I'm reading "Introduction to Eletrodynamics" by David J. Griffiths. And the definition of electrostatic pressure on a conductor makes me confuse.
As Griffiths wrote- because the field inside a conductor is zero, so, according to boundary condition, the field just outside must be $(\sigma/\epsilon_0)\hat{n}$ ,the average $(\sigma/2\epsilon_0)\hat{n}$ and the force per unit area:
$$
f = \frac{1}{2\epsilon_0}\sigma^2\hat{n}
$$
Expressing the pressure in terms of the field just outside the surface:
$$
P=\frac{\epsilon_0}{2}E^2
$$
This pressure tends to draw the conductor into the field.
As I see in the equation, does the Electric field just outside the conductor does not depend on external field? And the Pressure also is independent on the external field. But if external field is, let say, zero, then is there no field for "this pressure" to draw into? Am I missing something? Please enlighten me!