Planck's constant got from experiment lower than accepted value

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Homework Help Overview

The discussion revolves around an experiment related to the photoelectric effect, specifically focusing on the determination of Planck's constant using varying stopping potential values obtained from a mercury lamp. The original poster reports inconsistent results across multiple trials, with the calculated values of Planck's constant deviating from the accepted value.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to understand the reasons behind the variability in their experimental results and seeks insights into potential sources of error. Some participants suggest considering experimental uncertainty and user error as possible factors affecting the results.

Discussion Status

Participants are exploring various explanations for the discrepancies in the results, including equipment limitations and user error. The original poster has provided additional context about their experimental setup, which may facilitate further discussion.

Contextual Notes

The original poster mentions that the battery level remained consistent across trials, which raises questions about other potential sources of error or variability in the experiment.

lkl1226
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Homework Statement


I did a photoelectric experiment using the PASCO Model AP-9368 mercury lamp. The stopping potential I got was very low: 1.476V for ultra violet, 1.303V for violet, 1.181V for blue, 0.788V for green and 0.707V for yellow. And the Plansk's constant I got from this set of values was about 4.005e-34J.s which is nowhere close than the 6.626e-34J.s

But when I did this experiment again about four days later, the stopping potential values I got changed: I got 1.964V for ultra violet, 1.686V for violet, 1.488V for blue, 0.840V for green and 0.728V for yellow. And this time the Planck's constant was 6.184e-34J.s which is better.

And after another two days when I did it again, the stopping potential values changed again to the lower ones. I checked the battery for the apparatus and the battery level was the same all three times. Could anyone please give me an idea why this is happening? I'm so confused? Thanks!
 
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Experimental uncertainty and inaccuracy due to limitations and intrinsic bias of the equipment is always a good bet. User error is another possibility. hard to tell without looking at the experimental set up.
 
Don't worry about Introductory Level Physics labs. During your UG course you will not be expected to find anything new in Physics.
 
dauto said:
Experimental uncertainty and inaccuracy due to limitations and intrinsic bias of the equipment is always a good bet. User error is another possibility. hard to tell without looking at the experimental set up.

Thank you for the ideas. The experiment set up is like the image below.
 

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