If resistors are connected in series,the current through each resistor remains the same.

$(a)$ 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?

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

The units of resistance are ampere.

$1\, \mu A = \dots \dots \dots \,mA$

Which law is used to calculate the magnitude of the electric force between two electric charges?