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.*

E.g.1

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.

E.g.2

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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