Which of the following is/areNOT affected by a magnetic field? I. Alpha particle II. Cathode rays III. Neutrons

Analyze the question: We need to find which of the listed items is NOT deflected by a magnetic field.

Concept: A magnetic field exerts a force on a particle only if the particle carries electric charge and is moving. The force is F = qvB sinθ. If q = 0, then F = 0, so there is no deflection.

Step-by-step check:

• I. Alpha particles are helium nuclei (2 protons + 2 neutrons) with a charge of +2e. They are charged, so they ARE deflected.
• II. Cathode rays are streams of electrons with a charge of -e. They are charged, so they ARE deflected. (This is the principle used in the CRT and in J.J. Thomson's experiment.)
• III. Neutrons have no charge (q = 0). The force on them is zero, so they are NOT affected by a magnetic field.

Answer: Only neutrons are unaffected, so the answer is C. III only.

Tip: For "affected by a magnetic or electric field" questions, ask one thing: is it charged? Alpha (+), beta (-), cathode rays (-), and protons are deflected. Neutrons, gamma rays, X-rays, and other electromagnetic waves are not.