A beam consisting of -particles. -particles and -rays pass through a magnetic field at right angles to the…

Analyze the question: The beam has three kinds of radiation moving at right angles to a magnetic field. We need to decide which are deflected.

Concept: A magnetic field exerts a force only on moving charged particles. The force is

F = qvB sin θ

When the motion is perpendicular to the field (θ = 90°), sin θ = 1, so F = qvB. If q = 0, then F = 0 and there is no deflection.

Solution:

• α-particles are helium nuclei with charge +2e. They are charged, so they are deflected.
• β-particles are fast electrons (or positrons) with charge −e (or +e). They are charged, so they are deflected. They bend in the opposite direction to the α-particles, and more sharply because they are much lighter.
• γ-rays are electromagnetic waves (photons) with no charge. They are not deflected.

So the order is: deflected, deflected, not deflected.

**Answer: B.

Tip: Remember "charge = deflection". Alpha and beta carry charge, so they bend in electric and magnetic fields. Gamma has no charge, so it goes straight. Use Fleming's left-hand rule to find the direction of deflection for the charged ones.