$(a)$ 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?

(A) The instrument used to measure the potential difference between two points in a circuit is a Voltmeter.
The $SI$ unit of potential difference is the Volt $(V)$. It is defined as the amount of work done $(W)$ in moving a unit positive charge $(q)$ from one point to another,i.e.,$V = \frac{W}{q}$. Thus,$1 \text{ Volt} = 1 \text{ Joule} / 1 \text{ Coulomb}$.
The circuit symbols are:
$(i)$ Variable resistor: $A$ resistor symbol with an arrow pointing across it.
$(ii)$ Closed plug key: $A$ circle with a dot inside it.
$(b)$ $(i)$ Circuit $(I)$ has more resistance because the resistors are connected in series,and the equivalent resistance is the sum of individual resistances $(R_{eq} = R_{1} + R_{2} + R_{3})$.
$(ii)$ More current passes through circuit $(II)$ because the equivalent resistance in parallel is less than the individual resistances,leading to a higher total current from the source.
$(iii)$ In circuit $(II)$,the potential difference across each resistor is equal because they are connected in parallel.
$(iv)$ In circuit $(I)$,more heat is produced in $R_{1}$ because in a series circuit,the current $(I)$ is the same through all resistors,and heat produced $H = I^{2}Rt$. Since $R_{1}$ is the largest resistance,it will produce the most heat.