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Question:

Assume that two deuteron nuclei in the core of a fusion reactor at temperature T are moving towards each other, each with kinetic energy 1.5kT, when the separation between them is large enough to neglect Coulomb potential energy. Also neglect any interaction from other particles in the core. The minimum temperature T required for them to reach a separation of 4 × 10⁻¹⁵ m is in the range:

1.0 × 10⁹K < T < 2.0 × 10⁹K

3.0 × 10⁹K < T < 4.0 × 10⁹K

2.0 × 10⁹K < T < 3.0 × 10⁹K

4.0 × 10⁹K < T < 5.0 × 10⁹K

Solution:

Conservation of energy:
KE = PE
2(1.5kT) = e²/4πε₀d
3kT = (1.6 × 10⁻¹⁹)²/4π(8.84 × 10⁻¹²) (4 × 10⁻¹⁵)
T = 1.4 × 10⁹K