(a) Ohm's Law: The potential difference (V) across the ends of a conductor is directly proportional to the current (I) flowing through it, provided the temperature remains constant.
Mathematically: $V \propto I$, i.e., $\dfrac{V}{I} = R$ (constant), or $V = IR$
(b) 1 ohm: The resistance of a conductor is said to be 1 ohm if a current of 1 ampere flows through it when a potential difference of 1 volt is applied across its ends.
(c) Given: I = 0.5 A, V = 2 V
$$R = \frac{V}{I} = \frac{2\text{ V}}{0.5\text{ A}} = 4 \text{ Ω}$$
The resistance of the conductor is 4 Ω.
Source: Chapter 11 – Electricity, Sections 11.2 and 11.3
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