

Step 2: Work out the direction of deflection

A charged particle in an electric field is attracted towards the plate of opposite sign:

• Alpha particles ($+2e$) are attracted towards the negative plate (right-hand plate).
• Beta particles ($-1e$) are attracted towards the positive plate (left-hand plate).
• Gamma rays, having no charge, experience no electric force and are undeflected.

Step 3: Compare the size of the deflection

For a given electric field, the deflection produced depends on the charge-to-mass ratio: a larger charge or a smaller mass gives a greater deflection.

Alpha particles and beta particles carry a similar magnitude of charge, but an alpha particle is roughly 7000 times more massive than a beta particle. Since the beta particle has so much less mass (and so much less inertia) for a similar-sized force, it is deflected more than the alpha particle.

Step 4: Match to the correct option

• Alpha deflects towards the negative plate, small deflection.
• Beta deflects towards the positive plate, larger deflection than alpha.
• Gamma is undeflected.

This matches option A.

Why the other options are wrong

Final answers

• Alpha deflects towards the negative plate (small deflection); beta deflects towards the positive plate (larger deflection); gamma is undeflected, option A.