The potential difference between the plates of an air-filled parallel-plate capacitor with a plate separation of 4 cm is 51 V. What is the magnitude of the electric field between the plates of this capacitor

Electric field between plates of a parallel plate capacitor is uniform.

In a uniform electric field, relation between electric field and potential gradient is as follows

electric field = potential gradient [ E = - dV / dl ]

in the given case,

dV = 51 V,

dl = 4 cm

= 4 x 10⁻² m

E = 51 / 4 x 10⁻²

= 12.75 x 10² V / m

= 1275 V / m