If the physical conditions remain constant, the voltage across a resistor is directly proportional to the current through it.
V ∝ I
V = kI
k = R = resistance
V = IR
The physical conditions are temperature, pressure and humidity etc.
When a filament lamp is switched on, the most notable physical factor – the temperature – goes up. Is Ohm’s law still valid in these circumstances for a light bulb?
Yes, only after the temperature of the filament becomes steady at some point.
The voltage across a resistor is 8V and the current through it is 2A. Find the resistance.
V = IR
8 = 2R
R = 4 Ω.
Investigating Ohm’s Law
Change the strength of the power source, a battery in this case, and measure the current through the ammeter and the voltage across the resistor. Then plot a graph of V against I.
If the graph is a straight line that goes through the origin, it shows Ohm’s Law is correct.
The substances that obey Ohm’s Law are Ohmic conductors.
The substances that do not obey Ohm’s Law are Non-Ohmic conductors.
E.g. semiconductor diodes, filament lamps
The I/V graph for Ohmic conductors is a straight line; for semiconductors and filament lamp, they are curvy.