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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.

Ohmic Conductors

The substances that obey Ohm’s Law are Ohmic conductors.

E.g. metals

Non-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.


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