1. What are the variables that effect on a conductor resistance (I mean all of them)?
First of all I would like to say that I know how to calculate the resistance of a conductor using the method below:
$R=\rho\dfrac{\ell}{A}\,$
where $\ell$ is the length of the conductor, measured in metres [m], $A$ is the cross-section area of the conductor measured in square metres [m²], and $\rho$ is the electrical resistivity of the material, measured in ohm-metres $(\Omega·m)$.
Now I can calculate the resistance of a wire using the relation between the electrical resistivity of the material, the length of the conductor and the cross-section area of the conductor.
But there is a missed variable in the previous method, it's the temperature change $\Delta T$.
2. So I thought that if I would use the previous method with the temperature change variable calculated, then I should multiply it by temperature change in kelvin (because of the direct proportion between resistance and temperature change)? Is that correct?
$R=\rho\dfrac{\ell}{A}\times\Delta T$
Best Answer
I think that while Temperature is a major factor for resistance, the equation you wrote is not correct.
In a linear approximation, $$R=R_0(1+\alpha(T-T_0))$$ where $R_0$ is the resistance at temperature $T_0$.
Exactly all factors that affect resistance, I think they are many, but these are the major ones (electrical applications).