NEET Physics: Modern Physics questions with solutions

Q1. Which one moves with greatest speed:

1. \( \alpha \) -rays
2. \( \beta \) -rays
3. \( \gamma \) -rays
4. cathode rays

Answer: \( \beta \) -rays

Beta rays are streams of electrons, which have very small mass, so they can move very fast. Alpha rays are heavy helium nuclei and move much more slowly; gamma rays travel at light speed but are not material particles, and cathode rays are also electrons but are typically less energetic than beta rays in this context.

Q2. f a proton and anti-proton come close to each other and annihilate, how much energy will be released?

1. \( 1.5 \times 10^{-10} \mathrm{J} \)
2. \( 3 \times 10^{-10} J \)
3. \( 4.5 \times 10^{-10} J \)
4. \( 2 \times 10^{-10} \mathrm{J} \)

Answer: \( 2 \times 10^{-10} \mathrm{J} \)

When a proton and antiproton annihilate, their entire rest mass is converted into energy. Since they have equal mass, the released energy is twice the proton rest energy, which is about 2 × 10^-10 J.