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The current-voltage graph for a device is shown in figure. The resistance is negative in region:

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(a) AB
(b) BC
(c) ABC
(d) None of these

Answer
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Hint: Resistance is given as R=VI. Here the slope gives us the value m=ΔIΔV=I2I1V2V1which is nothing but m=1R. Therefore, try to figure for which part the slope will be negative.

Complete step by step answer:
From Ohm’s Law we have,
V=IRR=VI
And the graph is I vs V so the slope will be,
m=ΔIΔV=I2I1V2V1
On further simplification,
R=1m
Now from the graph we see that for part AB the slope is negative, for point B slope is zero and for part BC the slope is positive. Therefore, the part for which the resistance is negative is AB.
The correct answer is option A.

Note:Be careful in evaluating the slope. Read the axes carefully whether V is on x-axis or y-axis. Many times students evaluate slope m=R=V/I in a hurry. Well the answer remains the same and correct in case of multiple choice. But in case of subjective questions, marks will be deducted.