JEE Main · 2012 · 29 Apr 2012 · Offline · Modern Physics

Assume that a neutron breaks into a proton and an electron. The energy released during this process is (Mass of neutron

= 1.6725 \times 1 0^{- 27}

kg; mass of proton

= 1.6725 \times 1 0^{- 27}

kg; mass of electron

= 9 \times 1 0^{- 31}

kg)

(A)0.51 MeV✓ Correct
(B)7.10 MeV
(C)6.30 MeV
(D)5.4 MeV

Answer: (A)

Solution

The data deliberately set m(neutron) = m(proton), so the whole mass defect is just the electron's mass: Δm = (m_p + m_e) − m_n = m_e = 9 × 10⁻³¹ kg. The energy released is Δm·c² = (9 × 10⁻³¹)(3 × 10⁸)² = 8.1 × 10⁻¹⁴ J = (8.1 × 10⁻¹⁴)/(1.6 × 10⁻¹³) MeV ≈ 0.51 MeV — exactly the electron's rest energy.

JEE Main · 2012 · 29 Apr 2012 · Offline · Single correct option

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