# Chapter 11: Resistance and Resistivity

Ohm’s Law

A conductor obeys Ohm’s law if the current in it is directly proportional to the potential difference across its ends. For something to obey Ohm’s law the current flowing is proportional to the p.d. pushing it. V= IR so this means the resistance is constant. Resistor
The above figure 1 shows that when the p.d. is zero so is the current. When we increase the p.d. in one direction the current increases in that direction. If we apply p.d in reverse direction a current flows in the reverse direction. The red straight line shows that the current is proportional to p.d. and it obeys Ohm’s Law.
Filament Lamp
As shown in figure 2, at low values the current is proportional to p.d. and so, obeys Ohm’s law. As potential difference increases so does temperature. This increases the resistance and the graph curve.
Diode
According to figure 3, it shows us that increasing the p.d. increases current in one direction, but in reverse direction the p.d does not make a current flow. Since resistance changes, it does not follow Ohm’s law.

Different Resistors
Variable Resistors
A variable resistor is a resistor whose value can be changed.
Thermistor
Thermistor are components that are designed to have a resistance which changes rapidly with temperature. The resistance of a thermistor varied with temperature. At low temperature the resistance is high, at high temperatures the resistance is low.
Light Dependent Resistor
The resistance of the thermistor varied with light intensity, in din light the resistance is high and in bright light the resistance is low.

Resistivity
Resistivity is the property of material. It can be shown by following:
Resistance ∝ length/(cross sectional area)
R∝l/A
Resistance=ρL/A Where; ρ is constant.
ρ=RA/L
Which gives the resistivity.
So, the resistivity increases with temperature according to the above equation.