(a) Which of the following graphs (A, B or C) shows the correct relation between Potential difference (V) and Current (I) for an Ohmic resistor? Give a reason for your answer.
(b) Give an example of an Ohmic resistor.
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(a) Graph C shows the correct relation.
Reason — An ohmic resistor obeys Ohm's law, according to which the potential difference across the resistor is directly proportional to the current flowing through it (V ∝ I), provided the physical conditions remain unchanged. A directly proportional relation is represented by a straight line passing through the origin, which is the case only in graph C.
In graph A the line is straight but it does not pass through the origin, and in graph B the potential difference decreases with increase in current. Hence neither A nor B represents an ohmic resistor.
(b) All metallic conductors are ohmic resistors, for example a copper wire (or a resistor made of an alloy such as manganin or constantan).