Is Resistance Different For Different Values Of V

Is Resistance Different for Different Values of V? Understanding Ohmic and Non-Ohmic Conductors

The simple answer is: it depends. Resistance, the opposition to the flow of electric current, is generally considered constant for many materials, particularly at a specific temperature. This is the basis of Ohm's Law (V=IR), where V is voltage, I is current, and R is resistance. However, this assumption only holds true for ohmic conductors. Many materials exhibit non-ohmic behavior, where resistance changes with the applied voltage. This article explores the nuances of resistance and its dependence on voltage.

What are Ohmic Conductors?

Ohmic conductors, like most metals at room temperature, maintain a constant resistance regardless of the applied voltage. If you double the voltage across an ohmic resistor, you double the current flowing through it. The ratio V/I remains constant, representing the fixed resistance. This linear relationship is perfectly depicted by Ohm's Law and is easily visualized as a straight line graph of V against I.

Examples of Ohmic Conductors:

• Copper wires: Commonly used in electrical wiring due to their consistent and low resistance.
• Nichrome wire (at constant temperature): Used in heating elements, though their resistance changes significantly with temperature.
• Resistors (within their operating range): Designed to maintain a specific resistance value, are crucial components in electronic circuits.

What are Non-Ohmic Conductors?

Non-ohmic conductors, on the other hand, show a change in resistance with varying voltage. The relationship between voltage and current is non-linear; doubling the voltage may not double the current. This variation in resistance stems from the material's inherent properties and can be due to several factors:

• Temperature dependence: As current flows, heat is generated. This increase in temperature often alters the material's resistance. For example, incandescent light bulbs exhibit significantly higher resistance when hot compared to when cold.
• Material properties: Some materials, such as semiconductors and electrolytes, have resistance that depends on the electric field strength (proportional to voltage). The charge carriers' mobility and concentration are affected by the applied voltage, resulting in non-linear behavior.
• Non-linear devices: Diodes, transistors, and other semiconductor devices are explicitly designed to exhibit non-ohmic behavior. They only allow current to flow in one direction or their conductivity changes drastically with small voltage variations.

Examples of Non-Ohmic Conductors:

• Diodes: Allow current flow in only one direction.
• Light bulbs (incandescent): Resistance increases with temperature as the filament heats up.
• Semiconductors: Resistance changes significantly with temperature, voltage, and light intensity.
• Electrolytes: Resistance depends on the concentration of ions and the applied voltage.

Determining Resistance:

Measuring resistance directly using an ohmmeter only provides a value at a specific moment and under specific conditions (typically low voltage). It does not necessarily represent the resistance at higher voltages unless the material is confirmed to be ohmic. To determine the resistance at different voltages for non-ohmic materials, you need to measure both the voltage and current at various points and then calculate the resistance (R=V/I) for each point. This will show a varying resistance value depending on the applied voltage.

In Conclusion:

While Ohm's Law provides a simple model for many everyday electrical scenarios, it’s crucial to remember its limitations. For materials that exhibit ohmic behavior, the resistance remains relatively constant across various voltages (at a constant temperature). However, a vast range of materials demonstrate non-ohmic characteristics, with their resistance changing significantly depending on the applied voltage. Understanding this distinction is fundamental to comprehending the behavior of electrical circuits and the properties of different materials.