What are the conditions under which $Ohm's$ law is not obeyed?

How much work is done in moving a charge of $2 \, C$ across two points having a potential difference of $12 \, V$ (in $, J$)?

$(a)$ Name an instrument that measures electric current in a circuit. Define the unit of electric current.
$(b)$ What do the following symbols mean in circuit diagrams?
$(c)$ An electric circuit consisting of a $0.5 \ m$ long nichrome wire $XY$,an ammeter,a voltmeter,four cells of $1.5 \ V$ each and a plug key was set up.
$(i)$ Draw a diagram of this electric circuit to study the relation between the potential difference maintained between the points $X$ and $Y$ and the electric current flowing through $XY$.
$(ii)$ The following graph was plotted between $V$ and $I$ values. What would be the values of $V/I$ ratios when the potential difference is $0.8 \ V$,$1.2 \ V$ and $1.6 \ V$ respectively? What conclusion do you draw from these values?

$(a)$ What is an electric circuit?
$(b)$ Calculate the number of electrons that flow per second to constitute a current of one ampere. The charge on an electron is $1.6 \times 10^{-19} \ C$.
$(c)$ Draw an electric circuit for studying Ohm's law. Label the circuit components used to measure electric current and potential difference.

The resistivity of semiconductor materials is $..........$.

Describe the nature of the $V-I$ graph for a nichrome wire. ($V =$ Potential difference,$I =$ Current)
$A$ metallic wire of $625 \, mm$ length offers a $4 \, \Omega$ resistance. If the resistivity of the metal is $4.8 \times 10^{-7} \, \Omega \cdot m$,calculate the cross-sectional area of the wire.