Name and define the $SI$ unit of resistance. Calculate the resistance of a resistor if the current flowing through it is $200\, mA$, when the applied potential difference is $0.8\, V$.

(A) $(i)$ The $SI$ unit of resistance is $Ohm (\Omega)$.
$(ii)$ One $Ohm$ is defined as the resistance of a conductor such that when a potential difference of $1\, V$ is applied across its ends, a current of $1\, A$ flows through it.
$(iii)$ Given:
$I = 200\, mA = 0.2\, A$
$V = 0.8\, V$
Using Ohm's Law, $V = IR$:
$R = \frac{V}{I}$
$R = \frac{0.8\, V}{0.2\, A}$
$R = 4\, \Omega$