... what is the "electrical resistance nature" ... if there is additional resistance than we calc the current will be lower !
There is not additional resistance. In this case, there is actually less resistance, so the current is higher.
The "electrical resistance nature" is simply the phenomena that results from the physical properties of any material to a change of temperature. In this case, it is resistance. The values are consistent with tungsten. My comments were to explain why the calculated value is different from the closest selectable value on the test.
All materials have a certain specific resistance and they change resistance according to temperature by certain amounts. Most (metal) materials have a positive resistance coefficient (negative conductance coefficient) to temperature. This is generally known as the "alpha" or “sigma” constant. The positive coefficient means the material increases in resistance to current flow as the temperature increases. For a negative coefficient the material decreases in resistance to current flow, as the temperature increases.
(The inverse of resistivity is called conductivity. There are perspectives where the use of conductivity is more suitable).
This coefficient is a hypothetical constant --though it really is not linear, meaning the rate of resistance to a given temperature will increase faster than the rate of temperature change. (Some materials become extremely nonlinear such as super conducting materials). However, for small (and often large) changes in temperature, the rate of change is usually insignificant, and a standardized value is used for basic calculations.
For tungsten the alpha" (α) constant is 0.004403 ohms per degree C. The initial reference point is 5.6E-8 ohms per meter @ 20 C. This means an increase of 227C will increase the resistance by 1 ohm uniformly across the standardized area and length.
Tungsten filaments are designed to operate near the lower end of its vaporization point to efficiently produce the greatest light per unit of energy. This is approximately 2500 to 2800 Kelvins. When a bulb is operated at a lower voltage the energy is reduced and the operation temperature drops accordingly. The temperature drop will in turn lower the resistance. The lower resistance will increase the current flow in proportion. Keep in mind, too, that as the resistance drops and the current flow increases the power also increases. At some point this reaches an equilibrium.
In the example, assume the calculated and actual current flow is different by about 0.7 amp, this would correspond to a temperature reduction of 159C per standard unit. Most (higher wattage) bulbs have a coiled filament of about 20 to 22 cm (the cross sectional area is also smaller). Without knowing accurate specifications, the calculations are somewhat arbitrary, but two to three times this temperature is reasonable.
The concepts presented here are usually covered in more detail in the advanced physics classes. The phenomena is described in part at the quantum level, how electron motion is affected and mediated by short-range spin waves known as paramagnons.
A basic analogy to describe the effect: Consider how a person might navigate across a room full of people. If the people are standing relatively still then navigating is more efficient than if the people are randomly moving around. The heat in this case corresponds to the motion of the people moving around.
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