If three resistors of $5 \Omega$,$10 \Omega$,and $15 \Omega$ are connected in series,the equivalent resistance $R$ is equal to ........ (in $\Omega$)

What is the electrical resistivity of a material? What is its $SI$ unit? Describe an experiment to study the factors on which the resistance of a conducting wire depends.

Draw a circuit diagram of an electric circuit containing a cell,a key,an ammeter,a resistor of $2\,\Omega$ in series with a combination of two resistors ($4\,\Omega$ each) in parallel and a voltmeter across the parallel combination. Will the potential difference across the $2\,\Omega$ resistor be the same as that across the parallel combination of $4\,\Omega$ resistors? Give reason.

Draw a schematic diagram of an electric circuit consisting of a battery of two cells each of $1.5\, V$,$5\, \Omega$,$10\, \Omega$ and $15\, \Omega$ resistors and a plug key,all connected in series.

What is the commercial unit of electric energy?

The $V-I$ graph for series and parallel combination of two metallic resistors is as shown in the figure. Which graph represents parallel combination? Justify your answer.