Field strength is the negative potential gradient

Electric Fields - OCR A-Level Physics

Field strength is the negative potential gradient: E = -$\frac{dV}{dr}$.
The minus sign means E points in the direction of decreasing potential.
In a uniform field, $E = V/d (the magnitude$ of the potential gradient is constant).
On a V-r graph for a radial field, the gradient at any point gives the field strength at that distance.

E = -\frac{dV}{dr}

Examiner Tips and Tricks

If asked to find E from a V-r graph, draw a tangent at the required point and calculate its gradient.
Do not forget the minus sign - though for magnitude-only questions, just state the gradient magnitude.