Top 20 Viva Questions for Physics Practical – Resistivity Using V–I Graph
This article covers the most important viva questions and answers for the Physics practical: To determine the resistivity of 2/3 wire by plotting a graph between voltage (V) and current (I). Each question is followed by a concise answer and a short explanation to help you understand the concept thoroughly.
Viva Questions with Answers and Explanations
1. Question: State Ohm’s law.
Answer: Voltage is directly proportional to current at constant temperature.
Explanation: This means that the ratio V/I remains constant if the temperature of the conductor does not change, which is a key condition for Ohm's law to hold true.
2. Question: What is resistivity?
Answer: Resistivity (ρ) = R·A/L.
Explanation: Resistivity is a material-specific property indicating how strongly it resists current flow. It is calculated using the wire’s resistance, cross-sectional area, and length.
3. Question: What is the SI unit of resistivity?
Answer: Ohm‑metre (Ω·m).
Explanation: Since ρ = R·A/L, its unit becomes ohm multiplied by meter, which is Ω·m, reflecting resistance across a unit cube of material.
4. Question: Why do we plot a V vs I graph?
Answer: To find resistance and verify Ohm’s law.
Explanation: The slope of the V–I graph represents resistance. A straight-line graph confirms the material obeys Ohm’s law (ohmic behavior).
5. Question: What is the shape of the V–I graph?
Answer: A straight line passing through the origin.
Explanation: This linearity indicates that voltage increases proportionally with current, confirming constant resistance.
6. Question: How is resistance determined from the graph?
Answer: By calculating the slope (ΔV/ΔI).
Explanation: The slope of the voltage-current graph gives the resistance of the wire directly in ohms.
7. Question: How is resistivity calculated?
Answer: ρ = (R·A)/L.
Explanation: Once you know the resistance from the graph and have measured area and length, you can use this formula to calculate resistivity.
8. Question: How is the cross-sectional area measured?
Answer: Using a screw gauge to measure diameter, then A = π(d/2)².
Explanation: Measuring the diameter with precision allows you to calculate area accurately, which is essential for finding resistivity.
9. Question: What is the role of a rheostat in the experiment?
Answer: It is used to vary the current in the circuit.
Explanation: The rheostat adjusts resistance dynamically, enabling smooth changes in current without needing to reconnect wires.
10. Question: Why should temperature be kept constant?
Answer: To prevent resistance from changing.
Explanation: Resistance increases with temperature, so maintaining thermal stability ensures accurate results consistent with Ohm's law.
11. Question: Why use thick cotton-covered wires?
Answer: To minimize resistance and avoid short circuits.
Explanation: Thicker wires carry current more effectively, and insulation ensures safety and reduces potential sources of error.
12. Question: Is Ohm’s law applicable to all materials?
Answer: No, only ohmic materials.
Explanation: Some materials like diodes and thermistors do not obey Ohm's law because their resistance changes with current or temperature.
13. Question: What is an ohmic conductor?
Answer: A conductor that follows Ohm’s law.
Explanation: Its voltage-current graph is linear and passes through the origin, showing constant resistance.
14. Question: What are the main sources of error in this experiment?
Answer: Loose connections, heating, and instrument error.
Explanation: Any inconsistencies in connections or fluctuations in temperature can affect the accuracy of your measurements and final results.
15. Question: Why are multiple readings taken?
Answer: To increase accuracy and reduce errors.
Explanation: Averaging several readings helps eliminate random fluctuations and improves result reliability.
16. Question: What is the effect of wire length on resistance?
Answer: Resistance increases with length.
Explanation: Longer wires offer more opposition to current flow due to increased number of collisions among electrons.
17. Question: What happens if the wire thickness increases?
Answer: Resistance decreases.
Explanation: A larger cross-sectional area allows more electrons to flow, reducing opposition and hence lowering resistance.
18. Question: What does it mean if the V–I graph does not pass through origin?
Answer: It indicates a systematic error.
Explanation: This could be due to a loose connection or an internal resistance in the measuring instrument.
19. Question: Why use digital voltmeters and ammeters?
Answer: For higher precision and ease of reading.
Explanation: Digital meters eliminate parallax errors and display values more accurately than analog meters.
20. Question: What is the advantage of plotting a graph in experiments?
Answer: It helps visualize relationships and find slopes accurately.
Explanation: Graphs not only validate theoretical concepts like Ohm’s law but also simplify the process of calculating resistance.
Conclusion
The practical experiment to determine the resistivity of a wire using a V–I graph is a foundational activity in understanding electric current, Ohm’s law, and material properties. Through careful measurement, plotting, and analysis, students can visually verify theoretical concepts and apply formulae in real-world scenarios. Preparing for viva questions like the ones above helps build confidence and ensures you are conceptually sound.
Always ensure accurate readings, maintain a steady temperature, and document your observations systematically. With practice, interpreting graphs and understanding the physics behind them becomes intuitive. If you're looking to deepen your understanding, try experimenting with different wire materials or gauge sizes.
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