Resistor Nonlinear Nature

A resistor is an electrical component designed to oppose the flow of electric current. For ideal resistors, and many practical ones, the resistance value remains constant regardless of the voltage applied across them or the current flowing through them. This behavior follows Ohm's Law, $V = IR$, where $R$ is a constant value.

Understanding Nonlinear Resistors

However, there are certain types of resistors whose resistance value is not constant. Their resistance changes depending on the operating conditions, such as the voltage across them, the current through them, or the temperature. These components are known as nonlinear resistors because the relationship between voltage and current is not a simple linear one.

Analyzing the Options

Let's look at the given options:

• Resistance: This is a fundamental property of a material or an electrical component that quantifies its opposition to current flow. It is not a specific type of component classified by nonlinearity.
• Varistor: A varistor, also known as a voltage-dependent resistor (VDR), is a type of resistor whose resistance changes significantly with the applied voltage. Its resistance is high at low voltages but decreases sharply as the voltage increases beyond a certain threshold. This makes its voltage-current characteristic highly nonlinear. Varistors are often used for surge protection.
• Thermistor: A thermistor is a type of resistor whose resistance changes significantly with temperature. There are two main types: Negative Temperature Coefficient (NTC) thermistors, whose resistance decreases as temperature increases, and Positive Temperature Coefficient (PTC) thermistors, whose resistance increases as temperature increases. Thermistors are nonlinear because their resistance is not constant, but their nonlinearity is primarily temperature-dependent, not voltage-dependent in the same way a varistor is.
• Specific resistance: This term refers to resistivity, which is an intrinsic property of a material indicating how strongly it resists electric current per unit length and cross-sectional area. It is represented by the symbol $\rho$ (rho) and is used to calculate the resistance of an object made from that material ($R = \rho \frac{L}{A}$). It is not a component.

Identifying the Nonlinear Resistor Type

Among the given options, both Varistor and Thermistor are nonlinear resistors. However, the Varistor's nonlinearity is defined by its significant voltage dependency, which is a direct form of nonlinear resistance often contrasted with ideal linear resistors. While thermistors are also nonlinear (temperature-dependent), varistors are primarily known for their voltage-controlled resistance variation.

Conclusion

The component that is explicitly defined by its voltage-dependent, and therefore nonlinear, resistance characteristic is the Varistor. Thus, the resistor which is nonlinear in nature is called a Varistor.