Factors Affecting Resistance MCQs Quiz | Class 10

Understanding Factors Affecting Resistance

Resistance is a measure of how much an object opposes the flow of electric current. Every material offers some resistance to current flow, but the extent varies greatly depending on several factors. Understanding these factors is crucial for designing electrical circuits and selecting appropriate materials for various applications.

Key Factors Influencing Resistance:

1. Length of the Conductor (L): The resistance of a uniform conductor is directly proportional to its length. This means a longer wire will have higher resistance than a shorter wire of the same material and cross-sectional area. Think of it as electrons having to travel a greater distance, encountering more obstacles.
2. Area of Cross-section of the Conductor (A): The resistance of a uniform conductor is inversely proportional to its area of cross-section. This implies that a thicker wire (larger cross-sectional area) will have lower resistance than a thinner wire of the same material and length. A larger area provides more space for electrons to flow, reducing collisions and opposition.
3. Nature of the Material (Resistivity, ρ): Different materials have different inherent abilities to conduct electricity. This property is quantified by resistivity (ρ), which is a characteristic of the material itself. Good conductors like copper and silver have very low resistivity, while insulators like glass and rubber have very high resistivity. Materials like Nichrome, often used in heating elements, have a moderate to high resistivity.
4. Temperature (T): The effect of temperature on resistance varies with the type of material.
   • For Metals: For most metallic conductors, resistance increases with an increase in temperature. As temperature rises, atoms vibrate more vigorously, increasing the frequency of electron collisions, thus impeding current flow.
   • For Semiconductors: For semiconductors like silicon and germanium, resistance generally decreases with an increase in temperature. This is because higher temperatures provide enough energy for more electrons to break free and become charge carriers.

These relationships can be summarized by the formula:

R = ρL/A

Where:

• R is the Resistance (measured in Ohms, Ω)
• ρ is the Resistivity (measured in Ohm-meters, Ωm)
• L is the Length of the conductor (measured in meters, m)
• A is the Area of cross-section of the conductor (measured in square meters, m²)

Resistivity of Common Materials

Quick Revision Points:

• Resistance directly proportional to length (R ∝ L).
• Resistance inversely proportional to area of cross-section (R ∝ 1/A).
• Resistance depends on the material’s resistivity (ρ).
• For metals, resistance increases with temperature; for semiconductors, it decreases.
• The SI unit of resistance is Ohm (Ω), and resistivity is Ohm-meter (Ωm).

Extra Practice Questions:

1. What is the SI unit of resistivity?
   Answer: Ohm-meter (Ωm).
2. Name a material commonly used in heating elements due to its high resistivity.
   Answer: Nichrome.
3. Why are household electrical wires typically made of copper or aluminum?
   Answer: Because they have low resistivity, making them excellent conductors of electricity.
4. If a wire is stretched to double its original length, assuming its volume remains constant, how will its resistance change?
   Answer: Its resistance will increase four times (because length doubles and area halves, R ∝ L/A, so R’ ∝ 2L / (A/2) = 4 * (L/A)).
5. Does the resistance of an alloy typically change more or less with temperature compared to pure metals?
   Answer: Alloys generally show less change in resistance with temperature compared to pure metals, which is why they are preferred for standard resistors and heating elements.