Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it (V = IR), provided the temperature remains constant. However, this law is not a universal law of nature. It fails in several scenarios.
Not a Universal Law: Ohm's law only works perfectly for certain metallic conductors at a constant temperature.
Ohmic Conductors: Obey the law (e.g., copper wire, silver).
Non-Ohmic Conductors: Do NOT obey the law (e.g., Semiconductor diodes, thermistors, vacuum tubes).
Ohm's law assumes that resistance (R) remains constant. However, when a massive amount of current flows through a standard metallic wire (like a lightbulb filament), the wire heats up rapidly. As the temperature rises, the resistance of the metal also increases. Therefore, the V-I graph ceases to be a straight line, breaking Ohm's law.
Ohm's law completely fails for semiconductor devices like P-N Junction Diodes and Transistors. In a diode, if you reverse the direction of the voltage (reverse bias), the current does not flow proportionally in the opposite direction; it almost completely stops. The relationship between V and I is highly non-linear.
Gases (like in neon signs or fluorescent tubes) and liquid electrolytes do not obey Ohm's law. In gases, once the voltage reaches a certain breakdown point, the current suddenly spikes violently, which completely defies the V=IR linear relationship.
Ohm's law is limited because it is not universal. It fails when the temperature of a conductor changes (like a hot filament), and it completely fails for semiconductor devices (like diodes) and gases, where the V-I relationship is non-linear.
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