The $I-V$ graph for two resistors, resistor 1 ($R_1$) and resistor 2 ($R_2$), are shown below :
[Image of I-V graph]
Which one of the following statements about these resistors is not correct?
$R_1$ follows Ohm's law.
$R_2$ does not follow Ohm's law after voltage $V_1$.
Up to $V_1$, the resistance of $R_1$ is smaller than that of $R_2$.
Up to $V_1$, the resistance of $R_1$ is larger than that of $R_2$.
Answer is Right!
Answer is Wrong!
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UPSC Geoscientist – 2023
The correct answer is D) Up to $V_1$, the resistance of $R_1$ is larger than that of $R_2$.
The resistance $R$ of a component can be determined from its I-V graph as $R = V/I$. This is equivalent to $R = 1 / (\text{slope of the I-V graph})$ if the graph plots I on the y-axis and V on the x-axis, as shown in the image.
For resistor $R_1$, the graph is a straight line passing through the origin, indicating that $R_1$ is an ohmic resistor and follows Ohm’s law. Its resistance is constant. The slope of the $R_1$ line (I/V) is constant.
For resistor $R_2$, the graph is a curve. Up to voltage $V_1$, the graph is approximately linear but less steep than $R_1$. The slope of the $R_2$ curve (I/V) is smaller than the slope of the $R_1$ line in this region.
Since resistance $R = 1 / (\text{slope of I-V graph})$, a smaller slope corresponds to a larger resistance.
Comparing the slopes up to $V_1$: (Slope of $R_1$) > (Slope of $R_2$).
Therefore, (Resistance of $R_1$) < (Resistance of $R_2$).
Statement A is correct because $R_1$ graph is a straight line through origin.
Statement B is correct because the curve of $R_2$ after $V_1$ shows that its resistance (V/I or $1/$slope) changes (increases as V increases).
Statement C says "Up to $V_1$, the resistance of $R_1$ is smaller than that of $R_2$". This is consistent with our finding that $R_1 < R_2$ up to $V_1$. So C is correct.
Statement D says "Up to $V_1$, the resistance of $R_1$ is larger than that of $R_2$". This contradicts Statement C and our analysis. Therefore, Statement D is not correct.