The thickest line symbolizes the metal creating the homogeneous electric field. To get an estimate for the current we need to estimate the number of electrons colliding with the wall. We need the velocity of electrons, v, and the density of the electrons, r . On the other hand T is very small, so in effect the field emission will be very small. A very similar problem, albeit on a very different scale is the radioactive a -decay of heavy nuclei.
Many heavy nuclei are unstable.
The radioactive decay of Uranium and the lifetime of the Uranium nucleus can be approximately explained by the tunneling effect. The fact that the decay happens at all indicates that it is energentically possible for the a -particle to separate fromwhat is left, a Thorium nucleus. Then the question arises how does the Uranium nucleus hold together, since two nuclei, being positively charged, always have a strong Coulomb repulsion between them. The answer is that there is a strong, but very short range attractive nuclear force between these nuclear fragments. The potential energy corresponding to this force can be approximated by a square potential well of depth of 40MeV in which the a -particle has kinetic energy larger then 40Mev.