Question

Is ohm’s law universally applicable for all conducting elements? If not, give examples of elements which do not obey Ohm’s law.

Answer 1

The answer is no, Ohm's law is not universally applicable for all conducting elements.

Ohm's law is an empirical rule, not a fundamental law of nature. It accurately describes the relationship between voltage, current, and resistance for a specific class of materials, but many elements and devices do not follow this simple linear relationship.

What Ohm's Law States

First, let's be precise about what it means to "obey" Ohm's Law. A conductor obeys Ohm's Law if the ratio of the voltage ($V$) across it to the current ($I$) flowing through it is constant, provided the temperature and other physical conditions do not change. This constant is its resistance ($R$).

$V = I R \quad \text{(where R is constant)}$

Graphically, this means the V-I (Voltage vs. Current) graph for an ohmic device is a straight line passing through the origin. The slope of this line is the resistance, R.

Conductors that obey Ohm's Law are called ohmic conductors (e.g., a simple copper wire at constant temperature).
Conductors that do not obey Ohm's Law are called non-ohmic conductors. Their resistance is not constant; it changes with the voltage, current, or temperature.

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Examples of Elements and Devices that DO NOT Obey Ohm's Law

Here are several common examples of non-ohmic elements:

1. Semiconductor Diodes

A diode is designed to allow current to flow easily in one direction (forward bias) and to block it in the opposite direction (reverse bias).
* Why it's non-ohmic: Its resistance is not constant. It is very low in the forward direction (once a small threshold voltage is overcome) and extremely high in the reverse direction. The V-I graph is a sharp, non-linear exponential curve, not a straight line.

2. Filament of an Incandescent Light Bulb

While the material (tungsten) is a metal, its behavior in a bulb is non-ohmic.
* Why it's non-ohmic: Ohm's Law requires constant temperature. As current flows through the filament, it heats up to thousands of degrees. The resistance of tungsten increases significantly as its temperature rises. Therefore, if you double the voltage, you will get less than double the current because the resistance has increased due to the higher temperature. The V-I graph starts as a line but then curves as the filament heats up.

3. Thermistors

A thermistor is a resistor whose resistance is designed to change significantly with temperature.
* Why it's non-ohmic: As current flows through a thermistor, it generates heat ($P = I^2R$), which in turn changes its own resistance. This self-heating effect makes its V-I characteristic non-linear. For an NTC (Negative Temperature Coefficient) thermistor, the resistance drops as it gets hotter.

4. Transistors

Transistors are the building blocks of all modern electronics. They are fundamentally non-ohmic.
* Why it's non-ohmic: A transistor is an active component. The current flowing between two of its terminals (e.g., collector and emitter) is controlled by a small voltage or current at a third terminal (the base or gate). Its resistance is actively being changed, so it does not have a constant R value.

5. Gas Discharge Tubes (e.g., Neon Lamps)

These devices conduct electricity through an ionized gas.
* Why it's non-ohmic: A high voltage is required to start the conduction (ionize the gas). Once conduction begins, the voltage across the tube can actually decrease as the current increases, a phenomenon known as negative differential resistance. This behavior is highly non-linear and very different from Ohm's Law.

Summary Table

| Element / Device | Reason for Non-Ohmic Behavior | V-I Graph Characteristic |
| --------------------------- | ------------------------------------------------------------- | ------------------------------------------------------ |
| Semiconductor Diode | Allows current flow in only one direction (p-n junction). | Exponential curve, not a straight line. |
| Light Bulb Filament | Resistance increases dramatically with temperature. | A curve with a decreasing slope (if I vs V). |
| Thermistor | Resistance is intentionally designed to vary with temperature. | Non-linear curve due to self-heating. |
| Transistor | Resistance is actively controlled by a third terminal. | A family of curves, not a single line. |
| Gas Discharge Tube | Conduction through ionized gas; exhibits negative resistance. | Complex; not a line and not monotonic. |

In conclusion, Ohm's law is a very useful and accurate model for simple metallic conductors under stable conditions, but it is far from a universal law for all materials that conduct electricity.