I/V Graph Of Filament Lamp

The study of the electrical characteristics of a filament lamp provides insightful information about its behavior under different conditions, particularly how its resistance changes with temperature. To understand this phenomenon comprehensively, it’s essential to delve into the relationship between voltage (V), current (I), and resistance, as well as the underlying physics governing the behavior of the filament when it is heated.

Understanding the I/V Graph Of Filament Lamp

iv graph of filament lamp

The I/V graph, which plots current (I) against voltage (V), is a crucial tool for analyzing the electrical properties of a filament lamp. A notable observation from this graph is that as the current increases, the ratio of voltage to current (V/I) also increases. This ratio is indicative of the resistance of the filament lamp. In simpler terms, as more current flows through the lamp, its resistance increases. This is a departure from Ohm’s Law, which states that resistance remains constant regardless of the current in an ideal resistor. The behavior of the filament lamp is therefore non-ohmic, meaning its resistance changes with the current.

The Role of Temperature

The increase in resistance with the increase in current is primarily due to the rise in temperature of the filament. Here’s a step-by-step explanation of the process:

  1. Increasing Potential Difference: When the potential difference (voltage) across a filament lamp is increased, the current flowing through it also increases. According to the formula $P = IV$, where $P$ is power, $I$ is current, and $V$ is voltage, an increase in either current or voltage results in a higher power dissipation.
  2. Dissipation of Energy as Heat: The increased power results in more energy being dissipated in the form of heat. This heat energy is absorbed by the filament, causing its temperature to rise.
  3. Increased Resistance Due to Higher Temperature: As the temperature of the filament increases, its resistance increases. This is due to the intrinsic properties of the material from which the filament is made, typically tungsten. The resistance of most conductors increases with temperature.
  4. Increased Collision Rate: The mechanism behind the increase in resistance is the increased rate of collisions between free electrons and the lattice ions of the filament material. At higher temperatures, the lattice ions vibrate more vigorously. This increased vibration leads to a higher probability of collisions between the free electrons (responsible for electrical conduction) and the lattice ions. Each collision impedes the flow of electrons, effectively increasing the resistance of the filament.


This temperature-dependent resistance has practical implications for the use of filament lamps:

  • Brightness Control: By controlling the voltage across the lamp, one can adjust the brightness. A lower voltage means lower temperature, less resistance, and dimmer light. Conversely, higher voltage leads to higher temperature, increased resistance, and brighter light.
  • Energy Efficiency: Filament lamps are less energy-efficient compared to other lighting technologies such as LED or fluorescent lamps. A significant portion of the electrical energy is converted into heat rather than light.
  • Lifespan: The lifespan of a filament lamp is influenced by its operating temperature. Higher temperatures can lead to quicker degradation of the filament material, reducing the lamp’s lifespan.


The relationship between voltage, current, and resistance in a filament lamp, as well as the effect of temperature on resistance, highlights the complex interplay between electrical and thermal phenomena in conductive materials. Understanding these principles is essential not only for educational purposes but also for practical applications in designing and using electrical devices more efficiently and effectively.

Worked Examples

Example 1: The Temperature-Controlled Room

A filament lamp is used in a temperature-controlled room where the ambient temperature is kept constant. Initially, the lamp operates at a low voltage, but the voltage is gradually increased. Describe qualitatively how the brightness of the lamp and its resistance change as the voltage increases. Explain the reasons behind these changes.

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As the voltage across the filament lamp is increased, two main changes occur: the brightness of the lamp increases, and so does its resistance. The increase in voltage causes a higher current to flow through the lamp, leading to more power being dissipated as heat (according to $P = IV$). This heat raises the temperature of the filament, making it glow brighter. The rise in temperature also causes an increase in the resistance of the filament. This is due to the enhanced vibration of the lattice ions at higher temperatures, leading to more frequent collisions with free electrons and, consequently, higher resistance. Despite the room’s constant ambient temperature, the filament’s temperature increases due to electrical heating, affecting both its brightness and resistance.

Example 2: The Dimming Dilemma

An experiment is set up to study the dimming capability of a filament lamp by gradually decreasing the voltage applied to it. As the voltage decreases, explain what happens to the current through the lamp, the filament’s temperature, and the lamp’s resistance. Consider the impact of these changes on the lamp’s light output.

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As the voltage applied to the filament lamp decreases, the current through the lamp also decreases since $I = \frac{V}{R}$, where $R$ is the resistance of the lamp. With a lower current, less power is dissipated in the form of heat, leading to a decrease in the filament’s temperature. As the temperature of the filament drops, its resistance decreases as well because the ions in the filament’s lattice vibrate less vigorously, reducing the rate of collisions with free electrons. This results in a smoother flow of electrons, lowering the resistance. The lower temperature of the filament causes it to emit less light, dimming the lamp. Therefore, by decreasing the voltage, we can effectively dim the lamp, illustrating the direct relationship between voltage, filament temperature, resistance, and light output.

Example 3: The Lifetime Test

A filament lamp is tested for its lifespan by keeping it continuously on at a high voltage setting, close to its maximum rating. Discuss qualitatively the effects of operating at high voltage on the filament’s temperature, resistance, and the overall lifespan of the lamp. Consider the physical processes affecting the filament over time.

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Operating a filament lamp at a high voltage setting, close to its maximum rating, significantly affects its temperature, resistance, and lifespan. The high voltage increases the current through the lamp, leading to greater power dissipation as heat, which significantly raises the filament’s temperature. This elevated temperature increases the resistance of the filament due to more vigorous vibrations of the lattice ions, causing more frequent collisions with free electrons.

Over time, the high temperature degrades the filament material. Tungsten filaments, for instance, may gradually evaporate, thinning the filament and making it more susceptible to breakage. Moreover, the increased resistance over time means the lamp consumes more power to maintain the same light output, decreasing its energy efficiency. Consequently, operating the lamp continuously at high voltage shortens its lifespan due to the accelerated degradation of the filament and the increased likelihood of failure. This example highlights the trade-offs between brightness, energy consumption, and longevity in filament lamp design and use.

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