Ohms Law And Light Bulbs: An Exception Explained

Ohm's Law, which describes the relationship between voltage, current, and resistance, does not seem to apply to light bulbs. This is because the resistance of a light bulb changes with temperature. When a light bulb heats up, its resistance increases dramatically. This means that the resistance of a light bulb with current flowing through it is much higher than when the bulb is cool. This change in resistance with temperature is why Ohm's Law does not seem to work for light bulbs.

Ohm's Law applies to resistance

Ohm's Law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. The constant of proportionality is the resistance. Ohm's Law can be expressed in three ways:

• V = IR
• I = V/R
• R = V/I

Where I is the current through the conductor, V is the voltage measured across the conductor, and R is the resistance of the conductor.

Ohm's Law is an empirical relation that accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current. However, some materials do not obey Ohm's Law; these are called non-ohmic materials.

Ohm's Law is a useful tool for analysing electric circuits and is used often in the study of electricity and electronics. It is important to note that resistance cannot be measured in an operating circuit, so Ohm's Law is especially useful for calculating resistance.

Resistance is measured in ohms and is symbolised by the letter R.

Light bulbs are not resistors

Ohm's Law, V = IR, is a law and not an approximation. However, it is only applicable to simple resistors and does not apply to other electrical components like incandescent lamps, semiconductors, and diodes.

While a light bulb does behave like a resistor, its resistance is a strong function of the filament temperature. The filament in a light bulb is usually made from a thin tungsten wire that glows when heated. The wire has resistance, which limits the current automatically. This resistance is also what causes the filament to heat up.

The resistance of a light bulb is not constant, and it is non-linear. It is a function of temperature. When the temperature increases, the resistance of the light bulb also increases. Therefore, Ohm's Law, which assumes a constant resistance, does not apply to light bulbs.

In conclusion, light bulbs are not resistors, and Ohm's Law does not apply to them due to the changing resistance with temperature.

Resistance changes with temperature

Ohm's Law states that the voltage across an element is equal to the product of the current passing through the element and the resistance of the element. Mathematically, this is expressed as ΔV = I * R, where ΔV is the change in voltage, I is the electric current, and R is the resistance. While Ohm's Law is applicable to certain elements called "ohmic", it does not apply to "non-ohmic" materials, which exhibit a non-constant resistance.

The filament in an incandescent light bulb is an example of a non-ohmic element. As the voltage across the filament increases, so does the current, causing the bulb to heat up and glow. However, as the temperature of the filament increases, so does its resistance. This change in resistance with temperature is what makes it challenging to apply Ohm's Law to light bulbs.

The resistance of a light bulb filament is influenced by its temperature. As the temperature increases, the resistance also increases. This relationship can be modelled using the equation R = R0 * (1 + α(T - T0)), where R is the resistance at a given temperature, R0 is the resistance at a reference temperature T0, α is the resistance temperature coefficient, and T is the current temperature. By measuring the voltage and current of a light bulb and plotting the results, one can observe that the resistance of the filament increases as the bulb gets hotter.

For example, consider an old-style tubular incandescent bulb. When the bulb is off, its resistance is lower, but as it heats up and starts to glow, its resistance increases. At room temperature (around 294 K), the resistance of the bulb may be around 161 Ω. As the bulb gets hotter, the resistance increases; at a temperature of 748 K, the resistance may be around 490 Ω. This change in resistance with temperature is why Ohm's Law doesn't seem to work for light bulbs.

In summary, the resistance of a light bulb filament is not constant and changes with temperature. This variation in resistance makes it challenging to apply Ohm's Law, which assumes a constant resistance, to light bulbs. While Ohm's Law provides a good approximation for many resistive materials, it does not hold true for elements like light bulbs, where the resistance is strongly dependent on temperature.

Ohm's Law is not a law, but an approximation

Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperatures remain constant. The law was named after German physicist Georg Ohm, who first verified it experimentally in 1827.

Ohm's Law is considered a law because it is an empirical relation that accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current. However, it is not a fundamental law like the conservation of momentum. Instead, it is more akin to an approximation, as it does not apply universally and is dependent on certain conditions being met.

For example, Ohm's Law does not hold true if the temperature or other physical factors are not kept constant. In certain components, increasing the current raises the temperature, such as in the filament of a light bulb. As the temperature of the filament rises, its resistance increases dramatically, and Ohm's Law cannot be applied.

Additionally, Ohm's Law only applies to ohmic conductors such as iron and copper but not to non-ohmic conductors like semiconductors. Non-linear electrical elements, such as semiconductors, have a ratio of voltage to current that varies with changes in voltage, and therefore, Ohm's Law does not apply to them.

In summary, while Ohm's Law is considered a law due to its broad applicability and empirical nature, it is more of an approximation as it relies on specific conditions being met and does not apply universally.

Ohm's Law applies to metals at constant temperature

Ohm's Law describes the behaviour of metals at a constant temperature. It establishes the relationship between the current (I) and the voltage (V) between two points of a conductor. The law states that the current flowing through a conductor is directly proportional to the voltage applied between the two points.

Ohm's Law is not universal and has several limitations. It is only applicable in the case of conductors and does not apply to insulators or unilateral circuits. It also does not apply to superconductors, which are materials that abruptly lose all resistance below a particular critical temperature.

The resistance of a conductor is affected by its length and the cross-sectional area. Research has shown that resistance is directly proportional to the length of the conductor and inversely proportional to its cross-sectional area. The resistivity of a material, which is the constant needed to remove the sign of proportionality in the equation, depends on the type of material and its temperature. In the case of conductors, the resistivity increases as the temperature increases, which makes the resistance also variable. Therefore, for Ohm's Law to be applicable, the temperature must be kept constant.

In summary, Ohm's Law applies to metals at a constant temperature because the resistance of a conductor is dependent on temperature. When the temperature is constant, the relationship between current and voltage described by Ohm's Law holds true.

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