What will be the equivalent resistance between points $A$ and $B$ of the following electric circuit?

$(a)$ Name an instrument that measures potential difference between two points in a circuit. Define the unit of potential difference in terms of $SI$ unit of charge and work. Draw the circuit symbols for $(i)$ variable resistor,$(ii)$ a plug key which is closed.
$(b)$ Two electric circuits $(I)$ and $(II)$ are shown below.
$(i)$ Which of the two circuits has more resistance?
$(ii)$ Through which circuit does more current pass?
$(iii)$ In which circuit is the potential difference across each resistor equal?
$(iv)$ If $R_{1} > R_{2} > R_{3}$,in which circuit will more heat be produced in $R_{1}$ as compared to the other two resistors?

$Ohm's$ law is a relation between the power used in a circuit to the current and the potential difference.

How does the resistance of a wire vary with its cross-sectional area?

$A$ $4 \, \Omega$ resistance wire is doubled on itself. Calculate the new resistance of the wire.

Which of the following formulas represents voltage $(V)$?