Ohmic and Non-Ohmic Conductors: How Resistance Changes Explained
Ohmic conductors follow Ohm's law, where resistance remains constant regardless of voltage and current. Non-ohmic conductors have resistance that changes w
TL;DR: Ohmic conductors follow Ohm's law, where resistance remains constant regardless of voltage and current. Non-ohmic conductors have resistance that chan…
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Ohmic and Non-Ohmic Conductors: How Resistance Changes: Ohmic conductors follow Ohm's law, where resistance remains constant regardless of voltage and current. Non-ohmic conductors have resistance that changes w
Ohmic and Non-Ohmic Conductors: How Resistance Changes Explained Simply
Ohmic conductors follow Ohm's law, where resistance remains constant regardless of voltage and current. Non-ohmic conductors have resistance that changes w This is a key physics concept for Class 11.
Ohm's Law and Ohmic Conductors
Ohm's law states that voltage equals current times resistance: V = IR. For ohmic conductors, resistance R is constant, so current is directly proportional to applied voltage. If you double the voltage, you double the current. This relationship holds across a wide range of voltages. Resistance depends on material (resistivity), length, and cross-sectional area: R = ρL/A. Ohmic behavior means the material properties remain unchanged as current flows. Most metals like copper and aluminum are good ohmic conductors at normal operating temperatures. Resistors used in electronics are designed to be ohmic over their rated voltage and current ranges.
Non-Ohmic Conductors: Resistance Varies
Non-ohmic conductors do not obey Ohm's law because their resistance changes with applied voltage or current. Semiconductors like silicon exhibit non-ohmic behavior because they contain both free electrons and holes. Light bulbs with filaments are non-ohmic because as current heats the filament, its temperature rises and resistance increases. Diodes are non-ohmic because they allow current in one direction but block it in the other direction. Thermistors are temperature-sensitive resistors where resistance decreases significantly as temperature increases. The relationship between voltage and current is non-linear for these devices. A V-I graph for non-ohmic conductors is curved rather than a straight line.
Filament Lamps: Temperature-Dependent Resistance
Traditional incandescent light bulbs have tungsten filaments that are non-ohmic. At low currents, the filament is cool and has low resistance. As current increases, the filament heats up and resistance increases substantially. The relationship is non-linear: resistance can double or triple as the filament heats. The resistance at operating temperature (about 2,500°C for warm bulbs or 3,000°C for bright bulbs) is much higher than at room temperature. This means the steady-state current is much less than initially expected from low-temperature resistance. The non-linear relationship makes starting current (when the filament is cool) higher than steady-state current, which is why light bulbs can blow out when switched on. As filaments age and become thinner, resistance increases further, reducing brightness.
Semiconductors and Diodes
Semiconductors like silicon and germanium exhibit non-ohmic behavior due to their atomic structure. Pure semiconductors have four valence electrons and form four covalent bonds with neighboring atoms. At room temperature, some covalent bonds are broken, creating mobile charge carriers (electrons and holes). As temperature increases, more carriers are available, and conductivity increases dramatically. This makes semiconductors temperature-sensitive. Diodes are semiconductor devices that allow current in one direction (forward bias) but block current in the opposite direction (reverse bias). The non-ohmic IV curve of a diode shows almost no current below the forward voltage threshold (about 0.7 V for silicon), then current increases rapidly. This makes diodes useful for rectification and protection.
Applications and Practical Implications
Thermistors are resistors designed to be non-ohmic, with resistance highly dependent on temperature. NTC (Negative Temperature Coefficient) thermistors have resistance that decreases with temperature, useful for temperature sensing and compensation. PTC (Positive Temperature Coefficient) thermistors have resistance that increases with temperature, useful for self-regulation (current limiting as temperature rises). In circuits, including non-ohmic components requires more complex analysis than simple Ohm's law. Circuit designers must account for how resistance changes. For example, an LED (light-emitting diode) requires a series resistor to limit current, because the LED's resistance changes with current. Incandescent bulbs in circuits with other components can affect stability and performance due to their non-linear resistance.
Worked Examples
Example 1. A lamp filament has cold resistance of 50 ohms at room temperature. If 10 volts is applied, what is the initial current? If at operating temperature the filament has resistance 500 ohms, what is the steady-state current?
Solution: Initial current when cold: I = V/R = 10/50 = 0.2 amperes. Steady-state current when hot: I = 10/500 = 0.02 amperes. The non-ohmic behavior (resistance increases with heating) reduces current tenfold between initial and steady states.
Example 2. Why do incandescent light bulbs have higher initial current when first switched on?
Solution: The filament is cool with low resistance when first switched on. As current flows, it heats the filament, resistance increases, and current decreases to a lower steady-state value. This initial surge of current can stress the filament, which is why bulbs often blow when first turned on.
Where You See This in Real Life
- Traditional incandescent light bulbs are non-ohmic, with resistance increasing as the filament heats, limiting current and protecting the bulb from excessive current.
- LEDs used in modern lighting are non-ohmic semiconductors requiring a series resistor to limit current and prevent damage.
- Thermistor temperature sensors in appliances like thermostats use non-ohmic properties to measure temperature reliably.
- Power electronics in devices like phone chargers use semiconductor diodes to rectify AC to DC, relying on their non-ohmic directional conductivity.
Common Mistakes Students Make
- Assuming all conductors obey Ohm's law. Many important components like LEDs and diodes are fundamentally non-ohmic.
- Treating non-ohmic components as ohmic in circuit analysis. This leads to incorrect current and voltage predictions.
- Not accounting for temperature effects in resistance calculations. Temperature dependence is crucial for accurate analysis of real circuits.
Frequently Asked Questions
What is the difference between ohmic and non-ohmic conductors?
Ohmic conductors obey Ohm’s law — current is directly proportional to voltage and the V–I graph is a straight line (e.g. metals like copper). Non-ohmic conductors do not obey Ohm’s law and their V–I graph is not a straight line (e.g. a diode or a filament bulb).
Give examples of ohmic and non-ohmic conductors.
Ohmic: copper wire and most metallic conductors at constant temperature. Non-ohmic: a semiconductor diode, LED, filament lamp and transistor.
What does the V–I graph look like for each?
For an ohmic conductor the graph of voltage against current is a straight line through the origin. For a non-ohmic conductor the graph is curved or non-linear.
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- Thermal Expansion: How Objects Grow with Heat Explained
- Latent Heat: The Hidden Heat in Phase Changes Explained
- Specific Heat Capacity: How Materials Store Heat Differently Explained
- Laws of Reflection and Mirror Formula: How Light Bounces Explained
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