Electric Field at Surface of Earthed Sphere (using image charges)

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"A charged sphere is a distance d from the centre of an earthed sphere conducting sphere of radius a. An image charge (q') for this system is located at a distance b from the centre of the sphere where:

q'= -(aq/d) and b=(a^2)/d

Calculate the expression for the Electric field at point p being on the surface of the sphere directly above the surface."

Would the Electric field be zero. As the Electric potential would surely be zero on the surface of the sphere and the E field is just -grad of the potential?

 

[mfb]

The potential is zero on the surface (and in the interior), but non-zero everywhere else. This gives a non-zero electric field.
In a similar way, you can define the floor as area of zero gravitational potential - and still get a gravitational field.