Factors Affecting Resistance – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Define resistance. Explain its unit, what it signifies in a circuit, and compare materials with high and low resistance using everyday examples.
Answer:
Resistance is the property of a material that opposes the flow of electric current through it. It acts like a “traffic barrier” for electrons, slowing their movement.
The SI unit of resistance is the ohm (Ω). A resistance of 1 Ω means a conductor allows a certain amount of current to flow when a specific potential difference is applied.
Materials with low resistance (like copper and silver) allow current to flow easily, so they are used in wires and cables. Materials with high resistance (like rubber and glass) do not allow current to pass and are used as insulators for safety.
In everyday life, a metal wire of the same size conducts better than a rubber rod. A pencil lead (graphite) may heat and glow if enough current passes, showing it has moderate resistance.
Thus, resistance determines how easily electricity flows and how devices behave under current.
Q2. How does the length of a conductor affect its resistance? Use the formula R = ρL/A and real-life cases to explain.
Answer:
The resistance (R) of a wire is directly proportional to its length (L), expressed as R ∝ L. From the formula R = ρL/A, if L doubles while ρ and A remain constant, R also doubles.
Physically, a longer wire gives electrons more collisions with atoms, increasing opposition to their flow.
Examples:
Two copper wires of the same thickness: if Wire A is 1 m and Wire B is 2 m, Wire B has approximately 2R (double resistance).
Long extension cords have higher resistance, often reducing the brightness of bulbs or slowing fans.
Power transmission lines are very long; to reduce energy loss from resistance, they use very thick conductors.
In practical circuits, when wiring needs to run over long distances, we prefer shorter paths or thicker wires to keep resistance and heating low, ensuring better performance and safety.
Q3. Explain how the area of cross-section of a wire affects its resistance. Include analogies and everyday applications.
Answer:
The resistance (R) of a wire is inversely proportional to its area of cross-section (A), written as R ∝ 1/A. Using R = ρL/A, increasing A decreases R if ρ and L are constant.
Think of it like a road: a wider road reduces traffic jams; similarly, a thicker wire offers more pathways for electrons, lowering opposition.
Examples:
If Wire A has area A and Wire B has area 2A, then Wire B has about half the resistance of Wire A.
Household wiring is made thicker to keep resistance low, which minimizes heating and energy loss.
Devices drawing higher current (like geysers) require thicker wires to avoid excessive voltage drops and overheating.
Therefore, choosing the appropriate thickness is crucial for efficiency, safety, and performance in electrical circuits.
Q4. What role does the nature of material (resistivity) play in resistance? Compare copper, iron, nichrome, and rubber with suitable uses.
Answer:
Resistivity (ρ) is a material-specific constant that indicates how strongly a material resists current. In R = ρL/A, a higher ρ means a higher resistance for the same L and A.
Good conductors like copper and silver have low ρ, so they are used where efficient current flow is needed—e.g., house wiring, motor windings.
Iron has a higher ρ than copper, so it offers more resistance for the same dimensions and is less efficient for wiring.
Nichrome has high resistivity and heats up quickly, making it ideal for heating elements (irons, toasters, heaters).
Rubber has very high resistivity, acting as an insulator to prevent electric shocks.
Thus, the choice of material depends on whether we need low resistance (for transmission) or high resistance (for heating or protection).
Q5. Describe how temperature affects resistance in metals and semiconductors. Support with examples from daily life.
Answer:
In metals, resistance increases with temperature. As temperature rises, metal atoms vibrate more, causing more collisions with electrons, which increases opposition to flow.
In semiconductors, resistance usually decreases with temperature because more charge carriers become available when heated.
Everyday examples:
The filament of an electric bulb has a higher resistance when it is hot than when it is cold; it glows due to the heat produced.
A wire carrying high current warms up and its resistance increases, slightly reducing current.
Thermistors (temperature-dependent resistors) change resistance significantly with temperature and can be used as sensors.
Understanding the temperature-resistance relation helps in designing circuits to prevent overheating and in creating devices that can measure temperature changes accurately.
High Complexity (Analytical & Scenario-Based)
Q6. You must light a bulb placed far from the power source using copper wires. Justify whether you should choose a long thin wire or a short thick wire, using the resistance formula and practical reasoning.
Answer:
To keep the bulb bright, you need low resistance in the connecting wire so that more current reaches the bulb. From R = ρL/A, resistance:
Increases with length (L), and
Decreases with area (A).
Therefore, a short, thick wire is the best choice. A long, thin wire would have high resistance, reducing current and making the bulb dim.
Using copper (low ρ) already helps, but if the distance is large, even copper can cause a noticeable drop in current if the wire is thin.
A thicker wire lowers resistance, reduces heating, and avoids energy loss in the wire itself.
In practice, when the power source is far, electrical installers select low-ρ material, minimal length, and larger cross-section to maintain efficiency and safety.
Q7. An electric heater uses a long, thin nichrome coil. Analyze why this design is chosen with respect to length, area, resistivity, and temperature behavior.
Answer:
A heater’s goal is to convert electrical energy into heat efficiently. A long, thin nichrome coil achieves high resistance, which helps produce greater heating when current passes.
Factors:
Length (L): A longer wire increases R, aiding heat production.
Area (A): A thin wire (small A) increases R, further boosting heat.
Resistivity (ρ):Nichrome has a high ρ, so for the same size, it offers more resistance than copper or iron.
Temperature behavior: Nichrome can withstand high temperatures without melting and its resistance changes predictably with temperature, making it stable for heating.
This design ensures the coil reaches a high temperature quickly, remains durable, and does not draw excessive current like a low-ρ, thick, short wire would.
Q8. A fan connected via a very long, thin extension cord runs slower and the cord gets slightly warm. Explain the reasons using factors that affect resistance and practical implications.
Answer:
A long, thin cord has high resistance because:
R ∝ L: Long length raises resistance.
R ∝ 1/A: Thin wire (small A) raises resistance.
Due to higher resistance, part of the voltage is dropped across the cord, so less voltage reaches the fan. This lowers the current through the fan’s motor, making it run slower.
The wire’s resistance causes energy loss in the form of heat, which is why the cord becomes warm.
If the wire is too thin for the current, the heating can be dangerous, risking insulation damage.
The solution is to use a shorter and thicker extension made of a low-ρ material like copper. This ensures better performance, reduced energy loss, and improved safety.
Q9. Two students build circuits with wires of equal length and thickness: one uses copper and the other nichrome. Predict which bulb glows brighter and justify your answer using resistivity.
Answer:
For the same length (L) and area (A), the resistance depends on the material’s resistivity (ρ). Using R = ρL/A, the higher the ρ, the higher the R.
Copper has a lower resistivity than nichrome, so the copper wire’s resistance is lower under the same dimensions.
With lower resistance, more current flows in the copper circuit, causing the bulb to glow brighter.
In contrast, the nichrome wire has higher resistance, which reduces current and makes the bulb dim. This is the reason nichrome is preferred in heaters, where heat (not brightness) is desired.
Thus, the copper-wired circuit delivers higher current to the bulb and better brightness, directly reflecting the material’s resistivity.
Q10. When a bulb is switched on, it initially draws more current and then stabilizes. Explain this behavior using temperature dependence of resistance for metals.
Answer:
The bulb’s filament is made of a metal. At the moment of switching on, the filament is cold, so its resistance is low (since metal resistance increases with temperature).
With lower initial resistance, a larger current flows. This current quickly heats the filament, making its temperature rise sharply.
