Question

In van de Graff generator potential on outer field increases at least until :

• A. the breakdown field of air is reached
• B. the threshold voltage of air is reached
• C. both a and b
• D. none of these

Answer 1

The correct option is A the breakdown field of air is reached

On one hand, we have the equation $V=\frac{1}{4\pi {\epsilon }_0}\frac{q}{r}$ i.e, $V\propto \frac{1}{r}$, whereas on the other hand, the natural tendency for positive charge is to move higher to lower potential. Thus, provided we are somehow able to introduce the small charged sphere into layer one. We can in this way pile larger and larger amount of charge on the latter. The potential of the outer sphere would always keep rising, at least unitl it reaches the breakdown field of air.