Practical No. 04: Determination of specific resistance of given wire.
XIII. Interpretation of Results
- The specific resistance calculated from the formula and from the graph are consistent.
- This value is a constant for the material of the wire, as it remained unchanged for different lengths of the wire, confirming the theory.
XIV. Conclusions and Recommendations
- It is concluded that the specific resistance is an intrinsic property of the material.
- It is highly recommended to measure the diameter of the wire very precisely using a micrometer, as the area (A) is very sensitive to errors in diameter measurement.
XV. Practical Related Questions & Answers
Q1. State the factors on which specific resistance of the wire depends in your experiment.
- Answer: The specific resistance depends only on the material of the wire and its temperature. It is completely independent of the length and cross-sectional area of the wire.
Q2. For two wires of same length and different radii, does the resistance of wire change? Give reasons.
- Answer: Yes, the resistance changes.
- Reason: Resistance is inversely proportional to the cross-sectional area (A = 3.142 × r²). Therefore, the wire with the larger radius (larger area) will have a lower resistance.
Q3. Name the different methods of finding unknown resistance.
- Answer:
- Ammeter-Voltmeter method (using Ohm’s law).
- Metre Bridge method (Wheatstone bridge principle).
- Using a digital multimeter in resistance mode.
- Carey Foster’s bridge method.
Q4. Calculate the resistance of a copper wire 20 meter long and diameter of 0.05 cm. (specific resistance of copper = 1.678 × 10⁻⁶ Ω-cm.)
- Answer:
- Given Data:
- Specific resistance (rho) = 1.678 × 10⁻⁶ Ω-cm
- Length (L) = 20 m = 2000 cm
- Diameter (d) = 0.05 cm -> Radius (r) = 0.025 cm
- Step 1: Calculate Cross-Sectional Area (A)
- Area (A) = 3.142 × r²
- Area (A) = 3.142 × (0.025)² = 0.00196375 cm²
- Step 2: Calculate Resistance (R)
- Formula: R = (rho × L) / Area
- R = (1.678 × 10⁻⁶ × 2000) / 0.00196375
- R = 0.003356 / 0.00196375
- R = 1.71 Ohms (Ω)
- Given Data:
Q5. If the radius wire is doubled, will the specific resistance change? Explain.
- Answer: No, the specific resistance will not change.
- Explanation: Specific resistance is an intrinsic property of the material itself, not its shape or size. Doubling the radius changes the wire’s dimensions (increases Area), which changes its total resistance (R), but the material’s specific resistance remains constant.
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