Lesson 5 of 12 · 7 min
Limitations of Ohm's law
NCERT § "Limitations of Ohm's Law"
Riya's 100 W bulb has a resistance of 484 Ω when lit. Put a meter across it cold and you read something very different.
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The lesson in notes
In short
Ohm's law is an empirical rule that many metals obey closely, not a fundamental law of nature.
Deviation 1: the V-I relation may stop being linear, for example as a conductor heats up at larger currents.
Deviation 2: the relation may depend on the sign of V, so reversing the voltage does not simply reverse the current with the same size (as in a diode).
Deviation 3: the relation may not be single-valued, with more than one current for the same voltage (as in gallium arsenide).
Materials and devices that do not follow V = IR are called non-ohmic.