What is resistance?
We know that electric current is nothing but only continuous flow of charges. The electric potential (voltage) difference established between the two terminals encourages the movement of charges. For a charge the journey from terminal to terminal is not a direct path. Rather, it is a zigzag path that results from countless collisions with fixed atoms within the material. That is why the charge experience a hindrance to their movement. This hindrance to the flow of charges is known as resistance. So the electric current depends on voltage and resistance. Ohm's law gives the relation among them like below.
Voltage = Current x Resistance
From this relation we see that if we increase the voltage in a resistive system, the current also increases.
Negative Resistance:
From above discussion, we can determine that the resistance is the ratio of voltage and current. So in general for resistance in a circuit or device, if we increase the voltage, always there is an increase in current and vice versa. But in practice, sometimes a situation arises when in a circuit or device, if we increase the voltage, the current decreases. This nature is the opposition of resistance. That is why we describe this property as negative resistance (opposition of resistance) but remember there is no positive resistance.
Negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.
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