A student while verifying Ohm's law calculated the value of resistance of the resistor for each set of observation. However, the values of resistance were slightly different from the actual value. Is his experiment wrong? Justify your answer.
A student while verifying Ohm's law calculated the value of resistance of the resistor for each set of observation. However, the values of resistance were slightly different from the actual value. Is his experiment wrong? Justify your answer.
No, his experiment is not wrong. The slight variation in the calculated resistance values from the actual value is expected in practical physics experiments due to the following reasons:
- Heating Effect of Current: As current flows through the resistor during each observation, electrical energy is dissipated as heat (). This causes the temperature of the resistor to rise. Since the resistance of a metallic conductor increases with an increase in temperature, the resistance does not remain strictly constant throughout the experiment.
- Experimental and Instrumental Limitations: Minor errors such as the least count error of the ammeter and voltmeter, contact resistance at the connection terminals, and parallax errors while reading the meters can cause slight deviations from the theoretical value.
Marking Scheme
- 11 Mark: Stating clearly that the experiment is not wrong and is scientifically valid.
- 21 Mark: Providing the justification based on the heating effect of current (temperature rise increases resistance) and/or experimental errors (contact resistance, least count).
Hint
Think about what happens to the temperature of a wire when current flows through it for some time, and how temperature affects resistance.
Quick Oral Answer
The experiment is correct because passing current through a resistor heats it up, which increases its resistance. Also, contact resistance at the terminals and instrument least-count limitations always introduce minor variations in real-world measurements.
Analysis & Explanation
The student's experiment is completely correct and scientifically valid. In school laboratories, ideal conditions (like perfectly constant temperature and zero contact resistance) cannot be maintained.
According to Ohm's law, the ratio is constant only if the temperature of the conductor remains constant. However, when current passes through the nichrome wire or resistor, heat is generated due to the collisions of flowing electrons with the atoms of the conductor. This temperature rise increases the thermal agitation of the atoms, offering more obstruction to the flow of electrons, thereby increasing the resistance. Thus, the resistance values calculated from successive readings (especially at higher currents) will be slightly higher than the initial or actual value. This is a standard observation in Ohm's law verification.
Common Mistakes
- 1Students often think that any deviation from the theoretical value means the experiment is completely wrong or failed, ignoring the practical limitations of temperature variation and instrument precision.
Interesting Facts
Standard resistance coils used in high-precision laboratories are made of alloys like Manganin or Constantan because their resistance changes very little with temperature (they have a very low temperature coefficient of resistance).
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2014?
This question carries 2 marks in the CBSE Class 10 Science 2014 examination.
Which chapter does this question come from in Science?
This question is from the chapter "Electricity" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "Ohm's Law" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2014 paper?
This is a Short Answer question from Section SECTION-B in the CBSE Class 10 Science 2014 paper.