Quick Exam Notes
  • Model: Particle in a 1D infinite potential well
  • Energy Levels: \(E_n = \frac{n^2 h^2}{8mL^2}\), n = 1,2,3…
  • Wave Function: \(\psi_n(x) = \sqrt{\frac{2}{L}} \sin \left(\frac{n\pi x}{L}\right)\)
  • Probability: P = \( \int_0^L \psi_n^2(x) dx \)
  • Boundary Conditions: \(\psi(0) = \psi(L) = 0\)
  • Key Point: Energy ∝ \(n^2\); no zero-energy state
1. A particle is confined to an infinite potential well of width ’L’. Calculate the probability and energy of the particle between x=0 and x=L/4 in the first excited state?

The probability of the particle in 1-D box is given by

\[ P = \left( \sqrt{\frac{{2}}{L}} \right)^2 \int_{0}^{L/4} \sin^2 \left( \frac{2\pi x}{L} \right) dx \]

\[ P = \frac{2}{2L} \int_{0}^{L/4} \left( 1 - \cos \left( \frac{4\pi x}{L} \right) \right) dx \]

Here \( n = 2 \) is the first excited state.

\[ P = \frac{1}{L} \left[ x \Big|_0^{L/4} - 0 \right] \]

\[ P = \frac{1}{4} \]

The energy of the particle in 1-D box is given by

\[ E = \frac{n^2 h^2}{8mL^2} \]

\[ E = \frac{4h^2}{8mL^2} = \frac{h^2}{2mL^2} \]

2. Find the least energy of an electron moving in the dimension in an infinitely high potential box of width 1˚A, given mass of the electron 9.11×10-31 kg and h =6.63×10-34 J.s.

The energy of a particle in 1-D box is given by

\[ E = \frac{n^2 h^2}{8mL^2} \]

The least energy (\(n=1\)) can written as

\[ E = \frac{h^2}{8mL^2} \]

\[ E = \frac{(6.63 \times 10^{-34})^2}{8 \times 9.11 \times 10^{-31} \times (10^{-20})^2} \]

\[ E = \frac{(6.63 \times 10^{-31})^2}{8 \times 9.11 \times 10^{-31} \times 10^{-20}} \]

\[ E = 0.603 \times 10^{-17} \]

Converting it into eV

\[ E = 0.603 \times 10^{-17} \times \frac{1.6 \times 10^{-19}}{eV} \]

\[ E = 37 eV \]

3. A neutron (m = 1.67×10-27 kg) is confined in a one dimensional box of length 1 angstrom. Calculate the minimum energy of the neutron in this box using the particle-in-a-box model.

Given:

The length of the box \( L = 1 \, \text{Å} \)

Mass of the neutron \( m = 1.67 \times 10^{-27} \, \text{kg} \)

We have to find the minimum energy \( E_1 \) (\( n = 1 \)).

The energy of the particle in 1-D box is given by

\[ E_n = \frac{n^2 h^2}{8mL^2} \]

\[ E_1 = \frac{h^2}{8mL^2} \]

\[ E_1 = \frac{(6.625 \times 10^{-34})^2}{8 \times 1.67 \times 10^{-27} \times (10^{-10})^2} \]

\[ E_1 = 3.28 \times 10^{-21} \, \text{J} \]

\[ E_1 = \frac{3.28 \times 10^{-21}}{1.6 \times 10^{-19}} \, \text{eV} \]

\[ E_1 = 0.02 \, \text{eV} \]

4. If the ground energy of the for a particle in a 1D box is 18 eV, what is energy of its 5th excited state?

The energy of a particle in the \(n\)-th state in a 1-D box is equal to the ground state energy multiplied by the square of the principal quantum number \(n\).

\[ E_n = n^2 E_1 \]

Given:

n=1+5 =6 \[ E_6 = 6^2\times 18 eV \] \[ E_6 = 36\times 18 eV \] \[ E_6 = 648 eV \]