Limitations of Ohm's Law
Ohm's law has an important limitation: it does not describe every electrical component under every operating condition. It works when current is directly proportional to voltage and resistance remains constant over the range considered.
The main limitations involve non-linear voltage-current relationships, temperature-dependent resistance and devices that behave differently when voltage polarity is reversed.
What are the main limitations of Ohm's law?
The following five points explain the situations in which a constant-resistance model cannot fully describe an electrical component.
Current may not be directly proportional to voltage
Ohm's law assumes that increasing voltage produces a proportional increase in current. Some devices do not follow this relationship. Their voltage-current graph is curved rather than a straight line.
Example: A semiconductor diode.
In a diode, current can rise rapidly after the forward voltage reaches a suitable operating region. Doubling the voltage does not necessarily double the current.
Resistance may change with temperature
In many metallic conductors, resistance increases when temperature rises. If current heats a conductor, its resistance may no longer remain constant.
Example: A filament lamp.
The filament is relatively cool before the lamp is switched on. As it glows, its temperature and resistance increase. Therefore, the current is not directly proportional to voltage over the lamp's full operating range.
Some devices behave differently when voltage is reversed
For an ohmic resistor, reversing the voltage reverses the current while preserving the same resistance. Some electrical components do not behave this way.
Example: A semiconductor diode.
A diode normally permits much more current in its forward direction than in its reverse direction, before reverse breakdown. Its behaviour therefore depends on the polarity of the applied voltage.
Electrical behaviour can depend on other physical conditions
Some components respond to changes in illumination, electric field, temperature or other conditions. A single fixed resistance may not accurately describe their operation.
Examples: Light-sensitive semiconductor devices and gas-discharge devices under suitable conditions.
Their electrical response must be considered using the relevant operating conditions and measured voltage-current characteristics.
Ohm's law is not universal for all materials
Many conductors behave approximately ohmically only within a particular range of voltage, current and temperature. Other materials and electronic components have non-ohmic characteristics.
Examples: Diodes, LEDs and filament lamps over their full operating ranges.
Engineers must choose a model that describes the actual component and its operating conditions rather than assuming that every component has a constant resistance.
Difference between ohmic and non-ohmic devices
| Feature | Ohmic device | Non-ohmic device |
|---|---|---|
| Voltage and current | Directly proportional over the range considered | Not directly proportional over the range considered |
| Resistance V/I | Approximately constant | Changes with the operating point |
| V-I graph | Straight line through the origin | May be curved or asymmetric |
| Typical example | Fixed resistor under stable conditions | Diode or filament lamp over its full range |
V-I graphs showing the limitation
These simplified graphs show the difference between a constant-resistance conductor and a non-ohmic component. Voltage is on the horizontal axis and current is on the vertical axis.
Ohmic conductor
Current increases proportionally with voltage. Resistance remains constant.
Non-ohmic device
Current is not directly proportional to voltage throughout the operating range.
The graphs are conceptual illustrations, not experimental data for a specific device. Actual graph shapes vary depending on the component.
Examples of components that do not follow Ohm's law
| Component | Why it can be non-ohmic |
|---|---|
| Filament lamp | The filament heats up, changing its resistance. |
| Semiconductor diode | Current changes non-linearly with voltage and polarity. |
| LED | Current depends non-linearly on its forward voltage. |
| Gas-discharge tube | Ionisation and discharge conditions affect its current-voltage behaviour. |
| Light-dependent device | Its electrical response can change with illumination. |
Interactive demonstration
Compare an ideal resistor with a simplified filament-lamp model to see how a changing resistance affects current.
The lamp model is illustrative only. It is not a universal equation for real filament lamps.
Exam-ready answer: Limitations of Ohm's law
Question: State the limitations of Ohm's law.
- It does not describe devices whose current is not directly proportional to voltage.
- It is not applicable as a constant-resistance model when relevant physical conditions change the resistance.
- It does not describe the asymmetric voltage-current behaviour of components such as diodes.
- It cannot be assumed to apply universally to every material and electrical device.
Frequently asked questions
What is the main limitation of Ohm's law?
The main limitation is that it applies only when voltage and current are directly proportional under the relevant physical conditions. Many real devices do not satisfy this condition throughout their operating range.
Why does a filament lamp not obey Ohm's law?
Its filament becomes hotter as current increases. This changes the resistance, making current non-proportional to voltage over the full operating range.
Why is a diode called a non-ohmic device?
A diode has a non-linear current-voltage characteristic. It also behaves differently under forward and reverse voltage, so it does not have a constant resistance throughout its operating range.
Does Ohm's law apply to all conductors?
No. Some conductors behave approximately ohmically under stable conditions, but other materials and devices have non-ohmic characteristics.
Can V = IR be used for a non-ohmic device?
The ratio V/I can give the resistance at a particular operating point when current is non-zero. However, that does not mean the device obeys Ohm's law over its entire operating range.
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