Non-Ohmic Conductors: When Current Doesn't Play by the Rules
Understanding the Basics: Ohm's Law vs. Reality
To understand non-ohmic conductors, we must first understand the rule they break: Ohm's Law. Formulated by German physicist Georg Simon Ohm in the 1820s, this law is a cornerstone of circuit theory. It states that the current ($I$) flowing through a conductor between two points is directly proportional to the voltage ($V$) across the two points, and inversely proportional to the resistance ($R$) of the conductor.
Where:
• V = Voltage (in Volts, V)
• I = Current (in Amperes, A)
• R = Resistance (in Ohms, $Ω$)
A component that obeys this law perfectly is called an Ohmic Conductor. For these materials, the resistance ($R$) is a constant value. If you double the voltage, the current also doubles. If you plot voltage against current on a graph, you get a perfectly straight line. This is known as a linear relationship. Most simple metal wires at a constant temperature are good examples of ohmic conductors.
A Non-Ohmic Conductor, however, does not follow this simple rule. Its resistance is not constant. It changes depending on the voltage applied to it or the current passing through it. If you were to plot the voltage and current for a non-ohmic component, the graph would not be a straight line. It could be a curve, a sharp turn, or any other non-linear shape. This means the ratio $V/I$ is not fixed, and therefore, the resistance value $R$ changes.
A Tale of Two Behaviors: Comparing Ohmic and Non-Ohmic
The key difference lies in how resistance behaves. The following table provides a clear comparison:
| Feature | Ohmic Conductor | Non-Ohmic Conductor |
|---|---|---|
| Obey's Ohm's Law | Yes | No |
| Resistance (R) | Constant (at constant temperature) | Changes with voltage or current |
| V-I Graph | Straight line through the origin | Curved line or a line not through the origin |
| Examples | Metal resistors (e.g., carbon film), copper wire | Light bulb filament, diode, thermistor, LED |
Why Resistance Changes: The Science Behind Non-Ohmic Behavior
Resistance in a material is a measure of how difficult it is for electric current to flow. In non-ohmic conductors, this difficulty changes due to various physical factors. The main reasons for non-ohmic behavior are:
1. Temperature Change: This is one of the most common causes. The resistance of most conductors increases with temperature. Imagine a simple incandescent light bulb. When you first turn it on, the filament is cold and has a relatively low resistance. A large surge of current flows. As the filament heats up to a very high temperature to produce light, its resistance increases significantly. This is why the bulb draws less current when it's glowing brightly than at the moment it's switched on.
2. Semiconductor Properties: Components like diodes and Light Emitting Diodes (LEDs)[1] are made from semiconductor materials. These materials have a unique property: they allow current to flow easily in one direction (low resistance) but block it almost completely in the opposite direction (very high resistance). This behavior is fundamentally non-ohmic and is the basis for modern electronics.
3. Ionic Conduction: In materials like gases or electrolytes, current flow involves the movement of ions. The relationship between voltage and current in these mediums is often complex and non-linear, especially at high voltages where electrical breakdown can occur (like in a spark).
Non-Ohmic Conductors in Action: Real-World Examples
Non-ohmic conductors are not rare exceptions; they are everywhere in our daily lives and modern technology. Their unique properties make them incredibly useful.
The Incandescent Light Bulb: As mentioned, the tungsten filament in an old-fashioned light bulb is a classic example. Its resistance when off (at room temperature) is much lower than when it is on and white-hot. If you use a multimeter to measure the resistance of a bulb when it's cold, you'll get a value. If you then calculate the resistance using Ohm's Law while it's lit ($R = V / I$), you will find a much higher value, proving its non-ohmic nature.
Diodes and LEDs: A diode is a one-way street for electricity. If you apply a positive voltage (forward bias), it has very low resistance and current flows easily. If you reverse the voltage (reverse bias), it has extremely high resistance and almost no current flows. An LED works the same way but emits light when current flows through it in the forward direction. Their V-I graph shows no current up to a certain voltage (the turn-on voltage), and then current rises very steeply.
Thermistors: The name comes from "thermal resistor." These are special resistors whose resistance changes dramatically with temperature. There are two main types:
• NTC (Negative Temperature Coefficient)[2] Thermistors: Their resistance decreases as temperature increases. They are used in digital thermometers and as inrush current limiters in power supplies.
• PTC (Positive Temperature Coefficient)[3] Thermistors: Their resistance increases as temperature increases. They are often used as self-resetting fuses in circuits.
Varistors (Voltage Dependent Resistors): These components change their resistance based on the voltage applied. At normal voltages, they have a very high resistance. But when the voltage exceeds a certain threshold (like during a power surge from a lightning strike), their resistance drops suddenly, diverting the harmful surge away from sensitive electronics and protecting them.
Common Mistakes and Important Questions
Q: Is a wire always an ohmic conductor?
A: Not necessarily! A simple copper wire is approximately ohmic at a constant, low temperature. However, if you pass a very large current through it, it will heat up. As its temperature increases, its resistance also increases, and it starts to behave in a non-ohmic way. For practical purposes in simple circuits, we treat wires as ohmic, but it's important to know the limits.
Q: Can we still use the formula R = V/I for non-ohmic conductors?
A: Yes, but with a crucial clarification. The formula $R = V / I$ gives you the dynamic or instantaneous resistance at that specific moment for that specific voltage and current. It is not a constant value. For an ohmic conductor, this value is always the same. For a non-ohmic conductor, this value is only valid for the single point on the V-I curve where you measured V and I. If you change the voltage, you will get a different resistance value from the calculation.
Q: Are non-ohmic conductors bad or defective?
A: Absolutely not! In fact, their non-ohmic behavior is what makes them so valuable. If all conductors were ohmic, we wouldn't have diodes, transistors, LEDs, temperature sensors, or surge protectors. Modern electronics as we know it would not exist. Their variable resistance is a feature, not a bug.
Conclusion
Non-ohmic conductors are essential components that break the simple, linear relationship of Ohm's Law. Their resistance changes with voltage or current due to factors like temperature changes and semiconductor physics. Far from being mere curiosities, they are the building blocks of modern technology, found in everything from the humble light bulb to sophisticated computers and protection devices. Understanding the difference between ohmic and non-ohmic behavior is a critical step in moving from simple circuit theory to the real, dynamic world of electronics.
Footnote
[1] LED (Light Emitting Diode): A semiconductor diode that glows when a current is passed through it in the forward direction.
[2] NTC (Negative Temperature Coefficient): A property of a material where its resistance decreases as its temperature increases.
[3] PTC (Positive Temperature Coefficient): A property of a material where its resistance increases as its temperature increases.