Why is the plot of resistivity vs temperature for copper parabolic in nature? Doesn't copper obey Ohm’s Law?

Variation of Resistivity in Conductors

We know that current is the movement of free electrons from one atom to the other when there is a potential difference. In conductors no forbidden gap is present between the conduction band and valence band. In many cases both the bands overlap each other. The valence electrons are loosely bound to the nucleus in conductors. Usually metals or conductors have low ionization energy and so they tend to lose electrons very easily. When an electric current is applied the delocalized electrons are free to move within the structure. This is the case that happens in normal temperature.

When the temperature increases the vibrations of the metal ions in the lattice structure increases. The atoms starts to vibrate with higher amplitude. These vibrations in turn causes frequent collisions between the free electrons and the other electrons. Each collision drain out some energy of the free electrons and causing them unable to move. Thus it restricts the movement of the delocalized electrons. When the collision happens the drift velocity of the electrons decreases. This means that the resistivity of the metal increases and thus current flow in the metal is decreased. The resistivity increases means that the conductivity of the material decreases.

For metals or conductors, it is said that they have a positive temperature co – efficient. The value α is positive. For most of the metals, the resistivity increases linearly with increase in temperature for a range of 500K. Examples for positive temperature co – efficient include, silver, copper, gold etc.  

Temperature dependence on resistivity for metals