Potential Difference Questions

$AB$ is a part of an electrical circuit (see figure). The potential difference $V_A-V_B$ at the instant when current $i=2\text{A}$ and is increasing at a rate of $1\text{amp / second}$ is:

In a circuit, the current through a $4\Omega$ resistor is $1\text{A}$ when the points $P$ and $M$ are connected to a d.c. voltage source. The potential difference between the points $M$ and $N$ is: