Question

Explain the factors that affect electric resistance.

Answer 1

There are four factors that affect the resistance of a wire:

1. Resistance is proportional to length. If you take a wire of different lengths and give each a particular potential difference across its ends. The longer the wire the less volts each centimeter of it will get. This means that the 'electric slope' that makes the electrons move gets less steep as the wire gets longer, and the average drift velocity of electrons decreases. The correct term for this 'electric slope' is the potential gradient. A smaller potential gradient (less volts per metre) means current decreases with increased length and resistance increases.
   
2. Resistance is inversely proportional to cross-sectional-area. The bigger the cross sectional area of the wire the greater the number of electrons that experience the 'electric slope' from the potential difference. As the length of the wire does not change each cm still gets the same number of volts across it - the potential gradient does not change and so the average drift velocity of individual electrons does not change. Although they do not move any faster there are more of them moving so the total charge movement in a given time is greater and current flow increases. This means resistance decreases. This does not give rise to a straight line graph as cross sectional area is inversely proportional to resistance not directly proportional to it
3. Resistance depends on the material the wire is made of. The more tightly an atom holds on to its outermost electrons the harder it will be to make a current flow. The electronic configuration of an atom determines how willing the atom will be to allow an electron to leave and wander through the lattice. If a shell is almost full the atom is reluctant to let its electrons wander and the material it is in is an insulator. If the outermost shell (or sub-shell with transition metals) is less than half full then the atom is willing to let those electrons wander and the material is a conductor.
4. Resistance increases with the temperature of the wire. The hotter wire has a larger resistance because of increased vibration of the atomic lattice. When a material gets hotter the atoms in the lattice vibrate more. This makes it difficult for the electrons to move without interaction with an atom and increases resistance. The relationship between resistance and temperature is not a simple one. $R=R_0(1+\alpha T)$, where $\alpha$ is the thermal resistance coefficient.