Of which physical quantity is the $joule/coulomb$ a unit?

From the listed electrical devices,which releases heat energy that is not appreciated?

$100 \, J$ of heat is produced each second in a $4 \, \Omega$ resistance. Find the potential difference across the resistor. (in $, V$)

An electric lamp is marked $100 \, W, 220 \, V$. It is used for $5$ hours daily. Calculate: $(i)$ its resistance while glowing. $(ii)$ energy consumed in $kWh$ per day.

An electric iron has a rating of $750\, W; 200\, V$. Calculate:
$(i)$ The current required.
$(ii)$ The resistance of its heating element.
$(iii)$ Energy consumed by the iron in $2$ hours.

When a high resistance voltmeter is connected directly across an electric bulb,its reading is $2 \ V$. An electric cell is sending a current of $0.4 \ A$ (measured by an ammeter) in the electric circuit.
$(a)$ Draw the circuit.
$(b)$ Find the resistance of the electric bulb.
$(c)$ State the law that is applied for making these calculations. If a graph is plotted between $V$ and $I$,show the nature of the graph obtained.