Three points A, B and C lie in a uniform Electric field E of $2 \times 10^3 ~N/C$ as shown in figure. Find the potential difference between points A and C is :

80 V

Potential difference is 

$\Delta V = -\vec{E}.\vec{\Delta r} = -\vec{E}.(\Delta x\hat{i}+\Delta y\hat{j}) = - E.\Delta x = -E.AB = -2\times 10^3 \times 4\times 10^{-2} = -80V$

-ve sign shows that A is at higher potential.